A3845 management in organic vegetable production

A PRACTICAL REVIEW OF APPLIED RESEARCH

JED COLQUHOUN

University of Wisconsin-Extension I Cooperative Extension Contents

Foreword ...... 1

Problem avoidance: Preventive strategies . . . 2

Managing the weed seedbank . . . . . 4

Weed suppression with ...... 6

Cover and ...... 8

Reduced tillage and dark cultivation . . . . . 10

Thermal weed management ...... 12

Alternative . . 14

Crop competition for weed control ...... 16

In-season cultivation . . 18

Integrated weed management ...... 21 Foreword he intention of this manual is to Few things are harder to put provide an objective analysis of Torganic weed management up with than the annoyance strategies. One that is based on peer- reviewed and published applied of a good example. research. The strategies used in organic weed control have been Mark Twain around, in most cases, for centuries, and therefore current peer-reviewed research is often not discussed when weighing management options. With this in mind, the research is first objec - tively summarized for those who are interested in“just the facts,” the back - ground information is presented in more detail, and finally the research that serves as the basis for the sum - mary is reviewed.

The strategies approved for use in organic production by the United States Department of (USDA) National Organic Program, local organic grower organizations, and end-users change periodically. Consult the appropriate organizations for approved strategies and products prior to any use, as well as labels and registration status for products used as herbicides.

How to use this manual: I Interested in just the facts? Focus on the key points and background information for practical guidance and an overview of weed manage - ment options. I Interested in the details and eval - uation of options? Consider also reading the research sections for an overview of recent peer-reviewed studies conducted on vegetables.

1 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I It’s often easier to prevent the introduction of a new weed species than to eradi - Problem avoidance: cate it once established. Preventive strategies

I Consider potential sources y far, the best management strat - should be cleaned of and vegeta - of new —as contami - egy for any weed is to prevent tion before being moved between nants in , , Bintroduction and dispersal. There fields. are several routes for weed introduc - Historically, many weed species have or mulches; on equipment; tion, and the majority involve inadver - been introduced as a seed contami - and in open tent grower actions. Potential sources of weed seed on the include nant in crop seed. This source of water. , manure, mulches such as weeds is particularly prevalent when straw, equipment, open irrigation “saved” seed is replanted in the follow - I Continue to manage weeds water, and contaminants in crop seed ing year or on another farm without after and in field and transplant containers. cleaning or analysis by a seed certifi - cation agency. margins to minimize weed Occasionally, new invasive weeds are intentionally planted, as is the case seed production through Prevention is also important in mini - with kudzu, which was first marketed mizing the spread of weed species mowing or planting of for forage and in the already found on the farm. competitive, non-invasive southeastern United States, and vel - Management practices that minimize vetleaf, which was initially cultivated species in boundary strips. weed seed production are important as a source of fiber for cloth and rope. in fields after crop harvest and in sur - Consider all potential entry routes for rounding field margins, such as: new weed species and methods to I Till or mow after crop harvest but reduce weed seed and vegetative before weeds go to seed. propagule introduction and viability. I Identify weeds prior to using tillage Weed seed in compost, for example, for weed management—perennial can often be destroyed by prolonged species can be spread by cutting heat and adequate moisture in the below-ground vegetative tissue, pile. Some weed such as curly such as and roots. dock can pass through the digestive tracts of cows and other farm animals. I In field margins, consider planting and managing a boundary strip Consider the source of all compost, that is kept free of weed seed pro - manure, and straw, and question duction. The planted species could providers about weed contamination include a mix of those that are com - prior to purchasing. For example, was petitive with weeds and support a hay field weedy, and if so, when was beneficial wildlife or predators of the field harvested relative to weed other agricultural pests. Field mar - flowering? Were perennial weeds gin plantings might also be har - present that could be dispersed vested for hay. through root cuttings? , I Prevent weed seed production harvesters, and other field equipment along irrigation canals, reservoirs, are sources of vegetative root tissue and other open water sources. and seed for many weed species and

2 PROBLEM AVOIDANCE: PREVENTIVE STRATEGIES

Research Open irrigation water Planted field margin boundary as a source of weed seed strips for weed control Manure composting to Source: Catalan, B., J. Aibar, and C. Source: West, T. M., E. J. P. Marshall, and reduce weed seed viability Zaragoza. 1997. Weed seed dispersal G. M. Arnold. 1997. Can sown field Source: Tompkins, D. K., D. Chaw, and through irrigation channels. Proc. boundary strips reduce the ingress of A. T. Abiola. 1998. Effect of windrow 1997 Congress Spanish Weed Science aggressive field margin weeds? Proc. composting on weed seed germina - Society, pp. 187 –193. Brighton Crop Protection Conference, tion and viability. Compost Science and pp. 985-990. Utilization 6:30–34. Summary: Irrigation channels were sampled at three depths and weed Summary: Boundary strips planted Summary: Manure was composted in seed was tested in con - with grass and wildflower seed mix - a windrow with a core temperature of trolled conditions. Seed from 23 fami - tures were compared to unplanted 130°–150°F. After 2 weeks of compost - lies and 63 species germinated, strips surrounding agricultural fields. ing, the high temperatures killed primarily from the Asteraceae and The planted strips limited movement 100% of the seed of green foxtail, Poaceae families. Common species of weeds including Canada thistle, catchweed bedstraw, kochia, wild included Conyza spp., annual sowthis - catchweed bedstraw, poverty brome, buckwheat, field pennycress, and tle, smooth pigweed, Bromus spp., and and quackgrass into the fields. pineappleweed, and all but 3.5% of annual bluegrass. the seed of redroot pigweed. No seeds survived after 4 weeks of composting. This study demonstrated the effective - ness of even short periods of com - posting at eliminating the distribution Table 1. Germination of selected of viable weed seeds. weed species after 17 years of burial in Nebraska.

