Effect of Herbicide-Organic Mulch Combinations on Weed Control and Herbicide Persistence DISSERTATION Presented in Partial Fulfi

Effect of Herbicide-Organic Mulch Combinations on Weed Control and Herbicide Persistence

DISSERTATION

Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University

Upender Reddy Somireddy, M.Sc.

Graduate Program in Horticulture and Crop Science

******

The Ohio State University

2011

Dissertation Committee:

Professor S. Kent Harrison, Adviser

Professor John Cardina

Professor Mark Bennett

Professor Terrence Graham

Upender Reddy Somireddy

2011

Abstract

Integration of different weed control methods is essential to address the financial as well as environmental concerns being faced by nursery and landscape industry.

Herbicide and mulch combinations have been suggested to achieve longer weed control in nurseries and landscapes. Field and laboratory experiments were conducted to determine the effects of herbicide formulations, mulch materials, depths of mulches, and herbicide placement relative to mulches on herbicide efficacy and persistence. Two field experiments were conducted, the first in the fall of 2006 and 2007 and the second in spring of 2007 and 2008. Granular and liquid formulations of a trifluralin+isoxaben mixture were applied alone and in combination with pine nuggets and hardwood mulch at three depths, 3-, 6-, and 12-cm. Granular herbicides were applied alone (without mulch) and above the mulch; and granular-pretreated mulches were also included. Liquid herbicides were applied alone, over the mulch, under the mulch, or as herbicide- pretreated mulches. Trifluralin and isoxaben in all formulations were applied at the rate of 4.48 kg ai/ha + 1.12 kg ai/ha, respectively. Mulch alone treatments and untreated control (no mulch, no herbicide) were also included. Visual weed control ratings and weed fresh weight data were collected at 30, 60, 90, and 120 days after treatment (DAT) for the spring experiment and at 180 and 210 DAT for the fall experiment. Visual ratings were based on a scale of 0 (no control) to 10 (complete control), with 7 and above being ii commercially acceptable. Selected treatments from the spring experiment, including the granular formulation alone, the liquid formulation alone, liquid formulation applied under

6-cm mulches, and the liquid formulation-treated mulches at 6-cm were used to investigate herbicide persistence. Herbicide residue analysis using liquid chromatography and bioassay studies using oats and radish were part of the herbicide persistence studies.

All the treatments involving 6-cm and 12-cm mulch with or without herbicides provided efficacy ratings of above 7 in both experiments. Certain combinations of 3-cm mulch and herbicides, such as granular formulation over 3-cm pine nuggets, liquid formulation under 3-cm pine nuggets, and liquid formulation under 3-cm hardwood consistently provided at efficacy ratings ≥ 7 at all evaluation dates in both studies. This could be due to the longer residual activity of herbicides in those treatments. The time-course assay of herbicide dissipation in soil indicated that herbicides applied under the mulch persisted longer compared to herbicides applied alone. Results suggested that the persistence of herbicides largely depended on the physico-chemical properties of herbicides and mulches, as well as soil moisture and temperature. Weed control with greater mulch thickness could be largely due to the physical inhibition of weed germination and growth by the mulch, whereas at lower mulch thickness, the addition of herbicides to the mulch treatments was necessary to provide weed control equivalent to the thick mulch layer.

Mulches applied at the 12-cm depth are expensive and can be detrimental to health of desirable plants, even though it provided almost complete weed control. Depth of mulches could be reduced to 3-cm from commercially recommended depths of 5- to 8-cm when herbicides were combined with mulches.

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Acknowledgements

I would like to thank my advisor Dr. S. Kent Harrison for his invaluable guidance and intellectual support. I would like to thank Dr. John Cardina for the encouragement and guidance to finish my Ph.D. program. It is not possible without the help, support and guidance of Drs. Harrison and Cardina. I would like to thank Dr. Terry Graham for agreeing to serve as committee member and helping me in herbicide residue analysis.

Thanks to Dr. Mark Bennett for agreeing to serve as committee member and providing feedback on my dissertation.

I would like to express my sincere gratitude to Dr. Hannah Mathers for giving me opportunity to pursue Ph.D. program and providing me guidance and financial support.

My special thanks to Mathers lab members: Luke Case and Kyle Daniel for their help in set up field experiments as well as in data analysis. I would also like to thank other current and former members of Mathers lab, Alejendra Acuna, Dania Rivera, Jason

Parish, Cheryl, Denise Johnson, Phoebe Gordon, and Michele Bigger. I would also like to thank Tara Daniel for the constant encouragement and support during my Ph.D. program.

Thanks to Mark and Glenn of Waterman farm, and green house coordinators,

Dave and Jim for their help in field and green house studies. I would also like to thank

Elaine Grassbaugh for her help in my bioassay study, and Sourav Chakraborty, Stephen

Opiyo, and Jiye Chang of plant pathology department for their help with chemical analysis.

I would also like to thank Dr. David Gardener and his lab group, Aneta

Studzinska and Ed Nangle for their help and support with herbicide extraction. I am also thankful to rest of the HCS faculty, staff and students who made my stay at OSU more enjoyable and less painful.

Finally, I would like to thank my family and friends for their constant support and encouragement to successfully finish my studies at OSU.

Vita

2000 ...... B.Sc. Agriculture. A.N.G.R. Agricultural University, Hyderabad, India

2003 ...... M.Sc. Agronomy, A.N.G.R. Agricultural University, Hyderabad, India

2004 ...... Farm Supervisor, Mahyco Research Foundation, Hyderabad, India

2005 ...... Junior Research Fellow, A.N.G.R. Agricultural University, Hyderabad, India

2006 ...... Production Executive, Monsanto India Ltd. Sathupally, Khammam, India

June, 2006 to present ...... Graduate Research Associate, Department of Horticulture and Crop Science, The Ohio State University

Fields of Study

Major Field: Horticulture and Crop Science

Table of Contents

Page #

Abstract…………………………………………………...……………………...... ii

Acknowledgements……………………………………………………….……………....iv

Vita……………………………………………………………………………………...... vi

List of Tables……………………………………………………………………...…….. ix

List of Figures……………………………………………………………………………..x

Chapter 1: Literature Review

Impacts of weeds on crop production……………………………………..1 Weed management methods……………………………………...... 3 Herbicide disadvantages…………………………………………. ………9 New innovations for weed management…………………………. ……..14 Organic mulches………………………………………………….……...18 Allelopathy……………………………………………………………… 21 Herbicide and mulch combinations…………………………………….. 22 Literature Cited……………...…………….……………………………..27

Chapter 2: Effect of spring applied herbicide-organic mulch combinations on weed control

Introduction………………………………………………………………38 Materials and Methods…………………………………………………...44 Results and Discussion…………………………………………………..46

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Page #

Literature Cited…………………………………………………………………..70

Chapter 3: Effect of fall applied herbicide-organic mulch combinations on weed control

Introduction………………………………………………………………74 Materials and Methods…………………………………………………...80 Results and Discussion…………………………………………………..83 Literature Cited…………………………………………………………..96

Chapter 4: Effect of herbicide-organic mulch combinations on herbicide persistence in soil

Introduction……………………………….…………………………….102 Materials and Methods……………………………………………….…105 Results and Discussion…………………………………………………111 Literature Cited…………………………………………………………127

Chapter 5: Summary and conclusions………………………………………………….131

Literature Cited…………………………………………………………………………135

Appendix A: Wooden boxes used to treat mulches…………………………………….153 Appendix B: Wooden boxes used to apply mulches in the field……………………….154 Appendix C: Resulsts of orthogonal analysis for spring……………………………….155 Appendix D: Resulsts of orthogonal analysis for fall…………………………………..156

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List of Tables

Table Page #

1.1 Preemergence herbicides labeled for weed control in ornamental crops (Kuhns et al. 2007)……………………………………………..7

2.1 Weed fresh weight (g) and visual ratings of herbicide, mulch, and herbicide-mulch combined treatments in 2007 and 2008…………………….…58

2.2 Weed fresh weight (g) and visual ratings of herbicide, mulch, and herbicide-mulch combined treatments in 2007 and 2008……………………..…60

2.3 Weed fresh weights (g) of herbicide, mulch, and herbicide-mulch combined treatments at different DAT in 2008………………………………….62

2.4 Efficacy ratings of herbicide, mulch, and herbicide-mulch combined treatments at different days after treatments (DAT) in 2007…………………….64

2.5 Weed fresh weights (g) of herbicide, mulch, and herbicide-mulch combined treatments at different DAT in 2007……………...……..……………66

3.1 Weed efficacy ratings of fall applied mulch, herbicide, and mulch and herbicide combined treatments for experiments initiated in fall 2006 and 2007…………………………………………………………….92

3.2 Weed fresh weights (g) of fall applied mulch, herbicide, and mulch and herbicide combined treatments for experiments initiated in fall 2006 and 2007…………………………………………………….………94

4.1 Parameter values from exponential decay and sigmoid growth curves…………………………………………………………………...………126

List of Figures

Figure Page #

1.1 Trifluralin and isoxaben molecules…………………………………………...... 6

1.2 Commercially available herbicide premixed mulches…………………………...26

2.1 Air and Soil temperatures during experimental period for 2007 and 2008 experiments………………………………………………….68

2.2 Weekly precipitation received during experimental period for 2007 and 2008 experiment (OARDC 2010) along with historical precipitation data………………………………………………………………...69

4.1 Effect of herbicide application method on trifluralin dissipation in soil over time. An exponential decay function was fitted to trifluralin residue data for 2007 and 2008 experiments………………….……120

4.2 Oat root bioassay of trifluralin+isoxaben residues in soil as a sigmoidal function of days after treatment. …………………………………..121

4.3 Effect of herbicide application method on isoxaben dissipation in soil over time. An exponential decay function was fitted isoxaben residue data for (A) 2007 and (B) 2008 experiments………….……………….122

4.4 Radish root bioassay of isoxaben+trifluralin residues in soil as a sigmoid function of days after treatment…………………………………..…123

4.5 Oat and Radish bioassay to determine trifluralin and isoxaben residues, respectively…………………………………………………………...124

4.6 Weekly average tempratures (A) and precipitation (B) during the experiment period for 2007 and 2008………………………………………125 x

Chapter 1: Literature Review

Impacts of weeds on crop production. The greenhouse and nursery industry is the fifth largest agricultural sector in the United States in terms of economic output (USDA 2003), often experiencing growth and expansion even during recessionary periods (Hall et al.

2006). Increased production and maintenance costs have continuously been challenging and weed control is one of the major activities that require a large portion of the production costs.

Weed control is important in agronomic and horticultural crops as weeds compete for resources such as water, nutrients, space, and light, resulting in reduced crop growth.

Weeds can also harbor many type of insects (Cranston 1994) and pathogens (Hobbs et al.

1996). Some weeds produce allelopathic chemicals to suppress the growth of other plant species surrounding them (Agarwal et al. 2002; Kadioglu et al. 2005; Verma and Rao

2006). In the horticulture industry, weed competition and interference can result in ornamental plants with low vigor, reduced leaf size, and fewer flowers (Adams 1990;

Fletcher 1983). Plants with reduced growth result in lower prices, and some long-term crops may need to grow extra one year to reach saleable size (Adams 1990; Davison

1971). Moreover, consumers prefer weed-free container-grown plants over weedy container grown plants (Simpson et al. 2002).

Weeds and ornamental plants differ in their competitive ability for limited resources. Weeds often have the advantage over agronomic or horticultural plants due to their genetic traits for competition and reproduction. Some crop plants are extremely sensitive to weed competition, and weeds can reduce plant growth significantly in container and field nursery production. Norcini and Stamps (1994) reported 60% less growth of a ‘Convexa’ Japanese holly in 3.7-liter containers due to weed competition from large crabgrass. Shoot weight of ‘Fashion’ azalea was reduced up to 78% by

Eclipta spp. depending on the size of the container and number of Eclipta weeds per container (Berchielli-Robertson et al. 1990). One redroot pigweed (Amaranthus retroflexus) or large crabgrass (Digitaria sanguinalis) plant in a 2.4-liter container reduced the plant dry weight of Japanese holly (Ilex crenata) by 47% and 60%, respectively (Fretz 1972). Barnyardgrass (Echinochloa crus-galli L.), large crabgrass

(Digitaria sanguinalis L.), or giant foxtail (Setaria faberi Herrm.) at a population density of five weeds per container suppressed the shoot dry weight of Bailey's redosier dogwood

( Cornus x baileyi) as much as 72% after 83 days of weed interference (Walker and

Williams 1988).

Walker and Williams (1989) observed that bush cinquefoil (Potentilla fruiticosa) shoot dry weight was reduced 52% by a single grass weed and flowers were reduced up to 99% depending on the number of grass weeds per container. The same authors reported that shoot growth of container-grown ‘San Jose’ juniper (Juniperus chinensis L.

‘San Jose’) was reduced 43% following 83 days of interference from barnyardgrass at a density of six plants per container (Walker and Williams 1990). Wilbourn and Rauch

(1972) reported that growth of Pyracantha, Andorra juniper, and Japanese holly was suppressed by coffeeweed (Senna obtusifolia) and common cocklebur (Xanthium strumarium). Neely (1984) reported 50% growth reduction in landscape trees due to the competition from grasses for nitrogen.

In contrast to weed interference effects described earlier, some weeds appear to have little or no effect the growth of nursery plants. Pennsylvania bittercress (Cardamine pensylvanica) did not reduce the growth of Pyracantha, Andorra juniper, and Japanese holly (Wilbourn and Rauch 1972). Even if some weeds do not reduce the growth of nursery crops, a container-grown plant with weeds present is a less marketable product than a weed-free product (Norcini and Stamps 1994). Weeds also reduce the aesthetic value of landscapes (Altland et al. 2003; Derr 1995; Gilliam et al. 1992; Kuhns et al.

2007).

Weed management methods. Weeds in the landscape and nursery industry are generally controlled by physical, cultural, and/or chemical methods. Physical and cultural methods include cultivation, hand-weeding, mowing, mulching, planting cover crops, and selecting a weed free growing medium (Kuhns et al. 2007). Proper sanitation is also important to prevent the spread and regeneration of troublesome weeds (Fausey 2003).

Weed control in container nursery production is mainly achieved by the use of herbicides and hand-weeding. Mulching and cultivation are also sometimes employed to control weeds in field nursery production (Appleton and Derr 1990; Appleton and French 2000;

Calkins et al. 1996). Cultivation is an effective method to control weeds even it has some disadvantages including loss of soil structure due to compaction and soil erosion (Kuhns

3 et al. 2007). Cultivation can also damage storage organs and shallow roots just below the soil surface in herbaceous perennial species (Calkins et al. 1996).

At the present time, herbicides are the most widely used and effective tools for weed management in the commercial landscape and nursery industry. Before the introduction of synthetic organic herbicides, inorganic chemicals such as copper salts, sodium nitrate, and iron sulfate were used for chemical weed control. However, these chemicals were needed in large quantities and thus were expensive and not used frequently (Peterson 1967). Oils and kerosene were used between 1919 and the 1940s in tolerant crops like citrus, cranberry, and carrot (Timmons 1970). In the early 1940s the herbicidal properties of the synthetic organic compound 2, 4-D (2, 4-dichlorophenoxy acetic acid) were discovered and patented. By 1962, there were 100 herbicides in the market in 6000 different formulations (Peterson 1967). By 1996, the number of different herbicides reached 146 (Weed Technology 1996).

Herbicides are currently used on 90% of all U.S. cropping acreage (Gianessi and

Sankula 2003). The total amount of pesticides applied in 2000-2001 was over 2.2 billion and 5.5 million kg in the world and the U.S., respectively. The major portion of these total was comprised of herbicides (Kiely et al. 2004). In the U.S., herbicides represented

60% by volume and 65% of the money spent on pesticides (Donaldson et al. 2002).

However, there are a limited number of herbicides available for use in the ornamental plant industry (Case et al. 2005). The use of herbicides in container-grown nursery production became standard practice in the 1970s. Remarkable developments took place

4 in the 1980s in the form of new herbicides and formulations for nursery weed control

(Berchielli et al. 1988; Kalmowitz and Whitwell 1988; Whitwell and Kalmowitz 1989).

Although herbicides are widely used in nursery production areas, preventive weed control measures are strictly followed (Norcini and Stamps 1994). Generally, only preemergent herbicides are used in nursery and landscapes. However, postemegence herbicides are (such as glyphosate) are also used in non-crop areas such as container yards between crops and around shade structures.

No single herbicide can control all types of weeds. Prepackaged mixtures of herbicides or tank-mixing individual broadleaf herbicides with grass herbicides is a common practice to achieve broadspectrum weed control (Judge et al. 2004; Altland et al.

2003). Most herbicide premixes used in nurseries contain a dinitroaniline herbicide such as trifluralin, oryzalin, pendimethalin or prodiamine (Altland et al. 2003), as these herbicides offer a broad range spectrum of weed control and crop tolerance (Neal et al.

1999; Stamps and Neal 1990; Weber and Monaco 1972). Herbicide products such as

OH2 (Scotts, Marysville, Ohio), Rout (Scotts, Marysville, Ohio), and Snapshot TG (Dow

AgroScience, Indianapolis, Indian) are some of those premixed combinations. A list of herbicides registered for nursery crops as of July 2007 is provided in the Table 1 (Kuhns et al. 2007).

Snapshot (Dow AgroSciences, Indianapolis, Indiana) contains 0.5% isoxaben and

2% trifluralin (Figure 1.1), and it is a widely used preemergent herbicide in the nursery and landscape industry (Judge et al. 2003; H. M. Mathers, personal communication).

Isoxaben is a pre-emergent herbicide that controls a wide range of broadleaf weeds in

5 cereals, turf, and ornamentals. Isoxaben kills weeds by inhibiting cellulose biosynthesis in emerging seedlings (Schneegurt et al. 1994). It is marketed for pre-emergence weed control in field nursery crop production as a 75% active dry flowable (DF) and is labeled for use in over 400 crop species. It is typically applied at 0.56 to 1.12 kg/ha (Altland

2003). Neal and Senesac (1990) reported that isoxaben provided excellent control of many broadleaved weeds including pigweeds (Amaranthus spp.), common groundsel

(Senecio vulgaris), and dandelion (Taraxacum officinale), but poor control of annual grasses such as crabgrasses (Digitaria spp.), goosegrass (Eleusine indica), and fall panicum (Panicum dichotomiflorum). The advantage of using isoxaben over the diphenylether herbicide oxyfluorfen (Dow AgroSciences, Indianapolis, Indiana) or the triazine herbicide simazine (Syngenta Crop Protection, Greensboro, North Carolina) is that isoxaben is safe on a broader range of nursery and landscape crops. Trifluralin is a grass-active herbicide used commonly in nurseries and landscapes. It is a dinitroaniline

(DNA) herbicide that acts by inhibiting root formation. DNAs are most widely used in the nursery due to their effectiveness, low cost, and low leaching potential.

Figure 1.1. Trifluralin and Isoxaben

Table 1.1. Preemergence herbicides labeled for weed control in ornamental crops (Kuhns et al. 2007) Ground Best for covers and Herbaceous broadleaves Field Container Landscape vines plants Aatrex x

Broadstar/Sureguard x x

Gallery x x x x x Goal x x

Princep x

Ronstar x x x x x Sureguard x x

Best for grasses

Barricade x x x x x Devrinol x x x x x Dimension x x x x x Pendulum x x x x x Pennant Magnum x x x x x Predict x

Surflan x x x x x Treflan x x x x x XL (Surflan+Balan) x x x x x Best for nutsedge

Pennant Magnum x x x x x Combinations OH2 x x x x (Goal+Pendulum) Regal 0-0 x x x (Goal+Ronstar) Rout (Goal+Surflan) x x x x x Snapshot x x x x x (Gallery+Treflan) Regalstar G x x x x x (Ronstar+Barricade) Control some perennial weeds Casoron x x

Image x

Kerb x

Pennant Magnum x x x x x 7

Selective postemergence herbicides for broadleaf weed control are not available for most nursery crops and broadleaf weed control can be achieved with preemergence herbicides only (Derr 1994). Available postemergence herbicides for selective grass control in nursery crops are fluazifop-butyl (Fusilade; Syngenta), sethoxydim (Vantage;

BASF), or clethodim (Envoy; Valent). Postemergence herbicides specifically used for nutsedge control are bentazon, halosulfuron, and imazaquin (Altland et al. 2003).

Granular formulations of herbicides are often preferred in the nursery industry as they are less phytotoxic to nursery stock and easy to apply (Gilliam et al. 1992). Weed control in container-grown plant production is typically accomplished by broadcasting granular herbicides with a cyclone spreader over the nursery stock (Gilliam et al. 1990;

Mathers 2003). Many scientists have demonstrated that liquid application of herbicides, though effective in controlling weeds, can cause injury to nursery plants. Fretz et al.

(1980) and Weller et al. (1984) compared a liquid (2E) formulation of oxyflourfen with a granular (2G) formulation and found that liquid formulations provide superior weed control but caused greater injury than the granular formulation.