Weed seed longevity Species Germination, % Source: Burnside, O. C., R. G. Wilson, Annual grasses S. Weisberg, and K. G. Hubbard. 1996. Barnyardgrass 0 Seed longevity of 41 weed species Green foxtail 0 buried 17 years in eastern and west - Longspine sandbur 0 ern Nebraska. Weed Science 44:74–86. Yellow foxtail 0

Summary: This study examined the Annual broadleaves long-term survival of weed seeds and Common lambsquarters 7 reveals the importance of preventing Hairy nightshade 65 weeds from becoming established. Jimsonweed 90 Weed seeds were buried and then Redroot pigweed 1 dug up and tested annually to see if Velvetleaf 35 they would germinate. Following Biennial/perennial weeds 17 years of deep burial without tillage, Canada thistle 7 seed of 4% of annual grasses, 11% of Common mullein 95 annual broadleaf weeds, 30% of bien - Curly dock 61 nial weeds, and 8% of perennial weeds Dandelion 0 MAKING THE CASE germinated (table 1). Common Source: Adapted from Burnside et al. mullein proved to have superior 1996. FOR PREVENTION : longevity, with 95% of the seed germi - After 17 years underground, nating at the end of the study. 95% of the seeds of common 3 mullein still germinated. KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I The soil contains a seed - bank—a vast supply of weed seeds—of potential Managing the weed seedbank future weed problems. he soil contains millions of weed additional tillage into a seedbed that seeds and serves as a seedbank or has been depleted of germinable I Two techniques for Treservoir for future weed infesta - weeds. This technique is particularly depleting weed seeds: tions. Many of these seeds are dor - useful with small-seeded or slow- mant, and will not germinate until growing crops that are not very com - Stale seedbed technique, triggered by environmental or physio - petitive with weeds, such as where soil is tilled 20 to logical signals. Although seeds can and carrots. remain dormant for several years, The second technique, , 30 days before crop - weed pressure during the cropping solarization uses sheets of clear plastic to ing. Weeds that germinate season can be reduced through prac - cover the soil and trap solar energy tices that deplete those seeds that will during the delay period are near the soil surface. Intense heat and readily germinate before crop planting . killed shortly before seed - sunlight are required for effective There are two techniques for deplet - solarization, thus limiting the use of ing using flaming, mowing, ing weed seeds. In the first, known as this strategy in more temperate pro - or shallow tillage. the stale seedbed technique, the duction areas. Moist soil conducts seedbed is tilled and prepared for heat better than dry soil and promotes Solarization is the practice crop planting, then allowed to rest microbial activity. After about 4 weeks of covering soil with plastic while non-dormant weeds germinate of solarization, the plastic mulch is and emerge. Tillage stimulates germi - removed and the crop is planted or mulch for several weeks in nation of many weed species. Crop the ground is left fallow until the next the summer to kill . planting is delayed while the soil is growing season. Solarization is effec - After the plastic is partially exhausted of potential weeds. tive at killing annual grass and Shortly before crop planting, emerged broadleaf weeds near the soil surface, removed, crops are planted weeds are eliminated by using a flame but is less effective with large-seeded or the ground is left fallow. torch, by mowing near ground level, or and perennial weed species that are by performing a very shallow cultiva - capable of emerging from lower tion. It is essential to keep tillage to a depths. The length of solarization minimum following treatment to pre - directly determines the degree of vent stimulation of further weed ger - weed seed depletion. mination. The crop is then transplanted or direct-seeded without

4 MANAGING THE WEED SEEDBANK

Research had higher cucumber yields than the Benghal dayflower ( Commelina beng - seedbed prepared on the day of plant - halensis ), was increased by solariza - Stale seedbed: ing. Optimum seedbed preparation tion. When soil was solarized for 3 to 9 Technique comparison was 20 to 30 days before planting. The weeks, carrot and weed control Source: Caldwell, B., and C. L. Mohler. authors concluded that a properly were greater than in the non-solarized 2001. Stale seedbed practices for veg - managed seedbed improved in-season control plot. etable production. HortScience weed control while increasing crop 36:703–705. yield. Source: Silveira, H. L., R. Gomes, L. Summary: The authors examined the Soil solarization Aguiar, M. L. Caixinhas, J. Bica, and M. effectiveness of various stale seedbed Source: Marenco, R. A., and D. C. Bica. 1990. Soil solarization under poly - techniques. Flaming reduced the Lustosa. 2000. Soil solarization for ethylene film: Cultivation of lettuce number of annual broadleaf weed weed control in carrot. Pesquisa and onions. Plasticulture 85:35–42. species, while yellow nutsedge control Agropecuaria Brasileira 35:2025–2032. was poor. A single flaming after a 24- Summary: Soil was solarized with to 28-day stale seedbed delay period Summary: Researchers compared the clear polyethylene film for 8 weeks was as effective as multiple flame effect of soil solarization using poly - prior to planting lettuce and . In treatments during the period ethylene film for 3, 6, and 9 weeks on lettuce, solarization eliminated all but between seedbed preparation and weed populations and carrot yield. 2 of the 14 weed species; in onion, crop planting. Using a flexible tine Soil temperature was about 50°F solarization reduced the number of weeder at the end of the delay period greater at a 2-inch depth in solarized weed species from 7 to 4. While solar - did not control established weeds, and . Weed biomass and density were ization reduced weed cover, further heavier cultivation using a spring- reduced following solarization for weed management was necessary tooth weeder stimulated further weed about half of the tested species. Seed during the crop season. germination. germination of one weed species,

Stale seedbed: Timing of seedbed preparation prior to crop planting Source: Lonsbary, S. K., J. O’Sullivan, and C. J. Swanton. 2003. Stale seedbed as a weed management alternative for machine-harvested cucumbers (Cucumis sativus ). Weed Technology 17:724–730.