New granular herbicide premixes including oxadiazon plus prodiamine, and oxyfluorfen plus oxadiazon are becoming more popular for their broadspectrum weed control. Flumioxazin, a relatively new chemical in the nursery industry, is now registered for use as a preemergence and early postemergence herbicide in much of the U.S. In

Alabama, three common herbicides in container nurseries are oxadiazon and two combination herbicides, pendimethalin plus oxyfluorfen and oryzalin plus oxyfluorfen

(Gilliam et al. 1990).

The use of herbicides in container-grown ornamentals is limited due to the lack of registered herbicides for greenhouses and difficulty in assuring crop safety on the wide range of crops grown in ornamental nurseries (Boydston et al. 2008). Due to this reason, the number of herbicides available to the ornamental industry is low (Case et al. 2005). A wide diversity of landscape plants makes herbicide selection a challenging task because of differences in herbicide selectivity (Watkins and Heggers 1982).

Herbicide disadvantages. Herbicides alone cannot completely control all weeds.

Chemical weed control in the nursery industry must often be supplemented by hand- weeding, which is expensive and laborious. In 1990, Gilliam et al. reported that nurseries spent $608-$1401 per ha ($ 246 – $567 per acre) on hand-weeding depending on the size of the nursery, with hourly wages ranging from $3.53 to $3.97. Gilliam also reported that the mean number of herbicide applications in the nursery industry was 2.9 to 3.2 and that total weed control costs, including chemical application and hand-weeding, ranged from approximately $2000-$2800 per ha ($812- $1133 per acre). Mathers (2003) reported more recently that nursery growers spend $1200 to $9800 per ha for manual removal of weeds, depending on the weed species to be removed. Other concerns due to herbicide use are improper calibration, herbicide runoff, spray drift, development of herbicide resistance, phytotoxicity to crop plants, and the need for multiple applications (Li et al.

2003; Radosevich et al. 1997).

Season-long weed control often requires a combination of herbicides as well as multiple applications of those herbicide combinations (Wehtje and Gilliam 1991).

Wehtje and Gilliam (1991) also reported that timing of herbicide application can be more

9 important than herbicide selection, provided that herbicide combinations used contain both broadleaf and grass herbicides in sufficient quantity. Multiple applications of herbicides are sometimes needed to control weeds throughout the growing season since many herbicides do not have sufficient residual activity from a single application to control weeds for the whole season (Wehtje and Gilliam 1991). Herbicidal activity of most of the herbicides used in the Southeastern United States is limited to 10-14 weeks

(Gilliam 1989). Combinations of herbicides and multiple applications ultimately lead to accumulation of herbicides in the environment. Environmental fate of herbicides has been a major concern of the ornamental plant industry for at least 20 years. It is difficult to understand or determine the long-term effects of herbicides on human beings because of the complexities between bodily processes and pesticides (Case et al. 2005).

Oxadiazon, a commonly herbicide in the nursery and landscape industry, can cause cancer in some laboratory animals but has slight to moderate acute mammalian toxicity

(Mattern et al. 1991).

In nursery containers, off-target herbicide movement from broadcast applications may reach up to 86% depending on the spacing of containers and growth habit of the plants in the containers (Gilliam et al. 1992; Porter and Parish 1993). Due to the foliar coverage of the soil surface in containers, herbicide application is often non-uniform and incomplete as the herbicide granules tend to bounce off the foliage and do not reach the soil surface in containers (Gorski 1993). Herbicide that falls outside the containers is a potential water contaminant of water by leaching or runoff following irrigation or rainfall events (Riley et al. 1994). Significant herbicide contamination of irrigation ponds and

10 other off-target areas has been reported (Horowitz and Elmore 1991; Keese et al. 1994;

Riley et al. 1994; Riley 2003). Containment ponds or recirculation ponds, which are used to irrigate the nursery areas, can collect 90% of the runoff water after irrigation and can become reservoirs for off-target herbicide residues (Keese et al. 1994; Riley et al. 1994).

The water from these ponds can be used for irrigation and be phytotoxic to subsequent nursery crops (Gilliam et al. 1992; Horowitz and Elmore 1991).

Off-target herbicide loss varies with the formulation of herbicide. Losses are often higher for liquid formulations of herbicide than for granular herbicides (Mahnken et al. 1992). Herbicide formulations such as emulsifiable concentrations can make the herbicides more prone to losses by runoff or leaching (Gorski 2003). Herbicides that rapidly release active ingredients can also reduce the period of weed control efficacy compared to slow-release formulations (Gorski 1993). The concerns mentioned above show a clear need for new herbicide application methods and/or formulations that reduce to potential for off-target herbicide movement and environmental contamination (Gilliam et al. 1992)

Another disadvantage of herbicide use is the development of herbicide resistance in weeds. Herbicide resistance can be defined as the inherited ability of a plant to survive and reproduce following exposure to an herbicide dose that would normally be lethal to the wild type (Prather et al. 2009). Repeated use of herbicides with the same biochemical mode of action speeds the development of resistant weed populations (Mallory-Smith et al. 1999). The first species reported to be herbicide resistant in the U.S. was common groundsel (Senecio vulgaris), which became resistant to triazine herbicides in a tree

nursery in Washington in the year 1970. Since then, 368 herbicide-resistant biotypes have been reported, representing 200 species (115 dicots and 85 monocots) (HRAC

2011). Herbicide resistance in plants is increasing at an exponential rate, and the lack of new herbicide discovery is likely to increase the development of resistant biotypes

(Prather et al. 2000). Rotating herbicides with different site of actions can delay resistance development, but in minor crops there is a limited number of registered herbicides available.

No single herbicide is safe on all crops. In the ornamental plant industry, herbicides should be highly effective on weeds and at the same time they should have no or low phytotoxicity on ornamental crops because a premium is placed on aesthetic value of the crop. Herbicides can reduce the value of ornamentals by causing leaf scorch, chlorosis on leaves and stems, and reduce plant growth (Adams 1990). Herbicide selectivity is highly species-specific, especially in ornamental plants. Phytotoxicity of isoxaben plus oryzalin was observed on Gloriosa daisy (Rudbeckia hirta L.) but not on lanceleaf coreopsis (Coreopsis Lanceolata L.) (Porter 1991). In the same study, isoxaben was safe on Gloriosa daisy and lanceleaf coreopsis but injured Shasta daisy.

Simazine, a broadleaf herbicide commonly used in nurseries, caused phytotoxicity to Ligustrum, Forsythia, Salix, Weigelia, Philadelphus, and Prunu, Euonymus, Buxus,

Nandina, Rosa, Syringa,and others (Ahrens 1963, 1966; Briggs 1978; Chadwick 1960;

Gordinier 1960; Sherwood and Kemmerer 1964). Ryan et al. (1981) observed injury to

roadside plantings from a preemegence application of simazine at 6.7 kg/ha. Even at lower doses, simazine was injurious to ‘Lynwood Gold’ forsythia (Forsythia

×intermedia), weeping willow (Salix ×niobi), ‘Vanicek’ weigelia (Weigela florida), mock orange (Philadelphus virginalis), and flowering pink almond (Prunus glanulosa) (Ahrens

1963). Other studies also reported simazine sensitivity of woody plant species such as

Catawba rhododendron (Rhododendron catawbiense), Japanese andromeda (Pieris japonica) (Bing 1981), eastern arborvitae (Thuja occidentalis) (Davis and Minton 1982), and ‘Greenspire’ linden (Tilia cordata) (Smith 1980).

Calkins et al. (1996) tested 12 preemergence and 2 postemergence herbicides registered for turf and woody landscape species and reported that Hemerocallis ‘After

Dark’ and Phlox maculate ‘Omega’ were sensitive to all herbicides. Blue fescue

(Festuca ovina var. glauca) was sensitive to preemergence Gallery 75WDG + Surflan

4AS and post emergence applied Snapshot 80DF, Goal 1.6EC, and Rout GS; Narrow- leaved plantain lily (Hosta lancifolia) was injured by Ronstar 50WP, hosta was injured by post-applied Stomp 3.3EC, Ronstar 50WP, and Goal 1.6EC; Gayfeather (Liatris spicata) was injured by Goal 1.6EC; and Paeonia ‘Felix Crouse’ was injured by Ronstar

50WP. They also observed that no single herbicide was acceptable for use on all the species they tested, and herbicide phytotoxicity was species- and cultivar-dependent.

Some herbicides can cause damage to ornamental plants by leaching into the root zone. Derr and Salihu (1996) observed root growth damage in Abelia (Abelia x grandifl ora) following oryzalin applications. This may be have been caused by the large

macropores present in container media after potting, which could allow herbicides to readily enter the root zone (Altland 2002).

Postemergence herbicides have an advantage over preemergence herbicides in that postemergence herbicides can be selected based on weed species that are present.

Preemergence herbicides on the other hand are often applied without prior knowledge of weed species that are present in the soil, resulting in poor weed control and loss of revenue (Case et al. 2005).

New innovation for weed management. Increased costs of production and environmental concerns of herbicide usage are compelling scientists to find economically and environmentally sound weed management strategies. Some innovative techniques have been introduced to use herbicides more efficiently or to reduce or avoid herbicide usage. For example, scientists have developed physical barriers for weed control in container production (Appleton and Derr 1990). Some of these techniques involve the use of weed discs made up of different materials such as paper, plastic, semi-rigid plastic, geotextile, fiberglass discs, and herbicide impregnated cloth (Danielson 1967). Other developments include herbicide-impregnated string (Hamill et al. 1975), geotextile disks

(Appleton and Derr 1990), fabric (Tex-R Geodisc) (Saint-Elzear-de-Beauce, Québec,

Canada), pressed peat moss (Biodisc) (Premier Tech, Riviere-du-Loup, Québec, Canada), corrugated cardboard (Corrudisc) (St. Catharines, Allanburg, Ontario., Canada), plastic

(Enviro LID) (Enviro LID, Langley, B.C., Canada), Mori Weed Bag (Mori Nurseries,

Niagara–On-The-Lake, Ontario, Canada) (Mathers 2003) and slow-release herbicide tablets (Verma and Smith 1978; Ruizzo et al. 1983; Gorski et al. 1989; Gorski 1993).

Due to various factors such as poor weed control, insufficient demand, and/or high costs, only some of the weed control discs mentioned above are being manufactured currently, including Weed Guard (Connon Nurseries, Rockton, Ontario, Canada), Tex-R

Geodisc (Texel USA, Henderson, N.C.), Biodisc, and Enviro LID (Chong 2003).

Combination of landscape fabrics with an herbicide can provide better weed control in container production than fabrics alone. Appleton and Derr (1990a) reported that discs composed of two layers of a landscape fabric in which herbicides and fertilizers are placed, called ‘Herbicide collars’, have controlled weeds better and longer than standard herbicide applications or discs without any herbicide. Tex-R Geodisc has shown promising results in controlling weeds (Svenson 1998; Mervosh 1999). Tex-R Geodiscs are needlepunched, nonwoven polypropylene disks coated with Spinout (Appleton and

French 2000). Spinout contains copper hydroxide and is used in the nursery industry to curtail excessive root growth in plant containers (Struve and Rhodus 1990). Appleton and French (2000) recorded excellent weed control with Geodiscs (made with polypropylene fabric) for up to six months.

A possible alternative to chemical weed control in containers is Geodiscs

(Appleton and French 2000). Disadvantages of Geodiscs include size of the opening in the disc for the plant, and wind blowing the disc from the pot (Case et al. 2005). In one study, many weeds grew around the outside edges and through the hole cut in the center to fit around the liner or seedling (Appleton and Derr 1990). Corrudiscs do not hold up

15 well under irrigation and Biodiscs quickly degrade in normal container culture (Chong and Purvis 2000). Weed discs have been on the market for several years, but are not widely used due to the price, extra expense for the grower and time of placing the disc in each container (Tatum et al. 1999).

Tatum et al. (1999) reported good weed control in containers with a 3-cm layer of crumb rubber. PolyVulc of Vicksburg, Mississippi was planning to develop discs using crumb rubber (Tatum et al. 1999). Wulpack (Wilbro, Inc Norway, S.C.) and

PennMulchTM (Lebanon Seaboard Corporation, Lebanon, Pa.) are products similar to geodiscs that control weeds by covering the surface of the growth media (Mathers 2003).

Wulpack is composed of pelletized sweepings from shearing floors of sheep operations

(Wooten and Neal 2000), and PennMulch is a pelletized newspaper product with 1% nitrogen. Wooten and Neal (2000) reported that most weed species were controlled with

Wulpack and PennMulch but some weeds were better controlled by Geodiscs.

Herbicides are often combined with different carriers to reduce the amount of herbicide needed for container-grown plants, to enhance and/or extend efficacy (Derr

1994), to increase environmental and ecological safety, or to avert some current label restrictions (Mathers and Ozkan 2001). For example, slow-release tablets have been developed as porous pellets made up of inert material such as plaster of paris, dicalcium phosphate, starch, starch xanthide, pine kraft lignin plus a pre-emergent herbicide (Baur

1980; Koncal et al. 1981a; Koncal et al. 1981b; Riggle and Penner 1988; Ruizzo et al.

1983; Smith and Treater 1987; Smith and Verma 1977; Verma and Smith 1978; Verma and Smith 1981; White and Schreiber 1984). Small amounts of herbicide are released into

16 containers when the tablets get wet during irrigation. Slow-release tablets control weeds for a period of a few weeks to 14 months depending upon the herbicide concentration in the tablets, amount of inert material used, type of herbicide, size of container, and porosity of the tablets (Gorski et al. 1989; Koncal et al. 1981a; Koncal et al. 1981b;

Ruizzo et al. 1983; Smith and Verma 1977; Verma and Smith 1978; Verma and Smith

1981; Smith and Treaster 1987). Crossan et al. (1997) found that controlled release fertilizers coated with oxadiazon were effective at suppressing prostrate spurge and large crabgrass. A dicalcium phosphate tablet coated with propachlor, alachlor, naptalam, or chloramben provided good weed control when the right number of tablets was placed in the right sized containers (Ruizzo et al. 1983).

Advantages of slow-release herbicide tablets include reduced herbicide application rates, less potential for herbicide leaching and off-target herbicide losses, less labor for application, no calibration required, and reduced applicator exposure.

Difficulties associated with slow-release tablets include maintaining uniform herbicide distribution and sufficient release rate in a container (Derr 1994). Slow-release herbicide tablets could potentially eliminate overspray and drift and greatly reduce leaching

(Gorski 2003).

Herbicide-treated paper is another innovative technique to control weeds in container production (e.g., ‘Herbisheet’ from American Cyanamid, Princeton, N.J.).

Herbicide-treated paper can be cut to fit different size containers or landscape beds.

Irrigation or rainfall releases the herbicide from the paper into the soil or mulch surface.

A thin layer of mulch on herbicide treated paper can help to keep the paper intact (Derr

1994). Fertilizers coated with pre-emergent herbicides have also been investigated, but weed control was relatively short-lived and declined 130 days after treatment (Mathers

2003). The advantages of pre-emergent herbicide coated fertilizer products include reduced herbicide dose, enhanced efficacy and simple and safe application.

Organic mulches. One way to suppress weed growth is to apply some type of mulch to the soil surface (Robinson 1988). A mulch is any material other than soil, specifically placed at the soil–air interface to manage soil and water and create a favorable environment for plant growth (Lal 2002). Organic mulches are typically derived from plant materials. Different plant materials that have been used as mulch include hardwood and softwood bark, buckwheat hulls, rice hulls, cocoa-bean hulls, compost, crushed corncobs, leguminous hay, spent hops, lawn clippings, leaf mold, leaves, manure, spent mushroom compost, peanut hulls, peat moss, pecan shells, pine needles, sawdust, straw, wood chips, and newspaper. Organic mulches are attractive alternatives to herbicides for weed control, as 38 million metric tons of urban tree residues are produced every year in the USA alone (National Research Council 2000). Wood mulch is used to control weeds in organic and sustainable production systems where chemical herbicides are not desirable. Moreover, wood mulch is convenient, cheap, renewable, and often locally available (Rathinasabapathi et al. 2005). Fresh organic mulches are more effective for weed control than older, decaying mulches (Duryea et al. 1999). Suttle (1996) and

Svenson (2002) advocated the use of mulches such as hazelnut shells, peanut shells, cocoa shells, and oyster shells for suppression of certain weeds. Newspaper mulch

18 reduced weed growth as well as soil temperatures (Pellett and Heleba 1995). Ahn and

Chung (2000) achieved weed control in containers using rice hulls.

Controlling pests using the byproducts of agricultural and related industries (e.g., the biofuel industry) could increase the profitability of those industries (Boydston et al.

2008a; Boydston et al. 2008b; Miller 2006). Corn gluten meal, dried distiller grains with solubles (DDGS) and mustard seed meal (MSM) are some of those byproducts. Corn gluten meal, a byproduct of corn wet-milling, and dried distiller grains with solubles

(DDGS), a byproduct of ethanol production, have been shown to have herbicidal properties (Bingaman and Christians 1995; Boydston et al. 2008a, b; Liu and Christians

1997; Liu et al. 1994; McDade and Christians 2001; Nonnecke and Christians 1993).

Previous studies reported weed control with mustard seed meal (Ascard and Johansson

1991; Boydston et al. 2008b; Brown et al. 2006; Earlywine et al. 2007; Miller 2006; Rice et al. 2007). Mustard seed meal contains glucosinolates which eventually break down into isothiocyanates which have herbicidal and pesticidal properties (Al-Khatib and Boydston

1999; Borek and Morra 2005; Boydston and Al-Khatib 2006; Brown and Morra 1995;

Daxenbichler et al. 1991; Oleszek 1987; Oleszek et al.1994; Vaughn and Boydston

1997).

Mulches offer many potential benefits such as protection of desirable plants from extreme temperatures (Harris 1983), soil moisture conservation by reducing evaporation

(Faber et al. 2001; Fraedrich and Ham 1982; Iles and Dosmann 1999; Watson and

Kupkowski 1991), improvement of water infiltration (Faber et al. 2001; Hoyt and

Hargrove 1986), reduced soil erosion and enhanced soil properties such as organic

19 matter, nutrient content, porosity, water holding capacity, microbial population, and cation exchange capacity (Abdul-Baki and Teasdale 1993; Ashworth and Harrison 1983;

Greenly and Rakow 1995), weed suppression and delayed weed emergence (Autio et al.

1991; Bond and Grundy 2001; Harris 1983; Hussein and Radwan 2002; Teasdale and

Mohler 1993), and maintenance of optimum soil temperatures (Fraedrich and Ham 1982;

Montague and Kjelgren 2004). Raindrop impacts are a main cause of soil erosion (Borst and Woodburn 1942), but their detrimental effects can be eliminated with mulching

(Harris 1983). In one study, mulches reduced maximum soil temperatures by 2.3 to 3.3

0C and increased minimum temperatures by 1.1 to 2.2 0C (Skroch et al., 1992d). Mulches are effective suppressors of water evaporation (Scholl and Schwemmer 1982).

Foshee et al. (1996) reported that the growth of young pecan trees under five different types of mulches (hardwood leaves, pine nuggets, pine straw, grass clippings, and chipped limbs) increased 60 to 70% compared to trees where weeds were controlled with herbicide and sod, respectively. Wilen et al. (1999) reported 62% and 79% greater root and shoot dry mass, respectively, when bottle brush was grown under the organic mulches compared to non-mulched treatments. Mulches increased the quality and quantity of fruits in orchards (MacRae et al. 2007; Kalita and Bhattacharyya 1995), and can reduce the occurrence of some plant diseases (Gleason et al. 2001).

Mulching can greatly reduce weed growth and its negative effects (Autio et al

1991; Harris 1983). Organic mulches control weeds by inhibiting weed seed germination and suppressing weed growth (Borland 1990; Duryea et al. 1999; Skroch et al. 1992).

Mulches provide effective weed control only if they are applied at proper thickness

(Mohanty et al. 2002; Ligneau and Watt 1995; Robinson 1988; Singh et al. 1985).

Abouzinea et al. (2008) obtained 95-99% weed control with 2-3 layers of cattail mulch or rice straw mulch.

There are some disadvantages of using mulches for weed control, including increased soil acidity and reduced soil nitrogen (Billeaud and Zajicek 1989; Campbell

1950; McCool 1948). Most landscape plants do not grow well in acidic soils except for a few acid-loving species such as blueberries and azaleas (Korcak 1998; Marx et al. 1996).

As mulches decompose, they can become a growing medium for weeds (Derr and

Appleton 1989). Even though organic mulches applied 15 cm deep effectively controlled weeds, they were detrimental to the establishment and growth of landscape plants. Thick mulching may reduce aeration and water penetration into the soil or create saturated soil conditions that lead to the decline and death of many shallow rooted shrubs and trees

(Billeaud and Zajicek 1989).

Allelopathy. Allelopathy is the inhibition of seed germination and growth of plants by other plants that release chemicals into environment (Duryea et al. 1999). These allelopathic chemicals may be released into environment through volatilization, exudation, leaching from plant parts, or release from decaying plant parts. Allelopathic properties of certain plants can be utilized for the purpose of weed control. Duryea et al.

(1999) reported that water extracts collected from fresh and three-month- old eucalyptus, melaleuca, and pine straw mulches inhibited germination of lettuce seeds. Cregg and

Schutzki (2009) found that the growth of landscape shrubs was suppressed by cypress mulch and they attributed this to the allelopathic properties of the mulch.

Wood chips and leaf mulches of southern red cedar (Juniperus silicicola) and southern magnolia (Magnolia grandiflora) contained water-soluble allelopathic chemicals useful for suppression of large crabgrass and redroot pigweed in horticultural production systems (Rathinasabapathi et al. 2005). In a lettuce bioassay study, Ferguson et al. (2008) found that the leachates of oriental arborvitae (Thuja orientalis), southern red cedar, and southern magnolia significantly reduced lettuce seed germination.

Identification and isolation of allelopathic compounds from select species can potentially lead to development of bioherbicides (Appleton and French 2000).

Herbicide and mulch combinations. Mulching is a common weed control practice in landscapes, and the combination of mulches and herbicides holds promise as a method to control weeds in containers, field nurseries, and landscapes and reduce labor costs, concomitantly. Weed management strategies for field production and landscape environments requires extensive knowledge of weed biology, herbicide application and calibration procedures, herbicide efficacy against target weeds, landscape and nursery crop tolerances to herbicides, and correct timing of application (Altland 2003). The most common reasons for ineffectiveness of herbicides are improper application timing, improper application rates, and wrong selection of herbicide for the prevalent weed species (Altland et al. 2003). Herbicide-treated mulches can address most of these concerns and simplify weed control. The only calibration needed for herbicide treated mulches would be monitoring the mulch depth to ensure the optimum rate of application

(Mathers and Ozkan 2001).

Lanphear (1968) reported improved weed control when dichlobenil was incorporated into mulch compared to dichlobenil or mulch alone. Excellent weed control has been observed with a combination of pine nuggets in a single layer plus oxyfluorfen and pendimethalin at 2.g and 1.12 kg/ha, respectively (Derr 1994). Wells et al. (1987) reported a threefold increase in duration of weed control using pine bark mulches treated with herbicides rather than pine bark alone. Mulch plus lower than recommended herbicide rates suppressed weed growth for more than five months in the field (Ferguson et al. 2008). Samtani et al. (2007) compared herbicides alone with herbicide-treated leaf pellets, rice hulls, and pine bark applied at a 0.5 cm thickness and reported that the herbicide-treated mulches resulted in equivalent or, in a few cases, improved crop growth and weed control compared to herbicides alone.

Composted pine or hardwood bark is an important component of container media for growing landscape plants. Many researchers have used a layer of bark pretreated with preemergence herbicides and applied as a 2.5-cm layer in containers (Derr 1994).

Herbicide-treated mulches can be added to containers as a top layer during filling of pots in assembly line plantings (Derr 1994; Mathers 2003). Mathers (2003) conducted two- year studies on different mulches, discs, herbicides, and their combinations, and reported that the application of pre-emergent herbicide-treated bark nuggets resulted in increased and extended herbicide efficacy compared to the herbicides or mulches applied alone.

Herbicide-treated bark mulches, regardless of preemergence herbicides applied, provided excellent weed control. Four of the six most efficacious treatments were herbicide-treated

Douglas fir bark treatments. Wells and Constantin (1986) observed extended weed

23 control in containers with the application of pine bark mulch treated with various preemergence herbicides.

Herbicide-treated mulches can also reduce the phytotoxicity of herbicides to desirable species as there is little direct contact of herbicides with plants. Herbicide- treated mulch is safe on most nursery crops when the herbicide is applied to the mulch at the recommended dose (Derr 1994). Mathers et al. (2004) reported that hardwood or pine nuggets treated with acetochlor were not phytotoxic to ‘Care Free Beauty’ rose (Rosa spp.) or ‘Green Gem’ boxwood (Buxus spp.), but acetochlor-treated hardwood was slightly phytotoxic to ‘Little Princess’ spirea (Spirea spp.). Application of preemergence herbicides on mulches can reduce herbicide leaching up to 74% compared to the herbicides applied to the bare soil (Knight et al. 2001). Reduced herbicide leaching is attributable to adsorption of herbicides by the mulch material (Case et al. 2005).

Based on the many previous studies that proved the effectiveness of herbicide- treated mulches for weed control, mulches premixed with herbicides are now commercially available. Granular herbicide and mulch combinations were introduced for commercial and noncommercial landscape markets during 2006 and 2008 (Mathers

2010). Some of these products include Schultz Premium Mulch with Weed Stop®

(Schultz Co., Bridgeton, MO), Vigoro Premium Mulch Plus Weed Stop® (Spectrum

Group, St. Louis, MO), Preen Mulch® and Preen Mulch Plus® (Mulch Manufacturing,

Reynoldsburg, OH) (Figure 2) (Mulch and Soil Council 2009). These products contain either dithiopyr (Schultz and Vigoro) or isoxaben+trifluralin (Preen). Dithiopyr (3, 5- pyridine-dicarbothioic acid, 2- (difluoromethyl)-4-(2-methylpropyl)-6-(trifluoromethyl)-

S,S-dimethyl ester ) (Dimension®, Dow AgroSciences, Indianapolis, Indiana) is a pyridine herbicide that controls more than 45 grass and broadleaf weed species

(Anonymous 2011). It is recommended for established lawns, turf, container grown and field grown ornamentals, landscape ornamentals, commercial sod farms, and non- cropland and industrial sites (Anonymous 2010). Additional research and development is needed to determine what type of mulch-herbicide combinations will maximize weed control effectiveness while minimizing environmental impacts.

Figure 1.2. Commercially available herbicide premixed mulches.

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Chapter 2: Effect of spring applied herbicide-organic mulch combinations on weed control efficacy

Introduction

The greenhouse and nursery industry is the fifth largest agricultural sector in the

United States in terms of economic output (USDA 2003), often experiencing growth and expansion even during recessionary periods (Hall et al. 2006). Increased production and maintenance costs have continuously been challenging and weed control is one of the major activities that takes a large portion of the production costs. Weed control is essential in the nursery and landscape industry to keep weeds below economic threshold levels and to maintain aesthetics.

Nursery areas include containers, greenhouses for overwintering and propagation, and production fields. The nursery and landscape areas need to be maintained with optimum soil and nutrient regimes, which makes these areas ideal for weed infestations.

Weed problems in containers are more severe than in the field as the resources of nutrients, water and space are limited to the volume of the container. Nursery owners depend primarily on preemergence herbicides and hand weeding for weed control.

Preemergence herbicides are the most important tools for weed control even though they need to be applied multiple times in a year (Gilliam et al. 1990). There are various negative impacts that are associated with herbicide use such as herbicide resistance, 38 environmental pollution, and health concerns to humans and wildlife (Hoar et al.1986;

Nielsen and Lee 1987; EPA 1992). Some of the other problems associated with the use of herbicides are improper calibration, phytotoxicity to desirable species, leaching, spray drift, runoff and non-target loss. Non-target herbicide loss can be up to 86% depending on growth habit of the species and spacing of the container (Gilliam et al. 1992).

Chemical weed control using herbicides is usually supplemented by hand weeding as herbicides alone cannot provide complete weed control. Hand weeding expenses can range from $500-4000/acre depending on the type of weed species present (Mathers

2003). Due to these financial and environmental concerns coupled with stringent herbicide registration rules by environmental protection agencies, there is an urgent need to develop cost effective and environmentally safe weed control methods (Mathers 2003).

The availability of herbicides labeled for use in greenhouses is limited due to volatility issues, which further stresses the need for effective weed control strategies in nursery production.

The ideal weed control strategy suited to the current situation in nursery production would be effective utilization of both chemical and non-chemical methods that are currently available. Innovative techniques developed to enhance the efficacy of herbicides have included combinations of chemicals with physical barriers, such as herbicide impregnated cloth (Danielson 1967), herbicide impregnated string (Hamill et al.

1975), geotextile disks with a slow release formulation of trifluralin (Appleton and Derr

1990a), fabric (Tex-R Geodisc) (Saint-Elzear-de-Beauce, Québec, Canada), herbicide

39 collars (Appleton and Derr 1990a), and herbicide treated paper (‘Herbisheet’, American

Cyanamid, Princeton, N.J.) (Derr 1994).

Herbicides may be combined with different carriers to reduce the amount of herbicide needed to enhance and/or extend efficacy (Derr 1994), to increase environmental and ecological safety, and to avert some current label restrictions (Mathers and Ozkan 2001). Previous research demonstrated that organic mulches are effective carriers of herbicide (Case and Mathers 2006; Fretz and Dunham 1971; Fretz 1973;

Mathers 2003; and Mathers and Case 2010). Oliveira et al. (2000) proved that lignin, an important constituent of woody plant cell walls, was the best herbicide carrier for controlled release of herbicides. Organic mulches have already been widely used by landscape industry due to many advantages including weed control, soil moisture conservation, regulation of soil temperature, control of soil erosion and soil crusting, and increased aesthetic value of the landscape (Skroch et al. 1992). Weed control and soil moisture conservation, however, are the primary advantages (Robinson 1988). Organic mulches can inhibit weed germination and suppress weed growth (Borland 1990; Duryea et al. 1999; Skroch et al. 1992).

Herbicide treated mulch (HTM) is the combination of physical (mulch) and chemical (herbicide) weed control methods. Previous studies have demonstrated that

HTMs provided higher weed control efficacy (Fretz and Dunham 1971; Fretz 1973,

Mathers 2003), extended efficacy (Mathers and Case, 2010) and reduced phytotoxicity

(Mathers and Case 2010; Fretz and Dunham 1971) compared to herbicides alone. Case and Mathers (2006) found that herbicide-treated pine nuggets provided weed control for 40 up to one year in the field. Mathers (2003) obtained higher weed control efficacy with

HT Douglas-fir (Pseudotsuga menziesii) bark nuggets in containers versus a variety of other weed control methods.

The interaction of mulch thickness and herbicides can impact the weed control effectiveness of HTMs. Dunham and Fretz (1968) achieved only 50% weed control at

130 DAT with a 5-cm layer of licorice root mulch. In contrast, they obtained nearly complete control when dichlobenil was incorporated into or applied below 3 cm of licorice root mulch. Bing (1965) believed that mulches need to be applied at a 5 to 8 cm depth to get adequate season-long weed control, but when herbicides were impregnated on mulch, even a 1 cm depth of mulch was adequate for season-long weed control.

Most of the preemergence herbicides and mulches or their combinations do not provide perennial weed control. Dunham et al. (1967) reported that licorice mulch incorporated with dichlobenil or trifluralin provided effective control of annual weeds, but provided poor nutsedge control. Even though dichlobenil is recommended for nutsedge control, mulches treated with dichlobenil provided poor nutsedge control.

Dichlobenil-treated mulch at a 5-cm depth provided extended weed control of annual weeds for ten months but did not affect the growth of Canada thistle (Oliver 1971). Some of the researchers have achieved weed control with HTMs applied at a thickness as low as 1 cm. Samtani et al. (2007) reported that diuron- or oryzalin-treated leaf pellets, rice hulls, and pine bark applied at a thickness of 0.5 cm effectively reduce weed biomass in containers for up to 120 DAT.

The research on HTMs in field nurseries is limited and comparisons of granular herbicides versus liquids in combination with mulches have never been published.

Research on HTM thickness and its effect on herbicide efficacy and fate have also never been published. Two new commercial mulch products containing granular herbicide,

Preen mulch (active ingredients: Snapshot®, a combination of isoxaben and trifluralin;

Mulch Manufacturing, Reynoldsburg, Ohio) and Weedstop (active ingredient dithiopyr;

Vigoro Premium Mulch Plus Weed Stop, Spectrum Group, St. Louis, MO) were evaluated and compared to other prepared granular and mulch combinations in this trial.

Snapshot® (Dow AgroSciences, Indianapolis, Indiana) contains 0.5% isoxaben

(N-[3-(1-ethyl-1-methylpropyl)-5-isoxazolyl]-2, 6-dimethoxybenzamide) and 2% trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl p-toluidine), and it is a widely used preemergence herbicide labeled for use in the nursery and landscape industry (Judge et al.

2003; H. M. Mathers, personal communication). For this reason, Snapshot was the major preemergence herbicide we chose to compare with liquids in this study. Isoxaben is a pre-emergence herbicide that controls a wide range of broadleaf weeds in cereals, turf, and ornamentals. Isoxaben kills weeds by inhibiting cellulose biosynthesis in emerging seedlings (Schneegurt et al., 1994). It is marketed for preemergence weed control in field nursery crop production as a 75% active ingredient dry flowable (DF) and is labeled for use in over 400 species. It is typically applied at 0.5 to 1.0 lb/acre (Altland, 2003).

Neal and Senesac (1990) reported that isoxaben provided excellent control of many broadleaved weeds including pigweed, common groundsel, and dandelion, but poor control of annual grasses such as crabgrass, goosegrass, and fall panicum. The advantage

42 of using isoxaben over oxyfluorfen (Goal®, Dow AgroSciences, Indianapolis, Indiana) or simazine (insert chemical name here) (Princep®, Syngenta Crop Protection, Greensboro,

NC) is that isoxaben is safe on a broader range of nursery and landscape crops.

Trifluralin is a grass-active herbicide used commonly in nurseries and landscapes. It is a dinitroaniline (DNA) herbicide that acts by inhibiting root formation. DNAs are most widely used in the nursery due to their high adsorption to organic matter and their low leaching potential. Dithiopyr (3, 5-pyridine-dicarbothioic acid, 2- (difluoromethyl)-4-(2- methylpropyl)-6-(trifluoromethyl)- S,S-dimethyl ester ) (Dimension®, Dow

AgroSciences, Indianapolis, Indiana) is pyridine herbicide controls more than 45 grass and broad leaved weed species (Anonymous 2011). It is recommended for established lawns, turf, container grown and field grown ornamentals, landscape ornamentals, commercial sod farms, and non-cropland and industrial sites (Anonymous 2010).

We hypothesized that mulches pretreated with preemergence herbicides would provide superior weed control compared to herbicides applied on top of or beneath the mulch layer. We also hypothesized that for the herbicides tested, granular herbicide- mulch combinations would be relatively less effective compared to liquid herbicide- mulch combinations. Phytotoxicity of these treatments to crop species was studied in previous years at The Ohio State University; therefore phytotoxicity evaluations were not included in this experiment. Phytotoxicity evaluations with HTMs have shown repeatedly that HTMs reduce phytotoxicity on broad range of plants compared to conventional spray treatments (Case and Mathers 2006; Mathers 2003; Mathers and Case

2010).

The objectives of this study were to: (1) Compare the effect of granular herbicide versus liquid herbicide formulations alone and in combination with different mulches on weed control (2) determine the effect of mulch layer thickness on weed control; and (3) determine the influence of herbicide placement (on top of mulch, on soil surface under mulch, or herbicide-treated mulch) on weed control.

Materials and Methods

Field experiments were conducted at The Ohio State University’s Waterman

Agricultural and Natural Resources Laboratory, Columbus, Ohio, in spring 2007 and

2008. A field that had not received herbicide applications for at least two years was chosen. The soil type is a Kokomo silty clay loam with cation exchange capacity (CEC) of 12-20 and pH 6-7. Hardwood mulch and pine nuggets were used untreated or in combination with herbicides and were applied at three different thicknesses: 3, 6, or 12 cm. Snapshot 2.5TG was applied preemergence either directly to soil or to the surface of the mulch treatments. A liquid formulation consisting of trifluralin (Treflan HFP, Dow

AgroSciences) + isoxaben (Gallery 75DF, Dow AgroSciences) was applied at 4.48 kg ai/ha + 1.12 kg ai/ha directly to soil, on top of the mulch, under the mulch or as pretreated mulch as described in Case and Mathers (2006). Hereafter, the liquid formulation of herbicide is referred to as T+G. There were a total of 35 treatments including the untreated mulches, the two commercial granular herbicide plus mulch products (Preen

Mulch and Weedstop, applied at a thickness of 6 cm only), and an untreated control (no mulch, no herbicide).

The plots were 1 m2, and arranged in a randomized complete block design

(RCBD) with treatments replicated five times. Mulches were pretreated with T+G by placing the mulch treatments on a plastic sheet and spraying them using a CO2- pressurized backpack sprayer at 40 psi (R&D Sprayers, Opelousas, LA) and equipped with 2-8002evs flat fan nozzles. Two wooden rectangular boxes with dimensions of 1.0 m wide by 5.0 m long (appendix A) were constructed at heights of either 6 or 12 cm.

These boxes were kept on the plastic sheet and filled with mulch to the appropriate depth and then T+G was sprayed. The HTMs were allowed to dry for 48 hours before applying them to the plots. Two 1 m2 wooden boxes (appendix B) either 6 or 12 cm deep were used to apply the mulch treatments in the field. Granular herbicide was spread on the mulch manually ensuring uniform application over the treatment plot. Overhead irrigation was applied immediately after all the treatments were applied in the field and later as needed during the experimental period to ensure weed growth.

Evaluations were done at 30, 60, 90, and 120 days after treatment (DAT). The first study began in the third week of May, 2007 and the second study in the first week of

May, 2008. Data collected included weed fresh weights within a 0.25 m2 quadrat, collected by clipping the weeds at ground level. Visual weed control ratings were also recorded on a scale of 0 to 10 with 0 indicating no weed control in comparison with the untreated check, and 10 indicating complete weed control. Weed control ratings ≥7 were considered commercially acceptable. 45

The PROC GLM (general linear model) and PROC MIXED (mixed model) procedures of SAS (Statitistical Analysis System Institute Inc., Cary, NC) were used to perform the statistical analyses. Fisher’s least significant difference (Fisher’s LSD) was used to separate the means at α = 0.05 unless otherwise mentioned. Dunnett’s test was also performed to compare the treatments with the untreated control. Orthogonal contrasts were conducted to compare selected treatment groupings using the CONTRAST procedure of SAS.

Results and Discussion

Spring 2007 and spring 2008 results are presented separately as the year by treatment interaction was significant. Rated score data were subjected to arcsine transformation and analysis was performed. There was no difference between the

ANOVA of the transformed data and non-transformed data (data not shown), for ease of presentation only the non-transformed data are presented.

The predominant weed species present in the experimental field were yellow foxtail (Setaria glauca), barnyardgrass (Echnochloa crus-galli), giant foxtail (Setaria faberi) and hairy galinsoga (Galinsoga quadriradiata) and ladysthumb (Polygonum persicaria).

The weed fresh weights and visual ratings pooled across dates of evaluation within year indicated that 3-cm mulch treatments that provided 7 or above visual ratings produced equivalent weed fresh weights to the the 6- and 12-cm depth treatments (Table

2.1). There were no differences among the ten treatments that included 12 cm of mulch,

46 with complete or near complete weed control ratings in 2007 (9.5-10) and 2008 (9.6-10) and with weed fresh weights of 0 to 1.45g in 2007 and 0-0.05 g in 2008 (Table 2.1).

Mulch at 6 cm depth without herbicide provided commercially acceptable or better weed control ratings in 2007 (8.35 to 8.6) and 2008 (8.25 to 8.55). Weed weights showed that the 6-cm layer of pine nuggets alone and hardwood mulch alone reduced weed fresh weights 77% and 89% in 2007, and 87 and 91% in 2008, respectively (Table 2.1).

There were no significant differences in weed fresh weight among any of the 12 treatments that included 6 cm of mulch in spring 2007 (Table 2.1). However, in spring

2008 the Weedstop provided significantly less weed control compared to the other eleven

6-cm mulch treatments (Table 2.1). Weedstop provided significantly less weed control at each of the rating intervals in 2008 than all the other 6-cm mulch treatments with herbicides (Table 2.2). The 6-cm Weedstop treatment was statistically comparable to the untreated pine nuggets and hardwood mulch across all dates except at 60 DAT with hardwood and at 120DAT with pine nuggets (Table 2.2). Weedstop was also comparable to the T+G-treated, 3-cm pine nuggets and hardwood across all dates. Weed fresh weights in the Weedstop treatment were comparable to the 3-cm pine nuggets and 3-cm hardwood treatments at 30 and 90DAT, and to pine nuggets only at 60 and 120 DAT

(Table 2.3).

The lower level of weed control from Weedstop in 2008 may be attributed in part to the age of Weedstop that was used in 2008. Bags of Weedstop from 2007 were stored outside under a roof and used in 2008, and yearlong storage of product mulch may have resulted in dithiopyr degradation. The formulated end use product of dithiopyr is only

47 stable for one year when it is stored at 20-26.6 0C (68-80 0F) (EPA, 1991). The higher temperatures during spring and summer might have resulted in dithiopyr degradation.

This indicates a possible limitation of this product, since it is likely that some consumers who purchase Weedstop will probably store the product from one year to the next.

Previous research also demonstrated that fresh mulch is more effective in controlling weeds than old mulch (Duryea et al., 1999). Nonetheless, Weedstop in this study provided commercially acceptable weed control at 30, 60, and 120 days after treatment.

This level of weed control may be attributed mainly to the 6-cm mulch depth of the treatment.

By comparing the direct chemical applications and the 6- and 12-cm mulch alone applications, it appears that the efficacy resulting from the herbicide-treated mulches was due predominantly to the mulch alone. Mulches applied at the 6 and12 cm depth was thick enough to prevent light penetration and thus germination of some seeds, and it physically suppressed any weed growth that did occur. However, with the 3-cm mulch and herbicide combinations, the weed control efficacy was fairly evenly distributed between mulch and the herbicides.