Summary: Stale seedbeds were pre - pared 40, 30, 20, and 10 days before cucumber planting and were com - pared against a control seedbed pre - pared on the day of planting. Weeds were killed at the time of crop plant - ing using herbicides. Weed biomass at planting was greatest in the seedbed prepared 40 days earlier and cucum - Stale seedbed prepared 3 weeks before planting, with ber yield was reduced. Seedbeds pre - emerged weeds controlled with flaming (left) compared to a pared 10 to 30 days before planting conventional seedbed with weeds left uncontrolled (right).

5 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I Non-living mulches that suppress weeds include polyethylene (plastic), bark Weed suppression with mulches and wood chips, newspaper oth living and non-living mulches While weed suppression is often equal and cardboard, yard waste, can be useful for weed suppres - to or greater than that with plastic and straw or other crop Bsion. The basic objective of weed mulch, natural products can also serve management with mulches is to de- as sources of pathogens and as refuge residue. Plastic and natural prive emerging weeds of light. Mulch for pests such as rodents and insects. mulches both have advan - is most commonly used with trans - Additionally, natural mulches can alter tages and disadvantages. planted annual vegetables or with soil properties as they degrade in soil. perennial crops such as berries where For example, mulches can alter soil pH the crop is larger than seedling weeds. or temporarily reduce the supply of I Living mulches (intercrop - nitrogen to the point where it affects Researchers have tested a variety of ping) walk a fine line be - plant growth. Despite these draw - materials, both natural and synthetic, tween suppressing weeds backs, natural mulches can be very for their value as mulch ground covers. useful in suppressing weeds with min - and suppressing the crop. or is the most Polyethylene plastic imal environmental impact. common synthetic mulch. Weed man - agement with plastic is often excel - The strategy for using living mulches lent, with the exception of the hole in (also known as , inter - which the crop plant is transplanted. seeding, and strip-mulching) is to Dark plastic mulches raise soil temper - plant a cover crop that will out-com - atures early in the season, which helps pete weeds while not interfering with promote crop growth and earlier crop growth and yield. This fine line maturity. between weed suppression and crop suppression by living mulches has lim - The drawback to plastic is that the ited practical applications in many mulch needs to be removed at the cases. Living mulches have been very end of the crop season, and often can - successful in long-term perennial not be reused. Degradable plastic crops, such as tree and bramble fruit mulches have been developed but are production, where the crop has a dis - not widely used because the rate of tinct age and size advantage over the degradation is difficult to manage— mulch. For small crops, though, the liv - the mulch needs to last long enough ing mulch can become the crop’s for efficient weed control, yet degrade worst enemy as they jointly compete fast enough so that it doesn’t need to for water, light, and nutrients. Living be pulled and disposed of at the end mulch research has included several of the growing season. grasses, such as rye, and legumes, Natural mulch products, by contrast, including alfalfa and clover species. can be used to suppress weeds, while In addition to competing with the contributing organic matter to soil main crop, living mulches can also without the need for disposal at the serve as a refuge for pathogens, end of the crop season. Natural mulch insects, and rodents. Further research products used in vegetable produc - is necessary to refine the practical use tion include wood chips and pine nee - of living mulches for weed suppres - dles, paper and cardboard products, sion without also suppressing crop yard waste and leaf debris, and straw. 6 growth. WEED SUPPRESSION WITH MULCHES

Research Winter rye as a living mulch Weed control was greatest where in broccoli white clover or the living mulch mix Comparison of paper and Source: Brainard, D. C., and was grown. However, the living polyethylene mulches R. R. Bellinder. 2004. Weed suppression mulches reduced asparagus fern Source: Jenni, S., D. Brault, and in a broccoli-winter rye intercropping growth by 50% the first year. In the K. A. Stewart. 2004. Degradable mulch system. Weed Science 52:281–290. second year, asparagus fern growth as an alternative for weed control in reduction ranged from 25% (white lettuce produced on organic soils. Summary: This study demonstrates clover and mulch mixes) to 50% Acta Horticulturae 638:111–118. the fine line between suppressing (perennial ryegrass). weeds and suppressing the crop. Rye Summary: Lettuce yield was up to interseeded at the same time as broc - Legumes as living mulches 25% greater when grown with paper coli transplanting reduced weed in sweet corn or polyethylene mulch compared to growth, but also reduced crop yield Source: Miura, S., and Y. Watanabe. lettuce grown with no mulch. Yields compared to the weed-free treatment. 2002. Growth and yield of sweet corn were similar for both mulch types. In Interseedings 10 and 20 days after with legume living mulches. Japanese the cooler of two evaluation years, air transplanting did not lower broccoli Journal of Crop Science 71:36–42. and soil temperatures were highest yield, but they also failed to suppress under the polyethylene mulch and weeds. Summary: White clover, red clover, lowest in the non-mulched treatment. and alfalfa were planted in fall, and The authors note the advantage of Clover and perennial ryegrass sweet corn was planted without paper degradation at the end of the as living mulches in asparagus tillage into the intercrops the follow - growing season. Source: Paine, L., H. C. Harrison, and ing spring. While all three legumes A. C. Newenhouse. 1995. Establishment suppressed weeds, only white clover Comparison of organic, plastic, of asparagus with living mulch. succeeded at not limiting corn growth and paper mulches Journal of Production Agriculture or yield. Red clover and alfalfa were Source: Schonbeck, M. W. 1998. Weed 8:35–40. too competitive, affecting corn plant suppression and labor costs associ - size and reducing yield. ated with organic, plastic, and paper Summary: Perennial ryegrass, white mulches in small-scale vegetable pro - clover, and a mixture of the two duction. Journal of Sustainable species were evaluated as living Agriculture 13:13–33. mulches in asparagus production.