Overall efficacy ratings indicated that there was no difference between Snapshot and T+G; or between the 3-cm pine nuggets and hardwood mulch treatments. However, in 2007 Snapshot alone produced lower weed biomass than T+G alone, and in 2008 the

3-cm pine nuggets suppressed weed biomass more than the 3-cm hardwood mulch (Table

2.1). Mulches alone at the 3-cm depth did not provide acceptable levels of weed control across all dates in both years (Tables 2.2 and 2.4). However, they did reduce weed

48 biomass production at 30 DAT in both years and at 60 DAT in 2007. Herbicide alone treatments performed similarly to the 3-cm mulch alone treatments except in 2008 at 30

DAT. The only 3-cm mulch and herbicide combinations that provided consistent, commercially acceptable weed control were Snapshot over pine nuggets, T+G under pine nuggets, and T+G under hardwood (Tables 2.2 and 2.4). These treatments also reduced weed biomass compared to the control at all dates (Tables 2.3 and 2.5). Snapshot over 3- cm pine nuggets provided >95% weed control in 2007 and >86% in 2008. T+G under 3- cm pine nuggets provided >99% weed control in both years. T+G under hardwood provided weed control of >72% in 2007 and >86% in 2008. Snapshot over hardwood mulch failed to provide acceptable weed control in both years with the exception of 30

DAT in 2008. The remaining treatments involving the 3-cm mulch plus herbicides performed better in both years than the Snapshot over hardwood mulch.

It appears that the efficacy of 3-cm mulch plus herbicide treatments depends on various factors such as formulation of herbicide, placement of herbicide, and type or texture of mulch. These factors in turn may largely be influenced by rainfall and temperature. There were large differences in rainfall amounts and its distribution between

2007 and 2008 (Figure 2.2) whereas the soil temperature differences were minimal. The difference in the air temperature between the two years was also small (with maximum difference of 5 0C during first 30 days) (Figure 2.1). Precipitation received during 1-30,

31-60, 61-90, and 91-120 DAT was 31, 26, 76, and 120 mm respectively in 2007; and

125, 169, 23, and 17 mm, respectively in 2008 (Figure 2.2).

The T+G-treated and T+G over 3-cm pine nuggets and 3-cm hardwood mulch behaved differently in 2007 and 2008. The T+G-treated hardwood mulch applied at the 3- cm thickness performed better than the same thickness of T+G-treated pine nuggets in

2007. Conversely, in 2008 the T+G-treated pine nuggets performed better than the T+G- treated hardwood mulch (Tables 2.2 to 2.5). T+G applied over 3-cm pine nuggets in

2008 performed better than when T+G was applied over 3-cm hardwood mulch, but in

2007 the opposite results occurred. These conflicting results may be attributed to the differences in rainfall between the two years. Higher early rainfall and rainfall intensity in 2008 compared to the previous year might have dislodged more herbicide from pine nuggets into the soil. Higher availability of herbicide in the soil surface along with the comparatively large and heavy aggregates of pine nuggets mulch could explain the weed control effectiveness of those treatment combinations. Simmons and Derr (2007) observed similar results and reported that pendimethalin was released from pendimethalin-treated pine bark after regular application of irrigation water. They also found that the amount of irrigation water also influenced the movement and persistence of herbicide.

The higher rainfall in 2008 might have also had a negative influence on weed control provided by T+G-treated 3-cm hardwood mulch and T+G applied over 3-cm hardwood. Hardwood mulch is relatively fine-textured with much smaller average particle size than pine nuggets, and as it gets mixed with soil it tends to disintegrate and degrade faster than pine nuggets. It is also easy for hardwood mulch to be moved or partially washed away in the high intensity rainfall. In a comparatively dry year like

2007, the hardwood mulch plus herbicide treatments were much more effective for weed control. Three out of four 3-cm hardwood mulch plus herbicide combinations provided more than acceptable weed control in 2007 where as in 2008 only one hardwood mulch plus herbicide combination provided acceptable control. It is likely that the lower rainfall in 2007 resulted in the hardwood mulch staying intact for a longer period than in the following year.

Billeaud and Zajicek (1989) reported that the size of the mulch particles influences weed control, with small-sized particles giving the poorest weed control. Case and Mathers (2006) reported higher levels of weed control with the combination of herbicide and pine nuggets compared to the combination of hardwood mulch plus herbicides (both mulches applied in a single layer). They reported efficacy ratings ≥ 7 for

12 out of 15 herbicide plus pine nuggets combinations after one year, and in contrast all of the hardwood plus herbicide combinations rated below 7. The authors did not explain the reasons or provide weather data, but it seems likely that the hardwood mulch applied in a single layer could easily disintegrate and degrade faster than the same thickness of pine nuggets.

Snapshot applied over 3-cm pine nuggets was one of the most effective treatments among the 3-cm mulch treatments in both years. Snapshot applied over 3-cm pine nuggets provided better and acceptable weed control at all dates whereas Snapshot applied over 3-cm hardwood mulch gave poor weed control in both years (Tables 2.1 to

2.5). The possible explanation is that when Snapshot was applied on pine nuggets, the

Snapshot granules gradually settled down through the large pore spaces of the mulch

51 quickly and reached the soil surface. This could protect the herbicide from losses through runoff or photodecomposition. A study conducted by Wilson et al. (1995) provided support to this explanation. They studied the effect of formulation and ground covers on herbicide loss by surface runoff, and reported that minimal losses occurred when a granular formulation of herbicide was applied on gravel compared to treatments applied on a plastic or fabric surface. Opposite results occurred with a liquid formulation of the herbicide. In our study, pine nuggets could have acted the same way as the gravel in the study by Wilson et al. (1995), and the lower herbicide losses resulted in better weed control. Poor weed control with Snapshot or T+G applied over 3-cm hardwood might be mainly due to more rapid degradation and/or movement of the mulch in those treatments.

The herbicide-rich top layer of hardwood mulch could have been blown away by wind or dislodged by strong rainfall. Bing (1965) reported that some mulches such as cocoa hulls and peat moss can be blown away easily by wind. Fretz et al (1966) constructed a 20-cm- high tar paper barrier to reduce the wind erosion of mulches from experimental plots.

Herbicide placement relative to mulch placement also influenced weed control efficacy. The three 3-cm mulch treatments that consistently provided more than acceptable weed control in both years were Snapshot over pine nuggets, T+G under pine nuggets, and T+G under hardwood mulch. This could be attributed to longer herbicide residual activity as the result of lower herbicide dissipation from volatilization and photodecomposition. Dinitroaniline herbicides are a relatively volatile group of herbicides compared to most other herbicide families; and trifluralin is the most volatile compound (highest vapor pressure) among dinitroaniline herbicides (Weber, 1990).

Orthogonal contrast results. Orthogonal contrasts were performed to compare hardwood and pine nuggets (mulch alone, across all depths); three depths of mulches, and between granular (Snapshot) herbicide plus mulch combinations vs liquid (T+G) herbicide plus mulch combinations. Contrast analyses were also performed to see if there were significant differences among herbicide placements: granular-over, liquid-over, liquid-under, and liquid-treated. Contrast analysis showed that there was no difference between hardwood and pine nuggets in terms of visual ratings and weed fresh weights in both years (Appendix C). Visual ratings were lower and weed fresh weights were higher for mulches applied at the 3-cm depth (alone or in combination with herbicides) than those in the 6-cm and 12-cm depth mulch treatments in both years. Visual weed control ratings of mulches applied at the 6-cm depth were lower than for the 12-cm depth mulch treatments (mulch alone or in combination with herbicides) in both years, but weed fresh weights were higher only in 2007 for the mulch alone treatments only (p=0.0514).

Contrasts of granular plus mulch combinations versus liquid plus mulch combinations showed that the liquid plus mulch combined treatments provided slightly better weed control only in 2007, but there were no differences in the weed fresh weight data. Visual ratings were higher for liquid-treated versus liquid-over and liquid-under versus liquid- treated, liquid-under and liquid-over, liquid-under versus granular-over in both years. The visual rating for granular-over was higher than that of liquid-over only in 2008. Weed fresh weights of various herbicide placement treatments were not different from each other except in 2008 when weed weights for liquid-under were lower than liquid-over, and were lower for liquid-treated versus liquid-over. Overall results of contrasts revealed

53 that mulch thickness and herbicide placement relative to the mulch placement were the most important factors affecting weed control in this study. It appears that in addition to providing a physical barrier to weeds, mulches placed over the herbicide treated soil surface resulted in the highest weed control. It is possible that herbicides applied directly to soil provided maximum weed control early in the season during a period of maximum weed emergence, and the mulch then prevented germination and/or establishment of weeds later in the season. The cumulative effect resulted in higher levels of weed control over the season compared with other treatments in which the herbicide concentration in the seed germination zone was suboptimal for maximum control.

Our results are in general agreement with previous studies. Case and Mathers

(2006b) reported higher weed control efficacy 115 DAT with some herbicide-treated mulches in containers. They applied oryzalin and flumioxazin at two rates (1X and 0.5X) with seven types of mulches, including cocoa shells, cypress mulch, Douglas fir bark, hardwood, PennMulch, pine nuggets, and rice hulls (applied as single layer). Only 4 herbicide treated mulches, 0.5X flumioxazin-treated hardwood, 1X oryzalin-treated pine nuggets, and 1X and 0.5X flumioxazin-treated rice hulls resulted in weed control ratings

≥ 7 in the first year, but none of them were rated ≥ 7 the next year. Mathers (2003) evaluated various herbicides alone (oxyflorfen and oryzalin at 1x and 0.5x rate), mulches alone (two types of Douglas fir nuggets, big and little nuggets), herbicide-treated mulches

(also flumioxazin treated with big Douglas fir nuggets) and some other novel methods of weed control such as Geodiscs, Environ Lids, Penn Mulch, Mori Weeds bags, and

Wulpack mulch in containers. Weed control in seven out of 24 treatments was rated ≥ 7,

54 including the little Douglas fir nuggets treated with oryzalin at both rates, big Douglas fir nuggets treated with a 1X rate of oryzalin, big nuggets treated with flumioxazin,

Geodiscs, Mori weed bags, and Flumioxazin granular (Suregaurd).

In another study, Mathers and Case (2010) compared microencapsulated (ME) and emulsifiable concentrate (EC) formulations of acetochlor and alachlor with or without pine nuggets, Douglas fir bark, and hardwood mulch, and mulches alone in a container study. They also did not get consistent results across three years of the study.

Their data showed that Douglas fir bark is not effective as a mulch to be treated with either of those herbicides. Only one combination of herbicide and mulch, acetochlor EC- treated Douglas fir bark, resulted in a weed control rating >7 in the second year. Weed control in ME acetochlor-treated pine nuggets was rated ≥ 7 consistently in all three years. The same authors reported long term weed control of up to 303 DAT from

Douglas fir treated with either acetochlor or oryzalin (Case and Mathers, 2003).

To get a positive interaction for weed control from herbicide and mulch combinations, some herbicides may need to be applied at a higher than normal dose. As herbicide-treated mulches can reduce herbicide leaching and non-target losses, it may be possible that higher doses of herbicides can be used with mulches without significantly increasing off-target movement. Fretz (1973) obtained ≥ 85% weed control at 150 DAT when herbicides such as chlropropham, EPTC, diphenamid, and dichlobenil were impregnated on milled pine bark. All of these herbicides were applied in two doses but the lower doses of chlorpropham- and diphenamid-impregnated mulches did not provide satisfactory weed control. Dunham et al. (1969) achieved season long weed control in

55 landscape plantings with granular dichlobenil (Casoron) mixed with licorice root mulch applied at a 3-cm depth. Certain mulches alone, when applied at a 5-cm thickness, did not provide sufficient weed control. However, when herbicides were combined with them they provided excellent weed control even at lower thicknesses.

Visual ratings data do not always coincide closely with weed fresh weights data

(Tables 2.1 to 2.5). One reason for this is that some of the dicot weed species produced much higher biomass per weed than others. Some of the treatments such as T+G at 60

DAT in 2008, T+G over 3-cm hardwood at 90 DAT in 2008, untreated 3-cm hardwood at

120 DAT in 2008 produced higher than or close to the fresh weights of the control but their ratings were different than the control. This is also due to the higher biomass production of some dicot weed species.

It was anticipated that liquid herbicides would provide effective weed control at least up to 30 DAT and that granular herbicides would control weeds longer than liquid herbicides, but might be less effective than liquids during the first 30 DAT due to their slow release nature. In our study both herbicides, Snapshot and T+G generally controlled weeds effectively up to 30 DAT, but the efficacy ratings were < 7 in 2007. There could be possibility that some amount of Snapshot and T+G might have moved out of plots in the runoff water during overhead irrigation or rainfall. Higher moisture levels can also result in faster herbicide biodegradation and/or volatilization.

The data indicated that mulches applied at the 6- and 12- cm depths with or without herbicide provided at least 80% weed control. Equivalent levels of weed control were achieved when mulches applied at the 3 cm depth provided that herbicides were

56 combined with them. Many aforementioned previous studies also proved that mulch depth can be reduced to 3 cm or even less when herbicides were combined with mulches.

The recommended depth of mulch is usually 5 to 8 cm for adequate weed control.

Our study proved that mulch costs can further be reduced by combining herbicides. Even though the 12-cm mulch layer provided higher weed control, it is not recommended due to the high cost of mulch and its possible effects on health of desirable plants. Billeaud and Zajicek (1989) reported that mulch at applied at the 15 cm depth was effective in controlling weeds but that it also was detrimental effect to landscape plants. Over-mulching may cause reduced oxygen and water penetration into the soil, or create saturated soil conditions that lead to decline and death of shallow rooted shrubs and trees. Our results indicate that the proper combination of herbicides, mulch material, and herbicide placement relative to the mulch can provide acceptable weed control with a mulch layer as thin as 3 cm.

In contrary to our our hypotheses, liquid herbicides applied below mulch provided better weed control than herbicide treated mulches, and certain combinations of granular herbicide plus mulch provided better weed control than some of liquid herbicide plus mulch combinations.

Table 2.1. Weed fresh weight (g) and visual ratings of herbicide, mulch, and herbicide- mulch combined treatments in 2007 and 2008.

Name of the treatmenty Spring 2007z Spring 2008z Visual Fresh weight Visual rating Fresh weight ratingx (g)v (g)u Control 0 g w 160.92 a 0 o 138.5 a Snapshot (SS) 4.75 f 54.67 cde 6.05 kl 80.42 bc Treflan + Gallery 5.2 f 99.44 b 6.6 jk 108.38¥ ab Untreated 3-cm PN 5.05 f 90.2 bc 4.6 n 56.17 cde Untreated 3-cm HW 5.45 f 62.17 bcd 5 mn 99.35¥ b SS over 3-cm PN 9.35 ab 3.83 g 8.8 efg 5.87 h SS over 3-cm HW 5.35 f 52.64 cde 6.2 kl 47.66 cdef T + G over 3-cm PN 6.89 e 48.41 def 7.7 hi 44.01 defg T + G over 3-cm HW 8.35 d 5 g 5.65 lm 74.38 bcd T + G under 3-cm PN 9.75 a 0.29 g 9.35 abcde 0.54 h T + G under 3-cm HW 8.5 d 26.66 defg 8.4 g 10.75 gh T + G treated 3-cm PN 6.68 e 61.21 cd 8.15 gh 6.15 h T + G treated 3-cm HW 8.4 d 21.46 efg 7.2 ij 21.18 fgh Untreated 6-cm PN 8.35 d 36.63 defg 8.55 fg 11.99 gh Untreated 6-cm HW 8.6 cd 18.3 efg 8.25 gh 17.99 fgh SS over 6-cm PN 9.68 a 3.85 g 10 a 0 h SS over 6-cm HW 9.32 abc 10.43 fg 9.35 abcde 0.34 h T + G over 6-cm PN 9.8 a 9.76 g 9.25 cde 2.16 h T + G over 6-cm HW 8.75 bcd 4.31 g 9.15 def 3.51 h T + G under 6-cm PN 9.7 a 1.45 g 9.9 abc 0 h T + G under 6-cm HW 10 a 3.43 g 9.4 abcde 0.54 h T + G treated 6-cm PN 9.85 a 7.37 g 9.75 abcd 0.5 h T + G treated 6-cm HW 9.6 a 5.79 g 9.3 bcde 0 h 6-cm Weedstop 9.6 a 0.7 g 7.7 hi 25.15 efgh 6-cm Mulch with SS 9.85 a 2.25 g 9.9 abc 0 h continued

Table 2.1 continued

Name of the treatmenty Spring 2007z Spring 2008z Visual Fresh weight Visual rating Fresh weight ratingx (g)v (g)u Untreated 12-cm PN 10 a 0 g 9.85 abc 0 h Untreated 12-cm HW 9.6 a 1.45 g 9.95 ab 0.03 h SS over 12-cm PN 9.85 a 0 g 10 a 0 h SS over 12-cm HW 10 a 0 g 10 a 0 h T + G over 12-cm PN 9.95 a 1.28 g 9.7 abcd 0 h T + G over 12-cm HW 9.5 a 0.7 g 10 a 0.05 h T + G under12-cm PN 9.8 a 0.6 g 10 a 0 h T + G under 12-cm HW 9.75 a 0 g 10 a 0 h T + G treated 12-cm PN 10 a 0.12 g 10 a 0 h T + G treated 12-cm HW 9.9 a 0 g 9.6 abcd 0.03 h

z data pooled across four dates in both years. y Abbreviations: PN, pine nuggets; HW, hardwood; SS, Snapshot; T+G, treflan (trifluralin) + Gallery (isoxaben). x Visual ratings are based on 0-10 scale; 0, no control; and 10, complete control. w Within each column, means followed by the same letter are not significantly different using LSD at P ≤ 0.05. v Weed weights of all the treatments are different from control on the basis of Dunnett’s test at P ≤ 0.05. u Weed weights indicated by (¥ ) are not different from control on the basis of Dunnett’s test. at P ≤ 0.05.

Table 2.2. Efficacy ratings of herbicide, mulch, and herbicide-mulch combined treatments at different days after treatments (DAT) in 2008.

Treatmentz Visual ratingsy 30 DAT 60 DAT 90 DAT 120 DAT Control 0 hx 0 l 0 m 0 j Snapshot (SS) 8.8 cd 6.2 hij 4.8 jkl 4.4 hi Treflan + Gallery 9.2 bcd 6.4 ghi 5.8 hij 5 hi Untreated 3-cm PN 5.8 g 4.8 k 3.8 l 4 i Untreated 3-cm HW 6.2 g 5.4 jk 4 kl 4.4 hi SS over 3-cm PN 9.4 abc 8.8 bcd 8.2 bcdef 8.8 abcd SS over 3-cm HW 8 ef 5.8 ij 5.4 ijk 5.6 gh T + G over 3-cm PN 9.2 bcd 7.2 fg 7.2 egfh 7.2 ef T + G over 3-cm HW 7.4 f 5.4 jk 4.4 jkl 5.4 h T + G under 3-cm PN 10 a 9.8 a 8.8 bacd 8.8 abcd T + G under 3-cm HW 9.4 abc 8.8 bcd 7.4 defg 8 cdef T + G treated 3-cm PN 9.2 bcd 8.2 de 7 fgh 8.2 bcde T + G treated 3-cm HW 8.6 de 7 fgh 6.4 ghi 6.8 fg Untreated 6-cm PN 9.2 bcd 8.4 cde 7.4 defg 9.2 abc Untreated 6-cm HW 9.2 bcd 8.8 bcd 7.6 cdefg 7.4 ef SS over 6-cm PN 10 a 10 a 10 a 10 a SS over 6-cm HW 9.8 ab 9.4 ab 9 abc 9.2 abc T + G over 6-cm PN 9.6 ab 9.4 ab 8.6 abcde 9.4 ab T + G over 6-cm HW 9.8 ab 9.2 abc 8.8 abcd 8.8 abcd T + G under 6-cm PN 9.8 ab 9.8 a 10 a 10 a T + G under 6-cm HW 10 a 9.6 ab 8.8 abcd 9.2 abc T + G treated 6-cm PN 9.8 ab 9.8 a 9.6 ab 9.8 a T + G treated 6-cm HW 9.8 ab 9.4 ab 8.8 abcd 9.2 abc 6-cm Weedstop 8.8 cd 7.6 ef 6.8 fghi 7.6 def 6-cm Mulch with SS 9.8 ab 9.8 a 10 a 10 a continued

Table 2.2 continued

Treatment Visual ratings 30 DAT 60 DAT 90 DAT 120 DAT Untreated 12-cm PN 10 a 9.6 ab 9.8 a 10 a Untreated 12-cm HW 10 a 10 a 10 a 9.8 a SS over 12-cm PN 10 a 10 a 10 a 10 a SS over 12-cm HW 10 a 10 a 10 a 10 a T + G over 12-cm PN 10 a 9.8 a 9.4 ab 9.6 a T + G over 12-cm HW 10 a 10 a 10 a 10 a T + G under12-cm PN 10 a 10 a 10 a 10 a T + G under 12-cm HW 10 a 10 a 10 a 10 a T + G treated 12-cm PN 10 a 10 a 10 a 10 a T + G treated 12-cm HW 10 a 10 a 9.2 ab 9.2 abc

z Abbreviations: PN, pine nuggets; HW, hardwood; SS, Snapshot; T+G, treflan (trifluralin) + Gallery (isoxaben). y Visual ratings are based on 0-10 scale; 0, no control; and 10, complete control. x Within each column, means followed by the same letter are not significantly different using LSD at P ≤ 0.05.

Table 2.3. Weed fresh weights (g) of herbicide, mulch, and herbicide-mulch combined treatments at different DAT in 2008.

Treatmentz Weed fresh weight (g)x 30 DAT¥ 60 DAT 90 DAT 120 DAT Control 27.34 a 173.36 a 157.04 a 196.26 ab Snapshot (SS) 1.14 bc 89.6 b 26.32 efgh 204.6* ab Treflan + Gallery 1.02 bc 192.46* a 58.06 defg 181.98* abc Untreated 3-cm PN 2.24 bc 52.48 bcde 77.8 cd 92.16* cd Untreated 3-cm HW 2.46 bc 76.84 bc 61.28 cdef 256.82* a SS over 3-cm PN 0.2 bc 1.56 f 21.72 fgh 0 d SS over 3-cm HW 0.9 bc 25.06 efd 102.4* bc 62.26* d T + G over 3-cm PN 0.22 bc 7.56 ef 65.22 cde 103.02* abc T + G over 3-cm HW 3.96 bc 61.46 bcd 140.62* ab 91.48* cd T + G under 3-cm PN 0 c 0 f 0 h 2.16 d T + G under 3-cm HW 0.08 c 12.66 ef 2.42 h 27.82 d T + G treated 3-cm PN 0.08 c 16.76 efd 0 h 7.76 d T + G treated 3-cm HW 0.14 c 34.58 cdef 17.4 gh 32.6 d Untreated 6-cm PN 0.54 bc 10.36 ef 14.72 h 22.34 d Untreated 6-cm HW 0 c 3.34 f 4.82 h 63.8* d SS over 6-cm PN 0 c 0 f 0 h 0 d SS over 6-cm HW 0.16 c 0 f 0 h 1.18 d T + G over 6-cm PN 0.02 c 0.22 f 0 h 8.4 d T + G over 6-cm HW 0 c 7.58 ef 5.3 h 1.14 d T + G under 6-cm PN 0 c 0 f 0 h 0 d T + G under 6-cm HW 0 c 0 f 0 h 2.16 d T + G treated 6-cm PN 0.58 bc 0 f 0 h 1.42 d T + G treated 6-cm HW 0 c 0 f 0 h 0 d 6-cm Weedstop 0 c 27.58 def 40.66 defgh 32.34 d 6-cm Mulch with SS 0 c 0 f 0 h 0 d continued

Table 2.3 continued

Treatmentz Weed fresh weight (g)x 30 DAT¥ 60 DAT 90 DAT 120 DAT Untreated 12-cm PN 0 c 0 f 0 h 0 d Untreated 12-cm HW 0 c 0 f 0 h 0.12 d SS over 12-cm PN 0 c 0 f 0 h 0 d SS over 12-cm HW 0 c 0 f 0 h 0 d T + G over 12-cm PN 0 c 0 f 0 h 0 d T + G over 12-cm HW 0 c 0 f 0.2 h 0 d T + G under12-cm PN 0 c 0 f 0 h 0 d T + G under 12-cm HW 0 c 0 f 0 h 0 d T + G treated 12-cm PN 0 c 0 f 0 h 0 d T + G treated 12-cm HW 0 c 0 f 0 h 0.12 d

z Abbreviations: PN, pine nuggets; HW, hardwood; SS, Snapshot; T+G, treflan (trifluralin) + Gallery (isoxaben). x Within each column, means followed by the same letter are not significantly different using LSD at P ≤ 0.05.

¥ All the treatments in the same column are different from control on the basis of Dunnett’s test.

* Treatments in the same column are not different from control on the basis of Dunnett’s test.

Table 2.4. Efficacy ratings of herbicide, mulch, and herbicide-mulch combined treatments at different days after treatments (DAT) in 2007.

Treatmentz Visual ratingsy 30 DAT 60 DAT 90 DAT 120 DAT Control 0 jx 0 i 0 g 0 i Snapshot (SS) 6.2 ghi 5.8 gh 4.2 f 2.8 h Treflan + Gallery 6.8 fgh 5.2 gh 4.6 f 4.2 gh Untreated 3-cm PN 5.8 hi 4.6 h 5.2 ef 4.6 fg Untreated 3-cm HW 5 i 4.4 h 6.4 e 6 ef SS over 3-cm PN 9.8 ab 9.6 abc 8.8 abcd 9.2 ab SS over 3-cm HW 5 i 4.6 h 6.2 e 5.6 efg T + G over 3-cm PN 7.6 efg 6.8 efg 6.5 e 6.6 de T + G over 3-cm HW 8 edf 7.6 def 9 abcd 8.8 abc T + G under 3-cm PN 10 a 9.8 ab 9.8 ab 9.4 ab T + G under 3-cm HW 9.2 abcd 8.2 bcde 8 d 8.6 abc T + G treated 3-cm PN 6.2 ghi 6.4 gh 6.5 e 7.6 cd T + G treated 3-cm HW 8.4 bcde 8 cdef 8.6 abcd 8.6 abc Untreated 6-cm PN 8.2 cdef 8 cdef 8.2 cd 9 abc Untreated 6-cm HW 9 abcde 8.6 abcd 8.4 bcd 8.4 bc SS over 6-cm PN 9.6 abc 9.5 abc 10 a 9.6 ab SS over 6-cm HW 9.6 abc 9 abcd 9 abcd 9.6 ab T + G over 6-cm PN 10 a 9.6 abc 10 a 9.6 ab T + G over 6-cm HW 9.4 abcd 8.6 abcd 8.4 bcd 8.6 abc T + G under 6-cm PN 9.8 ab 9.6 abc 9.8 ab 9.6 ab T + G under 6-cm HW 10 a 10 a 10 a 10 a T + G treated 6-cm PN 10 a 9.6 abc 9.8 ab 10 a T + G treated 6-cm HW 9.8 ab 9.4 abc 9.8 ab 9.4 ab 6-cm Weedstop 9.8 ab 9.4 ab 9.6 abc 9.6 ab 6-cm Mulch with SS 10 a 9.8 ab 9.6 abc 10 a continued

Table 2.4 continued

Treatmentz Visual ratingsy 30 DAT 60 DAT 90 DAT 120 DAT Untreated 12-cm PN 10 a 10 a 10 a 10 a Untreated 12-cm HW 9.6 abc 9.2 abcd 9.6 abc 10 a SS over 12-cm PN 10 a 9.8 ab 10 a 9.6 ab SS over 12-cm HW 10 a 10 a 10 a 10 a T + G over 12-cm PN 10 a 10 a 9.8 ab 10 a T + G over 12-cm HW 9.6 abc 9.4 abc 9.6 abc 9.4 ab T + G under12-cm PN 9.8 ab 10 a 9.8 ab 9.6 ab T + G under 12-cm HW 9.8 ab 9.2 abcd 10 a 10 a T + G treated 12-cm PN 10 a 10 a 10 a 10 a T + G treated 12-cm HW 10 a 9.8 ab 10 a 9.8 ab LSD 1.45 1.69 1.49 1.48

Table 2.5. Weed fresh weights (g) of herbicide, mulch, and herbicide-mulch combined treatments at different DAT in 2007. Treatmentz Weed fresh weight (g)x 30 DAT¥ 60 DAT 90 DAT 120 DAT Control 0.52 bcd 106 a 253.46 a 283.68 a Snapshot (SS) 1.22 a 72 ab 102.8 bcdef 42.66 bcd Treflan + Gallery 0.12 dc 40.8* bc 252.4* a 104.44 b Untreated 3-cm PN 0.68 abc 25 cd 126.68* bcd 208.44* a Untreated 3-cm HW 0.76 ab 13.6 cd 190.6* ab 43.7 bcd SS over 3-cm PN 0 d 1.12 d 0 g 14.18 cd SS over 3-cm HW 0.48 bcd 15.6 cd 165.2* ab 29.26 bcd T + G over 3-cm PN 0.44 bcd 22 cd 171.2* ab 0 d T + G over 3-cm HW 0.18 dc 0 d 19.8 g 0 d T + G under 3-cm PN 0 d 1.16 d 0 g 0 d T + G under 3-cm HW 0 d 0 d 27 efg 79.64 bc T + G treated 3-cm PN 1.2 a 42.2 bc 144.6* bc 56.84 bcd T + G treated 3-cm HW 0.24 bcd 25.2 cd 47 defg 13.4 cd Untreated 6-cm PN 0.3 bcd 8.8 cd 115* bcde 22.4 cd Untreated 6-cm HW 0.22 bcd 3 d 56.5 cdefg 13.48 cd SS over 6-cm PN 0 d 0 d 15.4 fg 0 d SS over 6-cm HW 0 d 0 d 41.7 defg 0 d T + G over 6-cm PN 0 d 0.6 d 33.8 defg 4.64 cd T + G over 6-cm HW 0.26 bcd 3.6 d 4.2 g 9.16 cd T + G under 6-cm PN 0 d 0 d 5.8 g 0 d T + G under 6-cm HW 0 d 0 d 12.5 fg 1.2 d T + G treated 6-cm PN 0 d 0 d 0 g 29.48 bcd T + G treated 6-cm HW 0 d 9.2 cd 0 g 13.96 Cd 6-cm Weedstop 0 d 0 d 0 g 2.8 d 6-cm Mulch with SS 0 d 0 d 9 fg 0 d continued

Table 2.5 continued Treatmentz Weed fresh weight (g)x 30 DAT¥ 60 DAT 90 DAT 120 DAT Untreated 12-cm PN 0 d 0 d 0 g 0 d Untreated 12-cm HW 0 d 0 d 5.8 g 0 d SS over 12-cm PN 0 d 0 d 0 g 0 d SS over 12-cm HW 0 d 0 d 0 g 0 d T + G over 12-cm PN 0 d 0 d 5.1 g 0 d T + G over 12-cm HW 0 d 0 d 1 g 1.8 d T + G under12-cm PN 0 d 0 d 2.4 g 0 d T + G under 12-cm HW 0 d 0 d 0 g 0 d T + G treated 12-cm PN 0.46 bcd 0 d 0 g 0 d T + G treated 12-cm HW 0 d 0 d 0 g 0 d LSD 0.58 35.62 94.68 75.73

¥ All the treatments in the column are not different from control on the basis of Dunnett’s test.

* Treatments in the same column are not different from control on the basis of Dunnett’s test.

0 68

Mean temperature -2007 18

Mean temperature -2008 Temperature ( Temperature 15 Mean soil temperature - 2007

Mean soil temperature - 2008 13 Historical average temperature

10 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Week

Figure 2.1. Air and Soil temperatures during experimental period for 2007 and 2008 experiments. Temperatures were collected using sensors that were installed one meter above the ground for air temperatures and 5 cm below the ground for soil temperatures at experimental site. Historical temperature data were obtained from weather.com.

110

100 Historical average precipitation 90 Precipitation - 2007 80 Precipitation - 2008

69 50

Precipitation (mm) Precipitation 30

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

Week

Figure 2.2. Weekly precipitation received during experimental period for 2007 and 2008 experiment (OARDC 2010) along with historical precipitation data.

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Fretz, T.A., C.W. Dunham, and E.M. Rahn. 1966. The incorporation of herbicides in organic mulches for use on ornamental plantings. Proc. Northeast Weed Control Conference. 20:204-208.

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Chapter 3: Effect of fall applied herbicide-organic mulch combinations on weed control

Introduction

Weeds affect the aesthetics of landscapes while reducing the growth of landscape plants by competing for resources. Weeds also reduce the sale value of container-grown nursery plants (Fretz 1972). Weed control in nurseries and landscapes is commonly achieved by herbicides, predominantly preemergence herbicides, and/or hand weeding. In general, herbicides are applied 3-5 times per year by nursery container growers (Gilliam et al. 1990) and 2-3 times per year by nursery field growers and landscapers to achieve yearlong weed control. Multiple herbicide applications in a single year can lead to the development of herbicide resistant weed populations, environmental pollution, and health concerns to humans and wildlife (Hoar et al. 1986;

Nielsen and Lee 1987; EPA 1992). Some of the other problems associated with herbicide use are improper equipment calibration, phytotoxicity to desirable species, and off-target herbicide movement via leaching, spray drift, and runoff. Off-target herbicide losses in container nurseries can be up to 86% of the applied herbicide depending on growth habit of the species and spacing of the containers (Gilliam et al.

1992). Pimentel and Levitan (1986) reported that less than 0.1% of the pesticide applied 74 to crops reaches the target pests. The severity of herbicide contamination can be understood more clearly from research conducted by Koplin et al. (1998), who reported that 75% of the wells sampled in Iowa were contaminated by herbicide residues.

Chemical control is usually supplemented by hand-weeding as herbicides alone cannot provide sufficient control. Hand-weeding is expensive and laborious, and can damage the desired plants when their roots are intertwined with the roots of weeds

(Neal 1998). Nursery owners can spend approximately $1000-10,000/ha for hand- weeding depending on the weed species present (Mathers 2003). These financial issues, previously mentioned environmental concerns, limited availability of herbicides for greenhouse use, and stringent herbicide registration laws all create the need to develop cost effective and environmentally safe weed control methods (Mathers 2003).

An economically and environmentally sound and effective weed control strategy would integrate chemical and non-chemical weed control methods. Developing innovative techniques utilizing already available weed control methods and/or resources would be cost effective and time saving. Organic mulches have already been widely used in the landscape industry due to many advantages including weed control (Billeaud and Zajicek 1989; Mathers 2002; Powell et al 1987; Skroch et al. 1992), soil moisture conservation (Merwin et al. 1995), regulation of soil temperature (Ashworth and

Harrison 1983; Paterson et al. 1979), control of soil erosion and soil crusting (Ingle

1983), and increased aesthetic value of the landscape (Neely 1984; Skroch et al. 1992).

Mulches alone, if applied at the recommended depth, can provide effective weed control

75 for an extended period. Bark mulches applied at the 8-cm have provided effective weed control (Campbell-Lloyd 1986; Case and Mathers 2006) but are costly.

Earlier research by Oliveira et al. (2000) showed that lignin, a major constituent of secondary cell walls, was the best herbicide carrier for controlled release of herbicides. Herbicides are often combined with different carriers for various advantages such as reducing the amount of herbicide used, increasing and/or extending the period of herbicide efficacy (Derr 1994), increasing environmental safety, or avoiding some current label restrictions (Mathers and Ozkan 2001).

Previous research demonstrated that mulches treated with herbicides (known as

‘herbicide treated mulches’, HTMs) provide better weed control than mulch or herbicide alone treatments. Compared to mulches alone, HTMs provided higher weed control efficacy (Fretz and Dunham, 1971; Fretz 1973, Mathers 2003), extended efficacy (Mathers and Case 2010), and reduced phytotoxicity to non-target plants

(Mathers and Case 2010). Phytotoxicity evaluations with HTMs have shown repeatedly that HTMs reduce phytotoxicity with broad range of plants (Case and Mathers 2006;

Mathers 2003; Mathers and Case 2010). Case and Mathers (2006) also found that pine nuggets combined with various herbicides provided weed control for up to one year in the field. Mathers (2003) obtained higher weed control efficacy with HT bark nuggets in containers versus a variety of other novel weed control methods. HTMs can also reduce the leaching of herbicides. Knight et al. (2001) reported that herbicides applied on mulches reduced herbicide leaching up to 74% compared to applications on bare soil.

Application of herbicides during fall to control winter annual and perennial weeds the following spring season is a common practice in the nursery industry. This is due to the poor field and weather conditions and lack of labor availability in the spring

(Ahrens 1971). Fall herbicide applications reduce spring workloads while controlling winter annuals during their vulnerable vegetative stage (Hasty et al. 2004; Kadir and Al-

Khatib 2006; Krausz et al. 2003). Fall application of herbicides that provide adequate weed control later into the spring and summer would be an additional benefit to the nursery growers as spring is busy time and most of the sales occur during this period

(H.M. Mathers, Personal communication; Wright 1985). The effect of fall applications of HTMs in the field nurseries has never been investigated. We were interested to see how long herbicide and mulch combinations could provide weed control into the following spring.

The effect of different depths of HTMs on weed control also has never been investigated. Depth of mulch used usually depends on the type of mulch used and type of soil (Evans 2000). Most of the previous research with HTMs focused on single layer of mulch which was ≤ 3-cm thick (Dunham and Fretz 1968; Dunham et al. 1967; Fretz

1973; Fretz and Dunham 1971; Fretz et al. 1966; Mathers et al. 2002; Mathers 2003;

Case and Mathers 2006). However, the recommended depth of commonly used mulches such as hardwood and pine bark mulches is 5-8 cm or an average of 6 cm (Rose and

Smith 2011; Relf 2009). In contrast, the landscape industry commonly practices applying mulch at a layer thickness of 13 cm or thicker, which when placed around trees and other landscape plants creates the appearance of so-called ‘mulch volcanoes’

(Carlson 2001; Carlson 2002; Derr and Appleton 1989; Higgins 2009; ISA 2009). We were interested to see if weed control could be maintained with thinner layers of mulch and in combination with herbicides.

Research on the combination of granular herbicides with mulches versus liquid herbicides with mulches has never been published. Since granular herbicides are preferred over liquid formulation of herbicides by the nursery and landscape industry, it is important to investigate the efficacy of granular herbicides and mulch combinations.

Another factor that merits investigation is the effect of herbicide placement relative to the mulch: herbicides applied to soil below the mulch, herbicide applied to the top of the mulch, or when herbicides are applied as a pretreatment and incorporated into the mulch before the mulch is applied.

Snapshot® (Dow AgroSciences, Indianapolis, Indiana) is a widely used standard preemergence granular herbicide that contains 0.5% isoxaben (N-[3-(1-ethyl-1- methylpropyl)-5-isoxazolyl]-2, 6-dimethoxybenzamide) and 2% trifluralin (α,α,α- trifluoro-2,6-dinitro-N,N-dipropyl p-toluidine) for broadspectrum weed control in nurseries and landscapes (Judge et al. 2003; H.M. Mathers, personal communication).

Snapshot was chosen to compare with liquid formulations of the same active ingredients in this study. Isoxaben is a pre-emergent herbicide that controls a wide range of broadleaf weeds in cereals, turf, and ornamentals. Isoxaben kills weeds by inhibiting cellulose biosynthesis in emerging seedlings (Schneegurt et al. 1994). It is marketed for pre-emergence weed control in field nursery crop production as a 75% active dry flowable (DF) and labeled for use in over 400 species. It is typically applied at 0.6 to

1.1 kg/ha (Altland 2003). Neal and Senesac (1990) reported that isoxaben provided excellent control of many broadleaved weeds including pigweed, common groundsel, and dandelion, but poor control of annual grasses such as crabgrass, goosegrass, and fall panicum. The advantage of using isoxaben over oxyfluorfen or simazine is that isoxaben is safe on a broader range of nursery and landscape crops.

Trifluralin is used commonly in nurseries and landscapes and is a dinitroaniline

(DNA) herbicide that acts by inhibiting cell division in developing roots. DNA herbicides (e.g., pendimethalin, oryzalin, and prodiamine) are widely used preemergence herbicides in nurseries as they control most of the annual grasses and some small-seeded broadleaf weeds (Stamps and Neal 1990; Weber and Monaco 1972), and they are not phytotoxic to most nursery crops (Neal et al. 1999). Trifluralin volatilizes easily due to its high vapor pressure (1.1x10-4 mm Hg) (Weber 1990; Weber and Monaco 1972) and has lower leaching potential due to its low water solubility

(Tepe and Scroggs 1967). Trifluralin is also susceptible to photodecomposition when exposed to ultraviolet radiation (Tepe and Scroggs 1967).

Dithiopyr (3, 5-pyridine-dicarbothioic acid, 2- (difluoromethyl)-4-(2- methylpropyl)-6-(trifluoromethyl)- S,S-dimethyl ester ) (Dimension®, Dow

AgroSciences, Indianapolis, Indiana) is the herbicide in Weedstop® mulch, and is a pyridine herbicide that controls more than 45 grass and broadleaf weed species

We hypothesized that mulches pretreated with preemergence herbicides would provide superior weed control compared to herbicides applied over or below the mulch. We also hypothesized that granular herbicides with mulches would be relatively less effective compared to mulches treated with liquid formulations of the same herbicides. The objectives of this study were to (1) Compare the effect of granular herbicide versus liquid herbicide formulations alone and in combination with different mulches on weed control (2) determine the effect of mulch layer thickness on weed control; and (3) determine the influence of herbicide placement (on top of mulch, on soil surface under mulch, or herbicide-treated mulch) on weed control.