Summary: This study evaluated four types of mulches on tomatoes. Hay mulch applied to a depth of 4 inches Interseeding a cover crop with corn. several weeks after tomato planting suppressed annual weeds. Adding two layers of newsprint below the hay enhanced weed control. By contrast, composted yard waste (1.2 to 2 inches in depth) was less effective than hay in suppressing weeds. Paper mulches decomposed during the crop season, allowing weed growth to occur before harvest. Labor costs were higher for plastic mulch than for hay mulch due to the time spent removing the plastic at the end of the season. 7 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I Allelopathy refers to the effect of one plant on another through the release Cover crops and allelopathy of a chemical compound llelopathy is defined as the effect living mulch during the cropping sea - into the environment. of one plant on another through son to reduce weed interference; and Allelo pathic compounds are Athe release of a chemical com - (3) as an isolated compound from an pound into the environment. allelopathic plant, applied as a herbi - often considered “nature’s Allelopathic compounds, often con - herbicides,” and can be sidered plant-produced herbicides, Table 2. Selected common crops and produced by both crops can inhibit growth of nearby plants of weeds with reported allelopathic the same and/or other species. properties. and weeds. The observation of allelopathic plant Crops Weeds I suppression is not new. Theophrastus Allelopathic cover crops alfalfa Canada thistle observed that chickpea reduced have been used with mixed asparagus cocklebur nearby weed growth as early as 300 barley common lambs quarters success to suppress weed B.C., and Plinus Secundus (1 A.D.) bean field bindweed reported that grain was “scorched” by growth in the following beet foxtail sp. chickpea, barley, and bitter vetch. Still, crop season. Allelopathic broccoli jimsonweed there have been relatively few effec - cabbage kochia cover crops can also be tive demonstrations that have led to clover pigweed sp. reliable uses in agricultural pest man - used to suppress nema - corn quackgrass agement. todes and pathogens. cucumber ragweed sp. Many crop and weed species have oat smartweed sp. I Rye, oat, and mustard been observed to have allelopathic pea velvetleaf potato wild mustard species are commonly properties (table 2). Over 240 weed species have been reported to be rapeseed wild oat planted as cover crops for allelopathic to other nearby plants of yellow nutsedge allelopathic weed control. the same species (autotoxicity) or rye soybean This strategy can be an other crop and weed species. The use of allelopathy to favor the crop over sunflower important component of an weeds has been investigated in three tomato integrated weed manage - aspects: (1) as an allelopathic winter wheat cover crop that suppresses weeds ment system. before the cropping season; (2) as a

8 COVER CROPS AND ALLELOPATHY

cide. To date, the use of allelopathic cover crops such as rye and oat, has had the greatest success.

The effect of allelopathic crops on weed growth has been very difficult to quantify. Weed suppression by neigh - boring plants is a combination of allelopathy and physical interference. Physical interference includes compe - tition for light, water, nutrients, and other resource by the cover crop residue or living mulch. Allelopathic compounds are often complex and short-lived, and therefore are difficult Allelopathic mustard cover crop growing in potatoes. to isolate and identify. Research con - ducted in often doesn’t Oat allelopathy differs compounds leached (removed) from account for the effect of microorgan - among crop cultivars the residue. For rye, reductions in yellow isms, climate, and soil type, and thus Source: Grimmer, O. P., and J. B. foxtail emergence were attributed to often exaggerates the potential weed Masiunas. 2005. The weed control physical suppression alone. For barley, suppression compared to field condi - potential of oat cultivars. a combination of physical suppression tions. In field research, the effect of HortTechnology 15:140–144. and allelopathy interacted to reduce physical interference is difficult to sep - yellow foxtail emergence by 81%. arate from allelopathy. Other factors Summary: Most of the oat cultivars that cloud evaluation of the impact of tested (20 of 24) reduced common Allelopathic compounds allelopathic cover crops on weed lambsquarters germination, but the isolated from turnip-rape species include timing, growth stage, amount ranged from 10% to 86% Source: Petersen, J., R. Belz, F. Walker, soil type, and climatic conditions dur - among cultivars. Several cultivars also and K. Hurle. 2001. Weed suppression ing growth of the cover crop. reduced pea germination, indicating by release of isothiocyanates from that selectivity in suppression turnip-rape mulch. Journal Research between the crop and weeds may be 93:37–43. difficult. Rye residue for weed suppression Summary: The authors reported that Source: Putnam, A. R., and J. DeFrank. Physical suppression isolated allelopathic compounds from 1983. Use of phytotoxic plant residues versus allelopathy turnip-rape plants suppressed several for selective weed control. Crop Source: Creamer, N. G., M. A. Bennett, weed species, including smooth pig - Protection 2:173–181. B. R. Stinner, J. Cardina, and E. E. Regnier . weed, spiny sowthistle, and barnyard - 1996. Mechanisms of weed suppres - grass. Six different allelopathic Summary: Rye residue reduced emer - sion in cover crop-based production compounds were isolated from all gence of green foxtail by 80%, redroot systems. HortScience 31:410–413. parts of the turnip-rape plant, pigweed by 95% , common ragweed by although these compounds were very 43%, and common purslane by 100%. Summary: To find out how large a short-lived when incorporated in soil. Rye residue had no effect on yellow role allelopathic compounds play in foxtail emergence. weed suppression, the authors com - pared barley and rye residue with the compounds present and with the