Materials and Methods

Two field experiments were conducted at The Ohio State University’s

Waterman Agricultural and Natural Resources Laboratory, Columbus, Ohio. A field site that had not received herbicide applications for at least two years was chosen. The soil type was a Kokomo silty clay loam with cation exchange capacity (CEC) ranging from 12-20 and pH 6-7. Mulch treatments investigated were hardwood mulch and pine nuggets, either untreated or in combination with herbicides applied at three depths, 3, 6, or 12 cm. Herbicide treatments included Snapshot 2.5TG [isoxaben + trifluralin at 1.12 kg ai/ha + 4.48 kg ai/ha, respectively (Dow AgroSciences, Indianapolis, IN)] applied alone or on top of the mulch. A liquid formulation consisting of trifluralin (Treflan

HFP, Dow AgroSciences, Indianapolis, IN) + isoxaben (Gallery 75DF, Dow

AgroSciences, Indianapolis, IN) at 4.48 kg ai/ha + 1.12 kg ai/ha was prepared and applied directly to soil, below or on top of the mulch or as a pretreatment on the mulch as described by Case and Mathers (2006). Hereafter, the liquid herbicide formulation will be referred to as T+G. Snapshot 2.5TG was directly applied on top of the mulch in the field. There were a total of 35 treatments including the untreated mulches and two commercial herbicide-treated mulches, Preen Mulch® and Weedstop® applied at a thickness of 6 cm only, and an untreated control (no mulch, no herbicide).

The experimental plots were 1 m2, arranged in a randomized complete block design (RCBD) with five replications. Mulches were pretreated with T+G by placing the mulch on a plastic sheet at their three respective depths and sprayed with the herbicide rates mentioned above using a CO2-pressurized backpack sprayer (R&D

Sprayers, Opelousas, LA) equipped with 2-8002evs flat pan nozzles operated at 40 psi and spaced 41 cm apart. Two wooden rectangular frames of 1 m x 5 m (Appendix A) were constructed at heights of 6 and 12 cm. These boxes were kept on the plastic sheet and filled with mulch and then T+G was sprayed. The mulch was then allowed to dry for 48 hours before mixing and applying them to the plots. Two wooden 1 m2 square frames (appendix B) were constructed with a 6- and 12-cm depth and used as templates in which to apply the appropriate treatments in the field. The granular herbicide treatment was spread on the mulch manually to ensure uniform application. Plots were sprinkler-irrigated immediately after all the treatments were applied and later as needed during the experimental period to ensure weed growth.

The first experiment was initiated on 15th October, 2006 and was repeated the following year beginning on 13th September, 2007. Since the goal was to evaluate long- term effectiveness of herbicide and mulch combinations for weed control, evaluations were done at 180 and 210 days after treatment (DAT) in 2006 and 200 and 230 DAT in

2007. There was no weed growth at 180 DAT for the 2007 experiment due to saturated soil conditions, so evaluations on that date were not possible. Data collected included weed fresh weights, obtained by clipping weeds off at ground level within a 0.25 m2 quadrate. Weed fresh weights were measured on the same day as collection. Visual ratings were also recorded on a scale of 0 to 10, where 0 was no weed control, 10 was complete weed control and ≥7 was regarded as commercially acceptable.

Precipitation and average temperatures for March, April, and May in 2008 were

150 mm and 3 ºC, 39 mm and 12 ºC; 71 mm and 15 ºC, respectively (OARDC 2010). In

2007, the precipitation and average monthly temperatures for March, April, and May were 133 mm and 7.5 ºC; 67 mm and 9 ºC; 23 mm and 18 ºC, respectively.

Data were analyzed using the PROC GLM (general linear model) procedure of

SAS (SAS Institute Inc., Cary, NC). Means were separated by Fisher’s least significant difference (LSD) at α = 0.05. Dunnett’s test was also performed to compare the treatments with the untreated control. Orthogonal contrasts were conducted using the

CONTRAST procedure of SAS to compare selected treatment groupings.

Results and Discussion

Canada thistle (Cirsium arvense (L.) Scop.) control was evaluated with other weeds in the 2006 experiment, so results of Fall 2006 and Fall 2007 experiments are presented separately. In 2007, a weed wiper filled with glyphosate was used to control the Canada thistle. Weed control ratings and fresh weights for the DATs listed in the

Materials and Methods are presented in Tables 3.1 and 3.2, respectively with the exception of weed fresh weights at 180 DAT in 2006, due to a collection error.

Orthogonal contrasts results. Orthogonal contrasts were performed to compare hardwood and pine nuggets (mulch alone, across all depths); three depths of mulches, and between granular (Snapshot) herbicide plus mulch combinations vs liquid (T+G) herbicide plus mulch combinations. Contrast analyses were also performed to see if there were significant differences among herbicide placements: granular-over, liquid- over, liquid-under, and liquid-treated. Contrast analysis showed that there was no difference between hardwood and pine nuggets in terms of visual ratings and weed fresh weights in both years (appendix D). Visual ratings for mulches applied at the 3-cm depth (across all treatments) were significantly lower than those of the 6-cm and 12-cm depth mulch treatments in both years. Visual weed control ratings of mulches applied at the 6-cm depth were lower than for the 12-cm depth mulch treatments (across all treatments or mulch alone treatments) in both years. There was no difference in the visual ratings between granular plus mulch combinations and liquid plus mulch 83 combinations in both years. Contrasts of herbicide placement treatments showed that there were differences only between granular-over and liquid-under, liquid-over and liquid-under in both years, liquid-under and liquid-treated in 2008 only. Liquid-under treatments provided higher visual rating than those of granular-over, liquid-over and liquid-treated. All of the rest of the possible herbicide placement comparisons were not significant. Weed fresh weight data showed that there were differences only between 3- cm and 6-cm depth mulches, with higher weed weights with 3-cm in 2008 only.

In the experiment initiated in 2007, mulches at the 12- and 6-cm depth with or without herbicide provided visual weed control ratings >9 at 200 DAT and >8.2 at 230

DAT. The efficacy of these treatments was also reflected in the weed fresh weights, with more than 90% reduction in weed biomass at 200 DAT and more than 93% reduction at 230 DAT compared to the control. Weed growth was minimal at 200 DAT, which may be attributed to lower than average temperatures (monthly average temperature 3 ºC) and saturated soil conditions during March 2008 that resulted from150 mm precipitation.

In the experiment started in 2006, ratings averaged across both evaluation dates indicated that three treatments had weed control ratings > 9, 13 treatments had a rated score of >8, and 23 treatments had a rated score of >7. In contrast in the 2007 experiment, 19 treatments had average ratings >9, 26 treatments had ratings >8, and 32 treatments had ratings >7 (Table 3.1). Despite the fact that so many herbicide plus mulch treatments performed better in the 2007 experiment, the average weed weights of the control treatment were approximately the same or higher than at the same time the

84 previous year, averaging 44 g in 2006 at 210 DAT, and 47g in 2007 (averaged over 180 and 210 DAT). Many treatments performed better in the 2007 experiment compared to

2006, but early spring weed growth was much greater in the 2006 than the 2007 experiment. In 2007, all of the treatments involving 12-cm of mulch had a combined weed weight of less than 1g with visual ratings of at least 9.8 (Tables 3.1 and 3.2). In

2006, 6 of the 10 treatments with 12-cm mulch did not rank in the top 9 treatments

(Table 3.1).

The data revealed some of the inconsistencies between the weed fresh weights and their corresponding weed control ratings. The averaged data across dates for the

2006 experiment indicated that for each 4g of weed fresh weight there was a decrease in efficacy rating score by 1. Therefore, the untreated PN at 6 cm should have gotten a rating score 4 instead of 7.2 and the untreated HW at 6 cm should have gotten 5 score instead of 6.7. Also, Snapshot over 3-cm of PN mulch had a weed control rating of 5.7 when the treatment produced a weed biomass of 83.5g, which compared to the control that had only 44g of weed biomass and control rating of 0. The disagreement between weed control ratings (which were based on the percent of area covered by weeds) and weed biomass are largely attributable to the presence of Canada thistle in the plot area in the 2006 experiment. Canada thistle is a cool-season creeping perennial that produced relatively high weed biomass in early spring, even though its spread over the treatment plot was relatively low due to its erect growth habit. The Canada thistle was controlled with glyphosate prior to the 2007 experiment.

Canada thistle is a perennial weed with an extensive spreading root system

(Amor and Harris 1976) and is classified as a noxious weed in many regions of the U.S.

(MacDonald and Loughner 2009; USDA- NRCS 2010). Canada thistle spreads through seeds and adventitious root buds on extensively expanding root system (Amor and

Harris 1976; MacDonald and Loughner 2009). Canada thistle has an extensive network of both deep roots and horizontal spreading root that can spread 5 m or more from the mother plant and can produce new plants from adventitious buds on the roots at 5 to 15 cm intervals (USDA-NRCS 2010). None of the herbicides evaluated in this experiment were labeled for Canada thistle control, and the distribution of the Canada thistle was not uniform in the field, making it difficult to compare results from the 2006 and 2007 experiment. There is limited research that shows the control of perennial weeds with mulches, and mulches have never been recommended for perennial weed control.

Skroch et al. (1992) reported 50% reduction in the perennial weed growth when organic mulches such as pine bark, hardwood mulch, pine needles and cedar chips were applied in a 9-cm-thick layer. They didn’t find any difference between the mulches in terms of weed counts of tall fescue (Festuca arundinacea Schreb.), common blue violet (Viola papilionacea Pursh), wild garlic (Allium vineale L.), bermudagrass [Cynodon dactylon

(L.) Pers.], or yellow nutsedge (Cyperus esculentus L.). Weed fresh weight data of the

2006 experiment (Table 3.2) indicated that mulches even applied at 12 cm cannot provide Canada thistle control.

The results of 2007 experiment were similar to the results we obtained from our spring studies with same set of treatments. Mulches at 6 and 12 cm with or without

86 herbicides received weed control ratings >8 up to 120 DAT. Treatments involving the

3-cm mulch treatments also behaved almost the same way as in the spring experiment.

Treatments with 3-cm mulch that provided acceptable weed control ratings at 210 DAT were T+G applied under both mulches, Snapshot applied above both mulches, and T+G applied over pine nuggets (Table 3.1). However, the ratings of Snapshot over both 3- cm mulches and T+G over 3-cm pine nuggets were not significantly different compared to either of the untreated 3-cm mulches. Mulches and herbicides provided acceptable weed control only when applied in combinations. Weed fresh weights of all the treatments involving 3-cm mulch and herbicide alone treatments were reduced significantly compared to the control (Table 3.2).

Snapshot applied both below and above the 3-cm mulch provided acceptable control and this could be due in part to the long residual activity of trifluralin and isoxaben herbicides. This explanation is supported by the soil persistence half-life data reported for trifluralin and isoxaben by earlier researchers. Overall half-life values for trifluralin in the published literature from university trials ranged from 19-132 days

(Weber 1990), but in USEPA tests they ranged from 116-201 days when trifluralin was incorporated into soil and 29-149 days without soil incorporation (EPA 1996).

Published isoxaben half-life values have ranged from 88-201 days (Rouchaud et al.

1993 a, b), 76-122 days (Huggenberger and Ryan 1985), 152-183 (Huggenberger et al.

1982), 76-158 days (Walker 1987), and 82-186 days (Chandran and Derr 1999).

Different half-life ranges for a given herbicide are due to various environmental factors and soil properties. Half-life values mentioned above indicated that it is possible for

87 fall-applied trifluralin and isoxaben to persist well into spring months since herbicide degradation rates decrease substantially when soil temperatures decline in fall and winter months (Chandran 1997; Judge et al. 2003).

In 2007, T+G applied over 3 cm pine nuggets resulted in weed control ratings that did not differ significantly from the T+G-treated pine nuggets applied at the same thickness. Huang et al. (2006) reported stronger adsorption and slower desorption of isoxaben on mulches. Our studies confirmed the release of herbicide from pine bark nuggets into the soil surface; however, the rate of release depends on several factors, including irrigation and/or rainfall. It is probable that desorption and leaching of herbicides from organic substrates such as wood-based mulches is also dependent on timing and intensity of precipitation relative to when the herbicides were applied.

T+G-treated pine nuggets and hardwood applied at 3 cm in 2007 provided weed control ratings of 6.2 and 6.4, respectively. These ratings were below the commercially acceptable level and not significantly different than the mulch or herbicide alone treatments. These results were in contrast to previous findings by Case and Mathers

(2003), who reported acceptable weed control up to 303 days in containers with

Douglas fir mulch treated with acetochlor (Degree) or oryzalin. The same authors reported that acetochlor-treated pine nuggets provided least efficacy but obtained different results in another container experiment when pine nuggets were treated either with microencapsulated acetochlor (Degree) or the emulsifiable concentrate (Harness)

(Mathers et al. 2003).

In other research, Case and Mathers (2003) conducted a field experiment in which they compared five herbicides, oryzalin, flumiaxazin, dichlobenil, acetochlor, and oryzalin+flumioxazin, in combination with single layer of hardwood or pine bark mulch. They also tried different herbicide placement treatments, including over the mulch, under the mulch, pretreated mulch, mulch alone, and herbicide alone. There were 10 treatments that provided 7 or higher rating one year after treatment, and all of them involved pine nuggets plus herbicide combinations (ratings were averaged over two years). None of the hardwood mulch and herbicide combinations provided acceptable ratings after one year. One possible explanation for low efficacy of the hardwood mulch treatments was that the mulch in a single layer was dislodged by wind or water, or it degraded rapidly on the soil surface in contrast to pine nuggets, which are much thicker and more resistant to biodegradation.

Results from our study and others indicate that not all of the mulch and herbicide interactions have a positive effect on weed control. Positive effects of HTMs mainly depend on the adsorption and desorption characteristics of herbicide with mulch, which depends on mulch and herbicide chemistry. HTM effectiveness for weed control also depends on rainfall or irrigation as it effects desorption of herbicide from mulch, which in turn affects efficacy and biodegradation of the herbicides.

Data clearly indicates that when mulches at 6 and 12 cm are combined with herbicides, the effect of weed control is predominantly from the physical contribution of the mulch, whereas mulch layers of 3 cm required the addition of herbicides for acceptable weed control. This is evident from the efficacy ratings of herbicides plus

89 mulches applied at the 3 cm depth (average ratings of 7.1 to 8.7 in 2007 (Table 3.1)) versus the mulch alone treatments applied at 3 cm (average ratings of 6.6 to 7.2 in 2007,

(Table 3.1)). Even though the 12-cm or thicker mulch treatments can provide excellent weed control, they are not recommended considering the cost of mulch and its potential for negative impacts on the health of desirable plants caused by a lack of oxygen or water, or by maintaining saturated soil conditions (Billeaud and Zajicek 1989; Gouin

1984). In landscapes, excessive mulching often leads to suffocation of roots and root girdling, which eventually kills trees (Higgins 2009; Gillman 2009).

Our data and previous research suggest that even though the recommended depth of mulch is 6 cm for optimum weed control, mulch depth can be reduced to 3 cm when herbicides are combined with the mulches. The research presented here agrees with previous findings that mulch and herbicide combinations do not always produce a positive interaction, and suggests that further research needs to be conducted to determine the optimum mulch and herbicide combinations. Most research conducted thus far has focused on combining single herbicides with a mulch. Future research should also consider combining more than one herbicide in HTMs to increase the weed control spectrum. Optimum herbicide application rates in HTMs also need to be determined based on the mulch material, since sorptive properties differ among different mulches. Future development of HTMs would benefit greatly by understanding the factors and underlying mechanisms that regulate adsorption and desorption of herbicides by mulches.

Table 3.1. Weed efficacy ratings of fall applied mulch, herbicide, and mulch and herbicide combined treatments for experiments initiated in fall 2006 and 2007.

Treatmentz 2006 experiment 2007 experiment 180 DAT 210 DAT Average 200 DAT 230 DAT Average Control 0y gx 0 k 0 o 0 h 0 i 0 k Snapshot (SS) 6.8 def 5.6 j 6.2 lmn 8.8 cde 5.4 H 7.1 Hij Treflan + Gallery 7.4 bcdef 6.2 ghij 6.8 hijklmn 7.8 fg 5.6 h 6.7 ij Untreated 3-cm PN 7.6 bcdef 6 hij 6.8 hijklmn 8.2 ef 6.2 gh 7.2 hij Untreated 3-cm HW 6.8 def 5.8 ij 6.3 klmn 7 g 6.2 gh 6.6 j SS over 3-cm PN 5.8 f 5.6 j 5.7 n 9.2 abcd 7.4 efg 8.3 defg SS over 3-cm HW 7.2 bcdef 7.4 cdefghi 7.3 fghijklm 8.2 ef 7.2 fg 7.7 fgh T + G over 3-cm PN 7.8 abcdef 6.4 fghij 7.1 fghijklm 9 bcde 7.4 efg 8.2 efg T + G over 3-cm HW 6.3 ef 6.4 fghij 6.3 jklmn 8.8 cde 5.4 h 7.1 hij T + G under 3-cm PN 6.8 def 6.6 efghij 6.7 ijklmn 9 bcde 8.4 cdef 8.7 cde T + G under 3-cm HW 8.4 abcde 7.2 defghij 7.8 defghi 9 bcde 8.2 def 8.6 cdef T + G treated 3-cm PN 8 abcdef 7 defghij 7.5 efghijkl 9 bcde 6.2 gh 7.6 ghi T + G treated 3-cm HW 6 f 6.2 ghij 6.1 mn 8.4 def 6.4 gh 7.4 ghij Untreated 6-cm PN 7.2 bcdef 7.2 defghij 7.2 fghijklm 9.2 abcd 9.2 abcd 9.2 abcd Untreated 6-cm HW 7 cdef 6.4 fghij 6.7 ijklmn 9 bcde 8.8 abcd 8.9 bcde SS over 6-cm PN 9.4 ab 8.2 abcde 8.8 abcde 9.8 ab 9 abcd 9.4 abc SS over 6-cm HW 7.2 bcdef 7.2 defghij 7.2 fghijklm 9.6 abc 8.2 def 8.9 bcde T + G over 6-cm PN 7.8 abcdef 6 hij 6.9 ghijklmn 9.6 abc 9.2 abcd 9.4 abc T + G over 6-cm HW 7.8 abcdef 7.2 defghij 7.5 efghijkl 9.8 ab 9 abcd 9.4 abc T + G under 6-cm PN 8.8 abcd 7.4 cdefghi 8.1 cdefgh 10 a 10 a 10 a T + G under 6-cm HW 7.4 bcdef 7.6 bcdefgh 7.5 efghijkl 9.4 abc 9.4 abcd 9.4 abc T + G treated 6-cm PN 8 abcdef 7.2 defghij 7.6 defghijk 9.8 ab 9.6 abc 9.7 ab T + G treated 6-cm HW 9 abcd 8.6 abcd 8.8 abcde 9.2 abcd 8.6 bcde 8.9 bcde 6-cm Weedstop 8.8 abcd 7.6 bcdefgh 8.2 cdefg 9.8 ab 9.8 ab 9.8 ab

Continued

Table 3.1 continued

Treatment 2006 experiment 2007 experiment 180 DAT 210 DAT Average 200 DAT 230 DAT Average Mulch with SS @2.5" 8.8 abcd 7.6 bcdefgh 8.2 cdefg 9.8 ab 9.6 abc 9.7 ab Untreated 12-cm PN 8.6 abcd 8.2 abcde 8.4 abcdef 10 a 10 a 10 a Untreated 12-cm HW 8.8 abcd 7.8 abcdefg 8.3 bcdef 9.8 ab 9.8 ab 9.8 ab SS over 12-cm PN 10 a 9.2 ab 9.6 ab 10 a 10 a 10 a SS over 12-cm HW 7.2 bcdef 6.4 fghij 6.8 hijklmn 9.8 ab 9.8 ab 9.8 ab T + G over 12-cm PN 9 abcd 8.4 abcd 8.7 abcde 10 a 10 a 10 a T + G over 12-cm HW 8.8 abcd 8 abcdef 8.4 abcdef 9.8 ab 9.8 ab 9.8 ab T + G under12-cm PN 10 a 9.4 a 9.7 a 10 a 10 a 10 a T + G under 12-cm HW 9.4 ab 9 abc 9.2 abc 10 a 9.8 ab 9.9 a T + G treated 12-cm PN 9.2 abc 8.6 abcd 8.9 abcd 10 a 9.8 ab 9.9 a T + G treated 12-cm HW 7.8 abcdef 7.6 bcdefgh 7.7 defghij 10 a 9.8 ab 9.9 a LSD (p=0.05) 2.3 1.6 1.39 0.9 1.3 0.95

Table 3.2. Weed fresh weights (g) of fall applied mulch, herbicide, and mulch and herbicide combined treatments for experiments initiated in fall 2006 and 2007.

Treatmentz 2006-Fresh weight (g) 2007- Fresh weight (g) 210 DAT 200 DAT¥ 230 DAT¥ Average¥ᴪ Control 43.86 abcdey 11.05 a 82.95 a 47.0 a Snapshot (SS) 52.06 abc 0 c 19.78 b 11.0 bc Treflan + Gallery 12.58 bcde 4.1 b 5.42 bcd 4.8 bcde Untreated 3-cm PN 13.24 bcde 0.25 c 4.26 cd 2.5 bcde Untreated 3-cm HW 56.26 ab 1.7 bc 20.34 b 12.1 b SS over 3-cm PN 83.54 a 2 bc 9.24 bcd 6.0 bcde SS over 3-cm HW 2.96 e 0 c 0.2 d 0.1 e T + G over 3-cm PN 43.86 abcde* 2.075 bc 5 bcd 3.7 bcde T + G over 3-cm HW 23.08 bcde 0 c 7.74 bcd 4.3 bcde T + G under 3-cm PN 6.02 de 0 c 1.8 d 1 cde T + G under 3-cm HW 32.28 bcde 1.075 bc 0.06 d 0.5 de T + G treated 3-cm PN 31.28 bcde 0 c 19.28 bc 10.7 bcd T + G treated 3-cm HW 10.16 cde 0 c 7.72 bcd 4.3 bcde Untreated 6-cm PN 24.54 bcde 0 c 0 d 0 e Untreated 6-cm HW 20.48 bcde 0.875 c 2.76 d 1.9 bcde SS over 6-cm PN 0.86 e 0.75 c 5.56 bcd 3.4 bcde SS over 6-cm HW 27.88 bcde 0 c 0 d 0 e T + G over 6-cm PN 23.98 bcde 1 bc 0 d 0.4 de T + G over 6-cm HW 18 bcde 0 c 0.42 d 0.2 e T + G under 6-cm PN 10.74 cde 0 c 0 d 0 e T + G under 6-cm HW 31.96 bcde 0 c 0 d 0 e T + G treated 6-cm PN 26.02 bcde 0.25 c 1.42 d 0.9 cde T + G treated 6-cm HW 3.7 e 0 c 2 d 1.1 cde 6-cm Weedstop 6.76 cde 0 c 0 d 0 e Control 24.24 bcde 0 c 0 d 0 e

Continued

Table 3.2 continued

Treatment 2006-Fresh weight (g) 2007- Fresh weight (g) 210 DAT 200 DAT¥ 230 DAT¥ Average¥ᴪ Untreated 12-cm PN 10.18 cde 0 c 0 d 0 e Untreated 12-cm HW 30.86 bcde 0 c 1.4 d 0.8 cde SS over 12-cm PN 0 e 0 c 0 d 0 e SS over 12-cm HW 51.02 abcd 0 c 0 d 0 e T + G over 12-cm PN 0.32 e 0 c 0 d 0 e T + G over 12-cm HW 16.14 bcde 0 c 0 d 0 e T + G under12-cm PN 0 e 0 c 0 d 0 e T + G under 12-cm HW 0 e 0 c 0 d 0 e T + G treated 12-cm 12.06 bcde 0.975 bc 0 d 0 e T + G treated 12-cm 22.32 bcde 0 c 0 d 0 e PNLSD 45.5 (p=0.1) 3.2 (p=0.05) 15.45 (p=0.05) 10.40 HW z Abbreviations: PN, pine nuggets; HW, hardwood; SS, Snapshot; T+G, treflan (trifluralin) + Gallery (isoxaben). y Within each column, means followed by the same letter are not significantly different.

*Not different from control on the basis of Dunnett’s test.

¥ All the treatments are different from control on the basis of Dunnett’s test.

ᴪ Averaged across the two evaluation dates in the 2007 experiment.

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Chapter 4: Effect of herbicide-organic mulch combinations on herbicide persistence in soil

Introduction

Weed management is a major activity that consumes a significant portion of the production costs for the greenhouse and landscape industry, which is the fifth largest agricultural sector in the U.S. Preemergence herbicides are important tools for weed management in the landscape and nursery industry; however, to provide season-long weed control they must often be applied three to five times per year by nursery container growers (Gilliam et al. 1990) and two to three times per year by nursery field growers and landscapers (H.M. Mathers, Personal communication). Multiple herbicide applications in a single year can lead to the development of herbicide-resistant weed populations, environmental pollution, and health concerns to humans and wildlife.

Some of the other problems associated with herbicide use are improper equipment calibration, phytotoxicity to desirable species, and off-target herbicide movement via leaching, spray drift, and runoff from plastic containers and gravel container beds, field soils or landscapes. Off-target herbicide losses in container nurseries can be up to 86%

102 of the applied herbicide depending on growth habit of the species and spacing of the containers (Gilliam et al. 1992).

Although herbicides are important tools for a successful weed management program in the landscape and nursery industry, they often fail to control weeds completely. As a result, chemical weed control is usually supplemented by hand- weeding. Development of improved weed control methods could reduce the additional expenses associated with hand-weeding and improve profitability in the industry. Due to the potential negative effects of herbicides described earlier, many researchers have focused on developing new weed control strategies that either minimize herbicide use or utilize herbicides more efficiently in order to reduce the potential for negative effects.

The most effective yet environmentally sound weed control strategy in the landscape and nursery industry would likely be one that integrates the effective utilization of herbicides with non-chemical methods. One such potential strategy is the combined use of herbicides and organic mulches. Organic mulches have been used widely because they provide several benefits including weed control, soil moisture conservation, regulation of soil temperature, control of soil erosion and soil crusting, and increased aesthetic value of the landscape. Weed control and soil moisture conservation, however, have been cited as the most important advantages (Robinson

1988). Organic mulches inhibit weed seed germination and suppress weed growth

(Borland 1990; Duryea et al. 1999; Skroch et al. 1992).

103

Herbicide and mulch combinations can reduce the amount of herbicide needed to enhance and/or extend weed control efficacy (Derr 1994), increase environmental and ecological safety, and comply with current label restrictions (Mathers and Ozkan

2001). Studies conducted by Oliveira et al. (2000) showed that lignin, an important component in organic mulches, can be used as a medium for controlled release of herbicides. Previous studies demonstrated that herbicide and mulch combinations worked effectively in controlling weeds. Fretz (1973) and Fretz and Dunham (1971) reported higher weed control efficacy with herbicide-impregnated mulches than with mulches alone. Case and Mathers (2006) found that pine nuggets combined with various herbicides provided weed control for up to one year in the field. Mathers

(2003) obtained better weed control in containers with herbicide-treated bark nuggets than with bark nuggets alone.

Even though several studies have reported the positive effect of herbicide and mulch combinations on weed control, there has been no in-depth study that clearly identifies factors that make the combinations most effective. In the study presented here we investigated the efficacy and persistence of herbicides over time from different herbicide formulations and in various herbicide-mulch combinations. We hypothesized that when compared to soil-applied herbicides, herbicide-treated mulches would extend the period of herbicide persistence in soil by releasing the herbicides into the soil more slowly over time. We also hypothesized that herbicides applied in granular formulations would persist longer in the soil than herbicides applied in liquid formulations. 104

The objectives of this study were (1) to determine the persistence of isoxaben and trifluralin applied with and without organic mulches, and (2) to compare the persistence of isoxaben and trifluralin when applied in granular versus liquid formulations.

Materials and Methods

Snapshot® (Dow Agrosciences, Indianapolis, IN) is a premixed granular herbicide formulation containing 0.5% isoxaben (N-[3-(1-ethyl-1-methylpropyl)-5- isoxazolyl]-2,6-dimethoxybenzamide) and 2% trifluralin (α,α,α-trifluoro-2,6-dinitro-

N,N-dipropyl p-toluidine). It was chosen to combine with mulches in this study because it is a popular preemergence product widely used in nursery industry (Altland

2005). Isoxaben controls a wide range of broadleaf weeds in cereals, turf, and ornamentals, and trifluralin controls grasses and some small-seeded broadleaves.

Granular herbicides are preferred over liquid formulations by nursery and landscape managers because of reduced phytotoxicity to non-target plants and ease of application.

Pine nuggets and hardwood mulch were selected as mulch treatments because they are widely available and used extensively by the nursery and landscape industry.

Field experiment. A field experiment was conducted in the spring of 2007 and repeated in 2008 at the Waterman Agricultural and Natural Resources Laboratory of

The Ohio State University, Columbus, Ohio (40.016°N, 83.039°W). Two types of organic mulches, pine nuggets and hardwood mulch, were investigated. There were a 105 total of six treatments: Snapshot® 2.5TG (granular formulation), trifluralin+isoxaben

(T+I; liquid formulation), T+I soil-applied under pine nuggets, T+I soil-applied under hardwood, T+I-treated pine nuggets, and T+I-treated hardwood. Trifluralin and isoxaben in all formulations were applied at the rate of 4.48 kg ai/ha + 1.12 kg ai/ha, respectively.

The experiment was conducted in a randomized complete block design (RCBD) with three replications. The first experiment was initiated on May 15, 2007 and the second on May 3, 2008. The plot size was 1 m x 1 m. Herbicide-treated mulch treatments were prepared by placing the mulches to a depth of 6 cm in a rectangular wood frame resting on a plastic sheet. Herbicides treatments were then sprayed evenly over the top of the mulch and allowed to dry for 48 hours before mixing the mulches and applying them to the plots. All liquid herbicide formulations were applied with a

CO2-pressurized backpack sprayer (R&D Sprayers, Opelousas, LA) operated at 276 kPa and equipped with 2-8002EVS flat-fan nozzles spaced 41 cm apart. The granular herbicide Snapshot was applied manually ensuring uniform application over the entire plot. A 6-cm-deep, 1 m2 wood frame was used as a template to apply mulches uniformly in the field plots. Overhead irrigation was applied immediately after all the treatments were applied in the field and later as needed during the experimental period.

Plots were soil-sampled to a depth of 6 cm at 0, 30, 60, 90, 120 days after treatment

(DAT) using a 2.5-cm-diameter soil probe. Soil sampled at 0 DAT was collected on the same day of application after overhead irrigation was applied. Soil samples were preserved in airtight containers and stored at -20 0C until herbicide extraction and 106 quantification. Soil type at the experimental site was Kokomo silty clay loam with cation exchange capacity (meq/100g of soil) of 15.6 ± 1.2, pH 6.6 ± 0.2, and 4.8% organic matter.

Laboratory Extraction Procedure. Herbicides were extracted from soil following a procedure similar to those published by Garcia-Valcarcel et al. (1996), Riley et al.

(1994), and Drakeford et al. (2003). Soil samples were removed from the freezer and dried for 48 h at 40 0C. Each sample was mixed well and 10 g of soil were added to 20 ml of methanol in an Erlenmeyer flask and shaken for 1 hour. The extract was filtered using Whatman no.1 filter paper. The same procedure was repeated twice and extracts were combined. The extracts were evaporated to dryness using a rotary evaporator

(Rotavapor RE 121, Buchi, Switzerland) at 35 0C. The evaporatory flask was rinsed twice with 3 ml of methanol, then the rinsate was transferred to a 10 ml Reacti-vial

(Thermo-Fisher Scientific, Rockford, IL) to evaporate to 2 ml using a Reacti-Vap evaporation unit (Pierce Model- 18780, Rockford, Illinois). The concentrate was drawn into a syringe and then filtered into 2-ml vial using 0.2 µm filter discs attached to the syringe. The vials were stored at -20 0C until analysis.

Herbicide residue analyses. The herbicide extracts were analyzed by HPLC-MS

(Varian, Model 500-MS IT HPLC-Mass spectrometer fitted with a Varian Prostar

Model 410 autosampler and Varian Model 335 photodiode array detector). Compounds were separated on a C18 reverse-phase column (LiChrosorb, RP-18, 10µ, 4.6mm x

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250mm) with a mobile phase gradient beginning with 60:40 water:acetonitrile for 10 minutes, then 50:50 water:acetonitrile for 10 minutes, then 0:100 water:acetonitrile for

5 minutes, and finally 60:40 water:acetonitrile for 1 minute. The flow rate was

800µl/min. A 250µl aliquot of each extract was transferred to an autosampler vial, and the injection volume was 50 µl. The flow was split post-column, with half of the flow going to the photodiode array detector and the other half into the mass spectrometer

(ESI-Electrospray ionization). Isoxaben was quantified by MS and trifluralin was quantified by photodiode array detection at 280nm, and data are reported as herbicide concentration (μg 10 g-1 soil).

Standard solutions of 100, 50, 25, 12.5, 6.25, 3.125, 1.56 ppm isoxaben and trifluralin were prepared by serial dilution of a 500 ppm stock solution. Pure standards were purchased from Chem Service, West Chester, Pennsylvania. Standard curves were developed by regressing peak areas on herbicide concentrations for the standards, then the resulting equation was used to determine concentrations of isoxaben and trifluralin in unknown samples. Recovery percentages and minimum detection limits were also calculated by spiking the herbicide-free soil samples with standard solutions. Average recovery of trifluralin was 89 ± 8% and recovery of isoxaben was 92 ± 4.5%.

Bioassay procedure. Bioassays were conducted to determine the bioavailability of trifluralin and isoxaben in the soil samples that were collected during the field experiment. Oats and radish were used to determine trifluralin and isoxaben bioavailability, respectively. The bioassay procedure was adopted from Jacques and

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Harvey (1979) and Parker (1966). Bioassays were conducted using the soil samples from the same six treatments described for the field experiment above. An untreated control was also included. Soil samples were dried at 40 0C for 48 hours and mixed thoroughly. The soil was sieved through a 3 mm screen and samples of 80 g each were transferred into 100-mm-diameter x 15-mm-deep petri dishes, and 28 ml of water was added to each petri dish to bring the soil moisture to field capacity.

Five pre-germinated seeds each of oats and radish were placed in a row and pushed into the soil so that the root tips were even with soil surface. Radicles of all the pre-germinated seeds were aligned in one direction. Petri dishes were closed with lids and all the petri dishes were placed in growth chamber with an angle of 15 degrees from the vertical so that roots would grow downward against the lids. The temperature of the growth chamber was set at 25 0C with continuous fluorescent light that provided about

30 µmol PPFD. The position of the root tips on the lids of petri dishes was marked after

24 hours, and subsequent root growth distance from the original mark was measured after 48 hours. Root length data of all the treatments were presented as percentage of the untreated controls.

Statistical analyses. Persistence of pesticides in the field is usually described by a first-order kinetic model (exponential decay function) (Simonsen et al. 2008; Zhang et al. 2010). An exponential decay function was fit to the herbicide degradation data, except for the two herbicide-mulch combination treatments, in which herbicide degradation did not follow first-order kinetics. The exponential decay equation or first order kinetic model fit to the data was:

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Where, is the concentration of herbicide at time is the initial herbicide concentration, is the first order rate constant.

Root lengths obtained from the bioassay study were converted to percent root growth. Percent root growth is calculated by comparing the root growth from the herbicide treatments to that of the untreated control. The bioassay data were fitted into following logistic growth function or sigmoid growth curve:

( )

where is the growth at time X, is the maximum growth (upper asymptote), X0 is the inflection point, and is the rate constant. All curve fitting was done using Sigmaplot® software (Sigmaplot v. 12, Systat Software Inc., Chicago, IL). Regression equations of exponential decay and sigmoid growth curves were compared pairwise using the following F-test (Zar 1996):

( )( )

Where, SSt is the total residual sum of squares of the combined data, SSp is the pooled residual sum of squares, m is the number of independent variables, k is the number of

110 regression equations being compared, and DFp is the pooled residual degrees of freedom. The significance level for all statistical tests was α = 0.05.

Results and Discussion

Overall results. Exponential decay regression curves for trifluralin and isoxaben degradation in soil are shown in Figures 4.1 and 4.3, respectively. Sigmoidal root growth reduction curves for trifluralin and isoxaben are presented in Figures 4.2 and

4.4, respectively. The data for each herbicide are presented separately by year because there was a significant year by treatment interaction except for the trifluralin bioassay data, which were pooled across years. All regression model parameters are shown in

Table 4.1. Regressions overall provided a good fit (r2 > 0.96) to the herbicide residue and bioassay data for Snapshot®, trifluralin+isoxaben (T+I), T+I under pine nuggets, and T+I under hardwood mulch. Herbicide residue analyses and bioassay data from the herbicide-treated mulch treatments did not follow the exponential decay and sigmoid growth reduction functions, respectively, so those data were analyzed separately using a t-test. Overall, the chemical analysis data (exponential growth curves) and bioassay data (sigmoid growth curves) were in good agreement. Precipitation and temperature data for both years of the field study are presented in Figure 4.6.

Trifluralin dissipation. The recovery of trifluralin at zero days after treatment was 24-

38% compared to the amount of herbicide applied. Low recovery of trifluralin was most likely due to its high volatility (Duseja and Holmes 1978; Grover et al. 1988;

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Jacques and Harvey 1979; Spencer and Cliath 1974; Zimhdahl and Gwynn, 1977).

Dinitroaniline herbicides have characteristically high vapor pressures, and trifluralin has among the highest in the dinitroaniline family of herbicides (Weber 1990). Some of the applied herbicide might have also been lost in the runoff during the overhead sprinkler irrigation given immediately after treatment (Briggs et al. 1998; Horowitz and Elmore

1991; Riley 2003; Schreiber et al. 1993; Wilson et al. 1996). Initial herbicide losses were higher and faster than subsequent losses (Grover et al. 1988; Jury et al. 1980; La

Fleur et al. 1978; Spencer and Claith 1974). White et al. (1977) reported that 13% of the total volatilization losses of trifluralin occurred during application and soil incorporation. In our case, this number could be larger as trifluralin was not incorporated into the soil.

Pairwise comparisons of regression equations for trifluralin loss indicated that the only treatments that were not significantly different were Snapshot® and T+I under

PN in 2007 and Snapshot® and T+I in 2008 (Figure 4.1). The descending order of trifluralin dissipation rate among treatments based on the coefficient estimates (decay rates) in 2007 were T+I, Snapshot®, T+I under PN, and T+I under HW (Table 4.1). In

2008, the descending order of treatments based on the coefficient estimates were

Snapshot®, T+I, T+I under PN, and T+I under HW. Thus the trifluralin dissipation rate in the T+I under HW treatment was significantly lower than the other treatments across both years. In 2007, trifluralin dissipation rate was highest in the T+I treatment, and in

2008 dissipation rate was highest in the T+I and Snapshot® treatments. Trifluralin dissipation under pine nuggets was significantly faster than under hardwood mulch in

112 both years, but slower than the T+I treatment. Higher rainfall during 2008 compared to

2007 (Figure 4.6B) may have caused faster degradation of Snaphot® in 2008, and overall dissipation rate constants were generally higher for 2008 than 2007 (Table 4.1).

Increased herbicide degradation with higher soil moisture was reported in earlier studies

(Choi et al. 1988; Dinelli et al. 2000; Johnson et al. 1995; and Tao and Yang, 2011).

This is probably due to the readily available herbicide which is released from continuous granule wetting, as well as from soil particles (Keese et al. 1994). This readily available herbicide is easily subjected to various dissipation pathways such as volatilization (Grover et al. 1988), chemical, and microbial degradation.

Concentrations of trifluralin detected in the herbicide-treated mulch treatments were much lower than in the other treatments. The range of trifluralin concentration detected in 2007 with T+I treated pine nuggets was 0.6 to 1.74 µg/10g of soil, and with

T+I treated hardwood the range was 1.71 to 2.13 µg/10g of soil (Figure 4.1A inset). The concentration ranges detected in 2008 were 1.03 to 3.62 µg/10g soil with T+I treated

PN, and 1.66 to 2.47 µg/10g soil with T+I treated HW (Figure 4.1B inset). An overall comparison of herbicide treated mulches using a t- test showed that T+I treated HW released significantly higher amounts of trifluralin compared to T+I treated PN in 2007.

However, there was no difference in 2008. It is unclear if the lower concentrations of trifluralin detected in soil from the herbicide-treated mulch treatments was due to greater overall herbicide degradation or prolonged adsorption of trifluralin to the mulch material.

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Bioassay results using oats to detect trifluralin residues generally reflected the herbicide concentration levels obtained from chemical analyses. Root growth of oats, regardless of treatment, was less than 10% of the untreated control from the soil collected at 0 days after treatment, and less than 30 percent from the soil collected at 30

DAT (Figure 4.2). As most of the herbicide dissipated by 60 DAT, lower root inhibitions were observed in oats as well as in radish (Figure 4.5). Parameter estimates showed that the growth rate of oat roots (parameter b values are inversely proportional to growth rate) was lower with T+I under HW compared to rest of the treatments (Table

4.1). This could be due to the reduced dissipation rate of trifluralin under hardwood mulch. Comparison of sigmoid growth equations using the F-test showed that equations of all the four treatments were different from each other except Snapshot® and T+I under PN. The predicted maximum root growth was highest with T+I applied alone, which agrees with the faster dissipation rate of trifluralin observed under this treatment (Figure 4.1). The bioassay response of the herbicide-treated mulches for the pooled data also agreed with the 2007 dissipation data and showed greater oat root growth inhibition in the T+I-treated HW compared to the T+I-treated PN (Figure 4.2 inset).

Isoxaben. In contrast to trifluralin recovery, 66 to 125% isoxaben was recovered on zero days after treatment. Unlike trifluralin, isoxaben is low in volatility (Chandran and

Derr, 1999) and its primary breakdown mechanism is via microbial degradation (Corio-

Costet et al. 1991, Rouchaud et al. 1993, 1997). Lower recovery of isoxaben at zero

114 days after treatment in 2008 is (Figure 4.3) likely due to the precipitation that followed treatment application and before soil samples were taken.