9 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I Tillage often exposes many weed seeds to light and moisture, triggering germi - Reduced tillage nation. Reducing soil tillage and dark cultivation can reduce germination of some weed species. pring tillage stimulates weed I Punch planting attempts to reduce seed germination and a flush of the weed infestations common Conversely, continuous Sweed growth. Two strategies have within the row in strip-tillage by reduced tillage production been evaluated with the goal of disturbing the soil only in an area systems favor shifts toward reducing spring weed growth— small enough to plant the crop reduced tillage and dark cultivation. seed. In small-seeded crops such as perennial weed species. red beet, the punch plant hole is as Reduced tillage small as 1⁄4 inch in diameter. The I Annual vegetable crops can Three methods of reduced tillage have crop seed is dropped in the be difficult to establish been evaluated for weed manage - punched hole and the area without some soil tillage. ment in vegetable production: between holes within the crop row remains undisturbed. Punch plant - With this in mind, strip I In no-till production , crops are ing was researched thoroughly in tillage and punch planting planted without any primary tillage. the 1960s as a method for precision systems both allow plant - Annual weed emergence is often planting and has received renewed reduced by the lack of soil distur - interest as a no-till planting strategy ing with limited soil distur - bance. Continuous no-till produc - that allows for good soil-to-seed bance. The drawback is that tion can lead to a shift in weed contact with minimal soil distur - weed germination will also species that favors biennial and bance in the crop row. perennial weeds. No-till planting be promoted within the requires specialized equipment and disturbed area directly more power to penetrate hard soil. around the crop. I Strip-till planting disturbs soil only in the rows in which the crop is I Some weed species require planted. Strip-tillage is often con - ducted with a narrow rototiller and light for germination. To allows for greater and more consis - prevent exposure, tilling at tent crop emergence than with no- night or shielding the soil till planting, particularly on hard or crusted soils. The area between during planting has been planted rows remains untilled, proposed as a way to inhibiting emergence of annual reduce weed populations. weeds. Weed pressure, however, is often greater within the tilled and Research results have been planted crop row. very mixed and the practi - cal utility of such a strategy has been limited.

10 REDUCED TILLAGE AND DARK CULTIVATION

Dark cultivation Figure 1. Effect of cover crops and tillage on weed emergence 4 weeks after sweet corn planting. Adapted from Peachey et al. 2004. The germination of several weed Hairy nightshade species is promoted by light. For some no-till conventional till 140 species, only a brief flash of light is 140 140 Weed growth

) 12020 needed to trigger germination. Soil 2 120 m tillage in complete darkness has been / s

t 10000 100 n

proposed and investigated as a a l

p 8080 80 ( method to reduce germination of e c light-stimulated weeds. Results have n 6060 60 e g been very mixed. Not all species r e 4040 40 require light to stimulate germination. m E Even when tillage is conducted in the 2020 20 dark, weed seeds end up close 00 0 Rye Oat Barley No cover Rye Oat Barley No cover enough to the soil surface that suffi - Rye Oat Barley Noc croverop Rye Oat Barley Noc croverop crop cient quantity and quality of light trig - Hairy nightshade Powell amaranth crop gers germination. Additionally, many other cues can stimulate weed seed germination, such as lack of dormancy and sufficient soil moisture, all of Night tillage Punch planting which confound results of dark tillage research. In summary, dark tillage can Source: Gallagher, R. S., and J. Cardina. Source: Rasmussen, J. 2003. Punch at best reduce subsequent weed pres - 1998. The effect of light environment planting, flame weeding, and stale sure, but it is not consistent enough to during tillage on the recruitment of seedbed for weed control in row be dependable as a major strategy for various summer annuals. Weed Science crops. Weed Research 43:393–403. weed management. 46:214–216. Summary: Punch planting plus flame Summary: Night tillage (plowing and weeding reduced overall weed den - Research disking) was compared to tillage dur - sity by 30% compared to conventional No-till vegetable production ing the day in a 3-year study. The planting plus flame weeding. When effect of light on weed germination punch planting and flame weeding Source: Peachey, R. E., R. D. William, was more important in disking than were combined with delayed planting and C. Mallory-Smith. 2004. Effect of plowing. When disking was conducted (stale seedbed technique), the effect no-till or conventional planting and at night, only pigweed species and was intensified considerably. This cover crop residues on weed emer - giant foxtail were affected; all other study demonstrates the advantages of gence in vegetable row crops. Weed weed species were unaffected by the integrating multiple strategies for Technology 18:1023–1030. absence of light. The authors con - weed management. Summary: Vegetable crops no-till cluded that night tillage was not a reli - planted into cover crop residues were able weed management strategy due compared with conventional tillage to the variable and inconsistent that incorporated cover crops. No-till results. production reduced emergence rates of hairy nightshade and Powell ama - ranth by 99% and 87%, respectively (figure 1).

11 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I Thermal weed management (flaming, hot water, and steam) works by breaking Thermal weed management plant cell membranes, lead - ntensive heat breaks plant cell Hot water and steam have also been ing to plant death when membranes, leading to plant death used to break weed cell walls, disrupt - damage is extensive. Iwhen damage is extensive. The use ing normal plant growth and some - of heat for weed control has been times causing plant death. As with I Heat treatments primarily explored with flaming and steam or flame weeding, early postemergence hot water treatments. applications are more effective and control young annual require less energy than applications broadleaf weeds with Flame weeding involves the use of a at later growth stages. Broadleaf bank of burners at temperatures of exposed growing points; annual weeds are more effectively 1500° to 1800°F, usually fueled with controlled than perennial and grass by contrast, control of propane, that are pulled through the species. While hot water and steam field. This technique can be used with established annual weeds, can effectively control some weed stale seedbeds where weeds are species, practical applications in agri - grasses, and perennial flamed shortly before crop planting, culture are limited by the cost of weeds is often poor, with after crop planting but before crop energy required to heat water. emergence, or after emergence of tol - plants regrowing following Additionally, the safety of using steam erant crops such as corn. Flaming is under pressure should be considered treatment. most effective on emerged annual by the applicator . broadleaf species with growing points I Fuel costs often limit exten - that are not protected by the soil or by Research sive use of heat treatments. vegetation. Annual broadleaf species such as common lambsquarters, red - Flame weeding: Weed species and root pigweed, chickweed, and common growth stage groundsel are effectively controlled by Source: Ascard, J. 1995. Effects of flaming when weeds are young and flame weeding on weed species at succulent. Larger, established annual different developmental stages. weeds, perennial weeds, and grasses Weed Research 35:397–411. often survive flame treatments. The intensity of flaming and dose (calcu - Summary: Plant size and species lated in terms of fuel burned per unit determined the degree of control with area) are controlled primarily by flaming. Species such as common ground speed. The dose required for lambsquarters and common chick - optimum weed control increases with weed with up to four leaves were weed size, therefore early-season completely killed. Larger weeds flaming is critical to its practical required a significantly higher dose of use. In sensitive crops where the heat than smaller weeds to achieve growing point is exposed, 95% control. Shepherd’s purse has a selective flaming is possible protected growing point and re-grew only with shielded burners that after flaming. Annual bluegrass was protect the crop. In-row weed not controlled at any growth stage or control is poor when burners flaming dose. shield the crop row. Fuel costs often limit the economical use of flame weeding on an extensive basis. 12 THERMAL WEED MANAGEMENT

Hot water weed control Source: Hansson, D., and J. Ascard. 2002. Influence of developmental stage and time of assessment on hot water weed control. Weed Research 42:307–316.