The regression equations for isoxaben exponential decay were significantly different from one another except for T+I and T+I under PN, T+I and T+I under HW in

2007; and Snapshot® and T+I under HW in 2008. The descending order of rate constants (decay rate or parameter b) of isoxaben for 2007 was T+I under HW, T+I,

Snapshot, and T+I under PN; and for 2008, the descending order was T+I, T+I under

PN, Snapshot, and T+I under HW. Overall, the dissipation rates of herbicides were higher in 2008 compared to 2007. The higher dissipation rates in 2008 may be attributed to weather factors. The amount of rainfall over the first ten weeks of the experimental period was much higher in 2008 compared to 2007 (Figure 4.6B).

Precipitation received during 1-30, 31-60, 61-90, and 91-120 days after treatment was

31.24, 25.90, 76.2, and 120.40 mm respectively in 2007; and 125.22, 168.91, 22.60, and

17.01 mm respectively in 2008. Average temperature of first 30 days in 2007 was

20.77 0C, and in 2008 the average temperature for the first 30 days was 15.22 0C

(Figure 4.6A). Higher soil moisture and temperature conditions are favorable for isoxaben degradation (Rouchaud et al., 1993). Walker (1987) reported that isoxaben degradation in soil was positively correlated with soil moisture content. The reason for slower isoxaben dissipation in the T+I under HW in 2008 compared to 2007 might be attributed to the saturated soil conditions below the mulch in 2008 that could reduce the microbial activity. Despite the lower precipitation in 2007, moisture was conserved

115 under hardwood mulch which created ideal conditions for microbial growth and as a result, higher isoxaben degradation.

Similar to results with trifluralin analyses, isoxaben concentrations detected in soil from the herbicide-treated PN and HW mulch treatments were much lower than the other treatments and did not follow first-order decay kinetics (Figure 4.3A and4. 3B insets). The range of isoxaben concentrations with T+I treated pine nuggets in 2007 was 0.3 to 1.12 µg/10g soil; and in 2008, 0.74 to 1.88 µg/10g soil. With T+I treated

HW, the ranges were 0.3 to 1.64 µg/10g soil in 2007 and 0.54 to 1.16 µg/10g soil in

2008. Comparison of the two herbicide-treated mulches on the basis of a t-test showed that slightly more isoxaben was released into the soil by PN mulch in 2008 experiment but not in 2007; however, bioassay results showed no significant difference in isoxaben bioavailability between the treated mulch treatments (Figure 4.4A and 4.4B insets).

The bioassay data for the remaining isoxaben treatments generally agreed with the chemical analysis data. All four treatments resulted in less than three percent radish root growth at zero days after treatment in both years (Figure 4.4). Herbicide concentrations were higher at 30 DAT in 2007 compared to 2008 as reflected by reduced growth of radish roots. Radish root growth expressed as a percentage of the untreated control for Snapshot®, T+I, T+I under PN, T+I under HW at 30 DAT in 2007 was 1.30, 9.7, 2, 3.8%, respectively, whereas in 2008 root growth was 11.80, 57, 19.55, and 5% of the untreated control, respectively. Lower root growth from Snapshot® in

2007 and higher growth from T+I in 2008 can be clearly seen in the figures (Figs. 4.4A and 4.4B) as well as from the coefficient estimates (Table 4.1). By 60 DAT, higher

116 radish root growths (80% or higher) were observed (Figures 4.4 and 4. 5) across all the treatments in both years except for Snapshot® in 2007 due to the lower total concentration and bioavailability of isoxaben in the soil. It was not clear why the percent root growth of radish in Snapshot® at 60 DAT in 2007 was less than 2% despite the concentration of isoxaben was lower at 60 DAT (Figure 4.3A). However, there is a possibility that the slower rate of dissipation for Snapshot® in 2007 was due to the combined effects of the granular nature of the herbicide formulation and lower precipitation in 2007 compared to 2008. Lower percent root growths with Snapshot® in

2007 were also observed at 90 and 120 DAT.

In contrast to our results, Chandran and Derr (1998) obtained 59, 69, and 75 percent yellow rocket control three months after applying 0.5, 0.84, and 1.12 kg/ha of isoxaben, respectively. They conducted the experiment in plastic boxes filled with silt loam soil that contained 3% organic matter, so the difference with our results could be due to different soil physical and chemical properties, weather factors, and species used for bioassay. Their evaluations were based on the weed count where as our evaluations were based on the percent root growth. Grant et al. (1990) and Chandran and Derr

(1998) reported differential species sensitivity with isoxaben. Isoxaben applied at 0.52 to 0.84 kg/ha controlled 80 to 100% of common chickweed (Stellaria media (L.) Vill.), lawn burweed (Soliva pterosperma (Juss.) Less.), smallflower buttercup (Ranunculus arbortivus L.), large hop clover (Trifolium campestre Schreb.), and henbit (Lamium amplexicaule L.) whereas lower weed control (47-80%) was achieved with parsley-piert

(Alchemilla arvensis (L.) Scop.) and Carolina geranium (Geranium carolinianum L.)

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(Grant et al.1990). Chandran and Derr (1998) recorded acceptable yellow rocket

(Barbarea vulgaris R.) control (≥75) six months after fall applied isoxaben at 1.12 kg/ha. However, acceptable buckhorn plantain (Plantago lanceolata L.) control was achieved only up to three months after application. The difference could also be due to the amount of organic matter content, since our soil contained higher organic matter

(4.8%) than that reported by Chandran and Derr (1998).

Soil persistence of isoxaben, trifluralin or other herbicides in general depends on various soil and weather conditions. This was clearly indicated by the various half-life values reported in the previous research. Reported half-life values for trifluralin range from 19 to 132 days (Weber 1990), 116 to 201 days when trifluralin incorporated into soil (EPA 1996), and 29 to149 days without soil incorporation (EPA 1996). Reported isoxaben half-life values range from 88 to 201 days (Rouchaud et al. 1993 a, b), 76 to

122 days (Huggenberger and Ryan 1985), 152 to183 days (Huggenberger et al. 1982),

76 to158 days (Walker 1987), and 82 to 186 days (Chandran and Derr 1999).

The results of this experiment revealed that physicochemical properties and dissipation pathways are important factors to consider when choosing an herbicide to combine with organic mulches, especially when herbicides are applied to soil below the mulch layer. Mulch type also plays important role as different mulches appear to interact differently with different herbicides. This could be due to the different chemical nature of herbicides as well as physical binding nature of the mulch that contains lignin and other chemical surfaces capable of adsorbing herbicides. Trifluralin dissipation was slower under hardwood mulch, whereas isoxaben dissipated faster. This may be due to

118 the predominant dissipation pathways for trifluralin and isoxaben, and soil factors such as soil moisture, temperature, and texture can influence herbicides differently, depending on the herbicide’s major dissipation pathways.

Overall results of this experiment indicated that herbicides applied in the liquid formulation dissipated faster when applied alone compared to when they they were applied in the liquid formulation under mulch. It is also worth noting that herbicide- treated mulch treatments had lower herbicide concentrations in the soil. It is unclear if the amount of bioavailable herbicide supplied by the herbicide-treated mulch played the principal role in weed control, or if weed control was provided mainly by physical suppression from the 6-cm-thick mulch layer. It is possible that herbicides and mulches may contribute equally to weed suppression when herbicides are combined with mulches, or that herbicides play an increasing important role as mulch layer thickness is reduced (< 6 cm).

Future research can be extended to other available mulches and herbicides to find the most effective combinations for weed control. This is because different physical and chemical nature of herbicides and mulches. Much research has been conducted on the adsorption and desorption properties of herbicides in soil but limited research was done on the adsorption and desorption properties of mulch materials. In- depth studies are needed to understand the chemical nature of adsorption and desorption properties of various mulches in order to optimize the desirable properties of herbicide- treated mulches. It is also worth investigating the effect of environmental factors on the release of herbicide into the soil from herbicide-treated mulches.

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25 3.0 A T+I treated PN 2.5 T+I treated HW

20 2.0

1.5

1.0 15

0.5

0.0 10 0 30 60 90 120

5 Snapshot T+I 0 T+I under PN

Trifluralin concentration (µg/10g of soil) of (µg/10g concentration Trifluralin T+I under HW

0 30 60 90 120 Days after treatment

25 4.5 T+I treated PN T+I treated HW B 4.0

3.5 20 3.0

2.5

15 2.0 1.5

1.0

10 0.5 0 30 60 90 120

Snapshot T+I

Trifluralin concentration (µg/10gTrifluralin of soil) 0 T+I under PN T+I under HW

0 30 60 90 120 Days after treatment

Figure 4.1: Effect of herbicide application method on trifluralin dissipation in soil over time. An exponential decay function was fitted to trifluralin residue data for (A) 2007 and (B) 2008 experiments. Abbr. PN=pine nuggets, HW=hardwood, T+I=trifluralin+isoxaben. The general equation fitted to the data was , and regression parameter values for each treatment are shown in Table 1.

120

140

120 Snapshot T+I T+I under PN 100 T+I under HW

60 140

120 Time vs T+I treated PN 40 Time vs T+I treated HW

100 20 80

Oat root length (% of control) of (% length root Oat

0 60

40 0 30 60 90 120

0 30 60 90 120

Days after treatment

Figure 4.2: Oat root bioassay of trifluralin+isoxaben residues in soil as a sigmoidal function of days after treatment. Results shown are the combined data from 2007 and 2008 experiments. Abbr. PN=pine nuggets, HW=hardwood, T+I=trifluralin+isoxaben. The general equation fitted to the data was , and regression parameter values for each treatment are ( ) shown in Table 1. Inset graph shows the root length (% of control) with herbicide treated mulches (both years pooled).

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18 A 2.5 T+I treated PN T+I treated HW 16 2.0

14 1.5

12 1.0

10 0.5

0.0 8 0 30 60 90 120

4 Snapshot 2 T+I T+I under PN

Isoxaben concentration (µg/10g of soil) of (µg/10g concentration Isoxaben 0 T+I under HW

0 30 60 90 120 Days after treatment

18 3.0 B Snapshot 2.5 T+I treated PN 16 T+I T+I treated HW

T+I under PN 2.0 14 T+I under HW 1.5

12 1.0

0.5 10

0.0 0 30 60 90 120 8

Isoxaben concentration (µg/10g of soil) of (µg/10g concentration Isoxaben 0

0 30 60 90 120 Days after treatment

Figure 4.3: Effect of herbicide application method on isoxaben dissipation in soil over time. An exponential decay function was fitted isoxaben residue data for (A) 2007 and (B) 2008 experiments. Abbr. PN=pine nuggets, HW=hardwood, T+I=trifluralin+isoxaben. The general equation fitted to the data was , and regression parameter values for each treatment are shown in Table 1. 122

A 2007 140

Snapshot 120 T+I T+I under PN T+I under HW 100

60 110 T+I treated PN 100 T+I treated HW

40 90

20 70

Radish root length (% of control) of (% length root Radish 0 50

40 0 20 40 60 80 100 120 140

0 30 60 90 120 Days after treatment

B 2008 140 Snapshot 120 T+I T+I under PN T+I under HW 100

140 60 120 T+I teated PN T+I treated HW 40 100

20 60

Radish root length (% of control) of (% length root Radish 40 0 20 0 30 60 90 120

0 30 60 90 120 Days after treatment

Figure 4.4: Radish root bioassay of isoxaben+trifluralin residues in soil as a sigmoid function of days after treatment. The general equation fitted to the data was , and ( ) regression parameter values for each treatment are shown in Table 1. Inset graph shows the root length (% of control) with herbicide treated mulches. Abbr. PN=pine nuggets, HW=hardwood, T+I=trifluralin+isoxaben.

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A B

Figure 4.5: Oat and Radish bioassay to determine trifluralin and isoxaben residues, respectively. The first five seeds in each petri dish (left to right) are oats and next five seeds are radish. A. Root growth from soil collected 60 days after treatment from trifluralin+isoxaben under pine nuggets; B. Root growth from soil collected at 0 days after treatment from trifluralin+isoxaben under pine nuggets.

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29 A 27 25 23

21 C) 0 19 17 15 13

Temperature ( Temperature Mean temperature-2007 11 Mean temperature-2008 9 7 5 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Week

120 B Precipitation in 2007 Precipitation in 2008 100

40 Precipitation (mm) Precipitation

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 Week

Figure 4.6: Weekly average temperatures (A) and precipitation (B) during the experiment period for 2007 and 2008. Week 1 starts from May first week. Spring 2007 experiment started on 3rd week of May, and spring 2008 experiment started in first week of May.

125

Exponential Decay Curvez Sigmoid growth curvey 2 2 Treatment Parameter a Parameter b R P-value Parameter a Parameter b Parameter X0 R P-value Trifluralin – Spring 2007 Bioassay of Oats* Snapshot 11.60 (0.626) 0.020 (0.0021) 0.975 0.0011 89.758 (2.72) 15.70 (1.887) 41.59 (2.313) 0.993 0.0035 T+I 12.08 (0.337) 0.028 (0.0016) 0.995 0.0001 117.36 (2.78) 15.68 (1.248) 61.63 (1.487) 0.998 0.0010 T+I under PN 11.87 (0.309) 0.019 (0.0010) 0.994 0.0001 99.05 (6.59) 15.45 (3.785) 50.86 (4.660) 0.975 0.0127 T+I under HW 13.73 (0.810) 0.018 (0.0021) 0.968 0.0016 89.23 (5.38) 20.66 (3.522) 52.24 (4.517) 0.986 0.0070 Trifluralin – Spring 08 Snapshot 14.65 (0.616) 0.028 (0.0025) 0.987 0.0004 - - - - - T+I 13.37 (0.434) 0.025 (0.0016) 0.992 0.0002 - - - - - T+I under PN 18.28 (0.787) 0.023 (0.0019) 0.986 0.0005 - - - - - T+I under HW 12.83 (0.609) 0.019 (0.0018) 0.980 0.0008 - - - - - Isoxaben –Spring 2007 Bioassay of Radish – Spring 2007 Snapshot 13.05 (0.088) 0.025 (0.0004) 0.997 <0.0001 88.85 (0.92) 4.68 (0.604) 80.36 (1.300) 0.999 0.0002

126 T+I 12.25 (0.101) 0.032 (0.0006) 0.996 <0.0001 108.27 (4.89) 6.94 (1.667) 45.97 (3.708) 0.984 0.0077

T+I under PN 13.15 (0.147) 0.025 (0.0006) 0.998 <0.0001 111.60 (0.52) 5.93 (0.430) 53.67 (0.485) 0.999 <0.0001

T+I under HW 10.70 (0.303) 0.033 (0.0021) 0.995 <0.0001 101.72 (0.54) 5.37 (0.584) 47.40 (0.224) 0.999 0.0001 Isoxaben - Spring 2008 Bioassay of Radish – Spring 2008 Snapshot 10.20 (0.280) 0.033 (0.0020) 0.995 <0.0001 90.45 (2.45) 6.63 (1.156) 42.55 (2.458) 0.994 0.0029 T+I 8.63 (0.096) 0.070 (0.0030) 0.999 <0.0001 107.50 (1.02) 1.35 (4.0e+6) 29.82 (5.3e+5) 0.998 0.0007 T+I under PN 10.71 (0.495) 0.048 (0.0486) 0.992 0.0002 110.84 (2.51) 12.70 (1.200) 49.21 (1.558) 0.996 0.0017 T+I under HW 7.10 (0.495) 0.031 (0.0046) 0.966 0.0017 95.51 (3.89) 5.10 (1.643) 44.73 (4.905) 0.988 0.0060 Table 4.1 Parameter values from exponential decay and sigmoid growth curves. Values in the parenthesis are standard errors. Abbr. T+I = trifluralin+Isoxaben, PN=pine nuggets, HW=hardwood. *Combined data for 2007 and 2008. z Equation for exponential decay, ; Where, is the concentration of herbicide at time is the initial herbicide concentration, is the first order rate constant. y Equation for sigmoid growth function, ; where, is the growth at time X, is the maximum growth (upper asymptote), X0 is the inflection ( )

point, X is the time, and is the rate constant.

126

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Chapter 5: Summary and Conclusions

Integration of different weed control methods is essential to address the financial as well as environmental concerns being faced by the nursery and landscape industry.

The use of herbicide and mulch combinations has been suggested as way to extend the period of weed control in nursery and landscape crops. Field experiments were conducted to evaluate granular and liquid formulations of herbicides, alone and in combination with different mulch layer thicknesses, and herbicide placement methods on weed control.

Two separate field experiments were conducted. The first experiment was conducted in

Fall 2006 and it was repeated in Fall 2007 (Chapter 3). The second experiment started in

Spring 2007 and was repeated in Spring 2008 (Chapter 2). Granular (Snapshot®) and liquid formulations of trifluralin+isoxaben were applied alone or in combination with pine nuggets and hardwood mulch applied at three depths, 3, 6, and 12 cm. Granular herbicide was applied alone and on top of the mulch; and two granular pretreated commercial mulches were also included for comparison. Liquid herbicide was applied alone, on top of the mulch, to the soil surface under the mulch, and as a pretreatment mixed in with mulch materials before the mulches were applied. Trifluralin and isoxaben in both formulations were applied at the rate of 4.48 kg ai/ha + 1.12 kg ai/ha, respectively. Mulch alone treatments and an untreated control (no mulch no herbicide) were also included. Visual weed control ratings and weed fresh weights were recorded at 131

30, 60, 90, and 120 days after treatment (DAT) for the spring experiment and at 180 and

210 DAT for the fall experiment. Visual ratings were based on a scale of 0 (no control) to

10 (complete control), with 7 and above being commercially acceptable. All the treatments involving 6 and 12 cm mulch with or without herbicides provided efficacy ratings of above 7 in both experiments. Certain combinations of 3 cm mulch and herbicides, such as Snapshot over 3-cm pine nuggets, liquid trifluralin+isoxaben under 3- cm pine nuggets, and liquid trifluralin+isoxaben under 3-cm hardwood consistently provided at least acceptable efficacy ratings at all the evaluation dates in both studies.

This could be due to the longer soil persistence of herbicides in those combinations.

Weed control with greater mulch depth could be largely due to the physical inhibition of weed growth by the mulch, whereas at lower depths of herbicide-treated mulches, weed control was due to the additive effects of mulch and herbicide. Although application of mulches at the 12-cm depth provided the greatest weed control, it is an expensive treatment and can be detrimental to landscape plant health. Results showed that even though the recommended depth of mulch is 6 cm for acceptable weed control, this can be further reduced to a 3-cm depth with no loss in weed control when herbicides and mulches are combined.

The third study (Chapter 4) was conducted to determine the soil persistence of herbicides when herbicides and mulches are combined. Six treatments from the spring field experiment were selected for analyses and included the following: Snapshot alone, liquid trifluralin+isoxaben alone, liquid trifluralin+isoxaben under 6-cm of pine nuggets, liquid trifluralin+isoxaben applied under 6-cm of hardwood mulch, a 6-cm layer of liquid

132 trifluralin+isoxaben-treated pine nuggets, and a 6-cm layer of liquid trifluralin+isoxaben- treated hardwood. Soil samples were collected in each year (2007 and 2008) at 0, 30, 60,

90, and 120 DAT and stored at -20 0C until chemical analysis was done. Trifluralin and isoxaben were extracted from the soil using methanol. Herbicide extracts were analyzed using HPLC-MS equipped with an auto sampler and photodiode array detector (PDA).

Bioassays were also conducted using oat and radish seedlings to determine the bioavailability of trifluralin and isoxaben residues in the soil samples that were analyzed by HPLC-MS. Exponential decay and sigmoid growth regression models were fit to the herbicide residue and bioassay data, respectively, except for the herbicide-treated mulch treatments. Regression models did not fit the data from the herbicide treated mulches and there was no discernable trend in herbicide degradation, so those data were analyzed and plotted separately. Results indicated that dissipation of isoxaben and trifluralin largely depended on the physico-chemical properties of herbicides, as well as on soil and weather factors. Bioassay data (percent root growth) was generally in agreement with chemical analysis data, and suggested that herbicide-mulch treatments providing the longest period of weed control in the field did so by delaying the rate of herbicide dissipation relative to the other treatments.

Further research needs to be conducted on the combined use of herbicides and mulches, specifically herbicide-treated mulches. Since the physico-chemical properties of every herbicide and mulch are unique, the adsorption-desorption relationship between herbicide and mulch also vary and influence the dynamics of herbicide release into the soil, and thus its efficacy. Future research should focus on the physical and chemical

133 properties of mulches in relation to herbicide sorption in order to understand how physical and environmental factors affect herbicide behavior on mulch surfaces, and underlying mechanisms that regulate herbicide release and downward movement into soil. Finally, agriculture and many related industries produce large amounts of byproducts, some of which could possibly be used as mulches for weed control in nursery and landscape settings. As the pressure for non-chemical alternatives in agriculture is increasing, it is worthy to investigate the allelopathic/herbicidal properties of various mulches or other plant by-products. Discovery of new plant-derived products having natural herbicidal properties would be a potentially valuable contribution to weed management.

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