Summary: The authors evaluated effectiveness of hot water for weed control, measured as a 90% reduction Close-up of a flame weeder in use. in weed weight. Treating weeds at the six-leaf stage required three times Weed control by flaming and Soil steaming hot steam in apple more energy than applications made Source: Melander, B., and M. H. at the two-leaf stage. The earlier treat - Source: Rifai, M.N., M. Lacko-Bartosova, Jorgensen. 2005. Soil steaming to ment reduced energy costs while and R. Somr. 1999. Weed control by reduce intrarow weed seedling emer - increasing application ground speed. flaming and hot steam in apple gence. Weed Research 45:202–211. The amount of energy used was orchards. Plant Protection Science directly related to the duration of Summary: Steam applications to 35:147–152. weed control. In general, a second bring the soil temperature to 142°F treatment was required 25 days after Summary: Flaming gave poor control and 160°F reduced weed emergence the original hot water treatment for of perennial weed species. Multiple by 90% and 99%, respectively. effective weed control. Perennial treatments were more effective than However, the researchers concluded weeds regrew after treatment. single treatments, particularly for more that the high energy consumption and tolerant species and at later weed time required for the weed control growth stages. Hot steam treatment operation limit practical applications. did not effectively control weeds. Steam application: Weed species Flame weeding in response and steam rate organic onions and carrots Source: Kolberg, R. L., and L. J. Wiles. Source: Rifai, M. N., M. Lacko- 2002. Effect of steam application on Bartosova, and V. Puskarova. 1996. cropland weeds. Weed Technology Weed control for organic vegetable 16:43–49. farming. Rostlinna-Vyroba 42:463–466. Summary: Steam application rate was Summary: Flame weeding was more critical for weed control. Steam efficient and less labor intensive than applied at 2,850 lb/a reduced weed hand weeding. Two flame applications growth, but application at 1,400 lb/a in onions were efficient in terms of did not. Common lambsquarters and labor but reduced crop yield. In redroot pigweed control was greater carrots , the combination of a single than 90% when steam was applied at flame weeding and hand weeding the higher rate. Emergence of several reduced labor costs compared to broadleaf weed species was not hand weeding alone, although pre - affected by steam application. emergence flame weeding slightly reduced crop yield.

13 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I use of any sort is not normally approved or accepted according to the Alternative herbicides

National Organic Standards. evelopment of “natural” herbi - Research on clove oil for weed control Be aware that herbicides cides has been relatively limited. is extremely limited (see research approved by national stan - DCorn gluten meal and other described below). This product has “meals” made from plants have been been used for postemergence control dards might not be used for weed control with varying of small annual weeds. Clove oil is not approved by local or results. Use of corn gluten meal as a selective and will injure or kill any herbicide was discovered accidentally plant species that it contacts. regional organic certifica - by researchers investigating turf dis - Note: Consider labeling and organic tion organizations. Always eases. Corn gluten is a naturally occur - certification requirements prior to use check with your certifica - ring protein product of corn wet of any products for weed control. milling and is often used as a protein tion agency, the National source in animal feed. It has about Organic Standards, and the 10% nitrogen content. Research Organic Materials Review Corn gluten must be applied before Corn gluten: Institute (OMRI) before weed emergence (preemergence) as it Weed control in strawberry using any herbicide! has no effect on emerged or estab - Source: Dilley, C. A., G. R. Nonnecke, lished weeds. The nitrogen in corn and N. E. Christians. 2002. Corn-based I Corn gluten meal and clove gluten can actually stimulate growth extracts to manage weeds and pro - of established weeds or crops. Corn vide nitrogen in matted-row straw - oil have been suggested for gluten inhibits root growth after seed berry culture. HortScience weed control. However, germination, leading to plant mois - 37:1053–1056. peer-reviewed research on ture stress. However, with sufficient moisture, seedlings can overcome the Summary: This 4-year study found alternative herbicides is inhibitory effects of corn gluten. that grass weeds were not affected by extremely limited, particu - Selected broadleaf and grass weeds corn gluten meal during any of the are listed as controlled on product trial years. Results were mixed for larly for clove oil; the major - labels. broadleaf weeds: While July applica - ity of published research on tions of corn gluten meal reduced the Corn gluten is non-selective as a her - corn gluten meal has number of broadleaf weeds during bicide—it will injure or inhibit early 2 of the 4 years, the same treatment yielded mixed results. root growth of both weed and crop significantly boosted weed growth in species. Therefore, it is not appropriate comparison to the control—by 133% for direct-seeded crops after planting and 12%—during the other 2 years. but before the crop is emerged. Weed control results have been variable when corn gluten is used in vegetable and fruit production systems.

14 ALTERNATIVE HERBICIDES

Corn gluten: Application rate in strawberry Source: Nonnecke, G. R., and N. E. Christians. 2002. Effects of source, rate, and particle size of corn gluten meal on weed and strawberry growth and development. Proceedings of the 5th North American Strawberry Conference, pp. 124–128. Clove oil as a herbicide Summary: Weed control varied by corn gluten meal application rate and Source: Tworkoski, T. timing. Broadleaf and grass weed cover 2002. Herbicide effects of and number were not reduced when essential oils. Weed Science corn gluten meal was applied in July, 50:425–431. but an August application reduced Summary: Several plant-derived weed cover and broadleaf weed num - oils were screened for potential her - ber. Weed cover following an August bicidal activity in laboratory and application was reduced 30%, 33%, studies. Johnsongrass, and 53% compared to the untreated common lambsquarters, and common control when corn gluten was applied ragweed plants treated with clove oil at 891, 1,783, and 2,674 lb/a. were injured as early as 1 hour after treatment, and oil concentrations of Corn gluten: Weed control in vegetables 5 to 10% killed most weeds 7 days after treatment. By 30 days after treat - Source: McDade, M. C., and N. E. ment, some common ragweed plants Christians. 2000. Corn gluten meal— displayed regrowth, and the author a natural preemergence herbicide: suggests that re-treatment and thor - Effect on vegetable seedling survival ough plant coverage may be neces - and weed cover. American Journal of sary for optimum weed control. Alternative Agriculture 15:189–191.

Summary: Corn gluten meal was incorporated prior to planting veg - etable crops. An application of corn gluten meal at 3,564 lb/a reduced weed cover by up to 82% as com - pared to the untreated control. However, corn gluten meal rates as low as 891 lb/a reduced survival of onion, red beet, radish, bean, carrot, pea, and lettuce seedlings by 35% to 73%. The authors concluded that corn gluten meal should not be incorpo - rated prior to direct-seeding vegetables.

15 KEY POINTS WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

I Competition with weeds involves the ability to sup - press weed growth and/or Crop competition for tolerate nearby weeds with - weed control out compromising crop growth and yield. he ability of a crop to successfully over the entire field. compete against weeds reflects a can also reduce weed growth by I The crop production system Tcombination of crop suppression directing water to support crop of nearby weeds and tolerance of growth while the areas between can be altered to favor crops weed interference. For a crop to sup - the crop row remain dry. over weeds by partitioning press weeds, it must outcompete resources to the crop. nearby weeds for limiting resources Research such as light, water, and nutrients. The Corn population and following strategies have been Crop row spacing and den - velvetleaf competition designed to favor the crop over weeds : sity can be arranged to Source: Teasdale, J. R. 1998. Influence enhance competition for I Some crop cultivars are more com - of corn ( Zea mays ) population and row petitive than others. Competitive spacing on corn and velvetleaf light and other limiting cultivars typically emerge soon (Abutilon theophrasti ) yield. Weed resources, but can also after planting, grow rapidly early in Science 46:447–453. increase disease pressure. the season, and have a large leaf area and tall stature that shades Summary: Velvetleaf response to three corn populations (1x, 1.5x, and Cultural practices can be nearby weeds. Choosing cultivars that mature quickly also allows for 2x of normal population) was deter - managed to favor the crop, aggressive post-harvest weed man - mined. Velvetleaf emergence timing such as banding agement with mowing or tillage was compared to corn emergence. When velvetleaf emerged at the same near the crop or using drip before weeds reproduce. time as corn, velvetleaf seed produc - I Altering crop row spacing and plant irrigation instead of broad - tion was reduced 69 to 94% at the density within the row can be used 1.5x corn population and 99% at the cast irrigation. to close the earlier, thus lim - 2x corn population when compared to iting resources (primarily light) the conventional 1x population. Crop cultivars can be cho - available to weeds. Velvetleaf seed production was mini - sen that compete with I Fertilizer and irrigation water can mal when emergence was delayed, weeds through rapid early be targeted to favor the crop while and the authors attributed reduced restricting access to weeds. season growth and leaf area weed seed production to corn compe - Banding fertilizer, for example, near tition for light with the shorter vel - development, expansive the crop row favors access by the vetleaf below the crop canopy. root growth, and early crop over a broadcast application canopy closure.

16 CROP COMPETITION FOR WEED CONTROL

Comparison of competitive (left) and uncompetitive potato cultivars (right), 3 weeks after emergence.

Snap bean row spacing and Bean cultivar choice and row Summary: Three bean cultivars were weed competition spacing affects weed growth compared at three row spacings. Source: Teasdale, J. R., and J. R. Frank. Source: Malik, V. S., C. J. Swanton, and Cultivars OAC Gryphon and OAC Laser 1983. Effect of row spacing on weed T. E. Michaels. 1993. Interaction of reduced weed biomass (weight) by competition with snap beans white bean ( Phaseolus vulgaris L.) 10 to 35% compared to OAC Sprint (Phaseolus vulgaris ). Weed Science cultivars , row spacing, and seeding (figure 2). Narrower row spacings 31:81–85. density with annual weeds. reduced weed biomass further. Weed Science 41:62–68. Neither row spacing nor cultivar choice Summary: Snap bean row spacings of altered weed density, although weed 6, 10, 14, 18, and 36 inches were com - biomass decreased as crop size and pared while keeping total plant popu - density (bean leaf area index) increased . lation per area constant (beans were spaced farther apart within the row in Figure 2. The effect of bean cultivar on total weed the narrower row spacings). Narrow- biomass, quantified at the R-9 bean growth stage, at row snap beans closed canopy earlier Elora, Ontario in 2 years. Adapted from Malik et. al 1993. than wider-row snap beans, reducing weed growth. Row spacings of 6 to 1998 1999 14 inches reduced weed growth by 80008000 18% when weeds emerged with

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I In-row cultivators are specifically designed to weed within the crop row. In-season cultivation Implements such as finger ultivation serves as the main form crop row and then push a shallow and torsion weeders work of weed control in most organic layer of soil back over the crop row to best when weeds are very Cvegetable crops. Many different cover small weeds. This precise, light - types of cultivation implements have weight tool is often belly-mounted on small and the crop is estab - been developed to control weeds in a small tractor with the operator sit - lished or transplanted. and/or between the crop rows. The ting over the crop row to guide the Requires precision cultiva - necessity for reliable mechanical weed implement. control has led to the development of The is a very tion and slow driving many innovative cultivators. This torsion or rod weeder simple, affordable design consisting of speeds to limit crop injury. review focuses on some of the more two steel rods, one on each side of the commonly used tools for in- and crop row to uproot small weeds while I Between-row cultivators between-row weed control. pushing soil into the row. The torsion include spider gang or In-row cultivators weeder is very effective in controlling rolling cultivators, basket small in-row weeds. This tool can be Mechanical weeding within the crop combined with an existing between- weeders, and the brush . row requires selectivity between the row for improved in-row These tools tend to be more crop and weeds, and can be a difficult weed control. The distance between proposition given that both are often aggressive than in-row cul - the rods and the cultivated crop can at similar growth stages at the time of be easily adjusted based on the tivators, and can be oper - cultivation. In-row weeders require desired precision and growth stage of ated at higher speeds and very precise operation and are often the crop and weeds. operated at slow speeds with narrow remove larger weeds. (one or two crop row) equipment. Between-row cultivators Weed control is greatest when weeds I Broadcast cultivators weed Several implement designs are effec - are very small—often with two leaves tive at controlling weeds between the both over and between the or less. These tools usually need to be crop rows, ranging from traditional combined with more aggressive crop rows. They require S-tine cultivators to more innovative between-row cultivators. some size selectivity brush hoes. This review focuses on the between crops and weeds. The finger weeder includes three sets spider gang or rolling cultivator, the of ground-driven rotating fingers that basket weeder, and the brush hoe. Broadcast cultivators are pull soil and weeds away from the effective against weeds that have germinated but not Finger weeder yet emerged (white-thread stage), often prior to crop emergence in larger seeded, deeply planted crops such as bean and sweet corn.

18 IN-SEASON CULTIVATION

Torsion weeder with shares

Spider gang or rolling cultivators established weeds and provide some use toothed “wheels” that rotate verti - residual weed control by creating a cally across the ground. The wheels dust layer that reduces further weed are angled to uproot weeds between germination. Due to their high pur - the crop row while throwing a small chase cost relative to other cultivators, amount of soil over weeds within the brush hoes are perhaps best suited for crop row. Wheel angle is easily use on high-value crops. adjusted based on the crop and weed growth stage and the desired cultiva - Broadcast cultivators tion aggressiveness. This implement In- and between-row cultivators have can be operated at fairly high ground been used for many years in wheat, Brush hoe speeds, but can occasionally become oat, and other agronomic crops and plugged with plant residue. they’ve become increasingly common The basket weeder consists of a in vegetable production. Flex-tine series of rolling wire baskets with cultivators use a series of flexible space between the baskets to accom - tines mounted in overlapping rows modate the crop row. Minimal soil is vibrate as they drag through soil, rip - pushed into the crop row. This cultiva - ping out small weeds. These tools are tor effectively controls small weeds, most effective when weeds are in the but does not perform well on crusted “white thread” stage (single white or high-residue soils. roots are visible on weeds when soil is disturbed, often before leaf appear - The brush hoe is best described as a ance). Established weeds and grasses Flex-tine harrow combination of a street sweeper and a at any stage are often not controlled. hook-and-ladder fire truck. A series of Conversely, the crop needs to be bet - brushes, similar in appearance to ter established or at a later stage of those on a street sweeper, are growth to allow selectivity in remov - mounted on a horizontal axle. The ing smaller weeds. Lightweight flex- PTO-driven brushes rapidly rotate and tine cultivators can be operated at rip weeds from the soil, while the crop high speeds and with varying tine is protected by shields mounted intensity based on the crop between the brush units. Soil is growth stage. pushed under the shields, covering small weeds within the crop row. The brush hoe is very aggressive and pre - cise, often requiring two operators— one to drive the tractor, the other to steer the shields over the crop row. The brush hoe can remove fairly

19 WEED MANAGEMENT IN ORGANIC VEGETABLE PRODUCTION

Research was most difficult in sweet corn, given the crop’s limited ability to compete Mechanical cultivation in broccoli, with weeds early in the season and sweet corn, and snap beans the long crop season. Source: Colquhoun, J. B., R. R. Bellinder, and J. J. Kirkwyland. 1999. Efficacy of Carrot tolerance of between-row cultivators mechanical cultivation with and with - out herbicides in broccoli ( Brassica Source: Ascard, J., and B. Mattsson. oleracea ), snap bean ( Phaseolus vul - 1994. Inter-row cultivation in weed- garis ), and sweet corn ( Zea mays ). free carrots: The effect on yield of hoe - Weed Technology 13:244–252. ing and brush weeding. Biological Agriculture and 10:161–173. Summary: Five cultivation implements were compared: flat and round flex- Summary: A conventional cultivator tine harrows, a spider gang cultivator, was compared with a brush hoe at Rolling cultivator a brush hoe, and a conventional shovel two cultivation distances from the cultivator. Weed control and crop yield crop row leaving the untilled strips of In-row were similar among implements in 5 and 10 cm. Carrot yields were not transplanted broccoli, largely because affected by type of cultivator or width Source: Ascard, J., and R. R. Bellinder. of the short crop season. In snap bean, of the untilled strip. In 2 of 3 years, cul - 1996. Mechanical in-row cultivation in weed control was greatest where tivation increased yield as compared row crops. Proc. Second International early-season flex-tine harrowing was to non-cultivated, weed-free carrots; Weed Control Congress pp.1121–1126. followed by either a shovel cultivation cultivation reduced yield during the Summary: Finger and torsion in-row or brush hoeing. When used individu - third year. In addition, undesirable car - weeders were compared with a broad - ally, weed control was greatest with rot branching was greater where the cast flex-tine harrow in onion and the brush hoe, spider cultivator, or crop was cultivated in 2 of 3 years sugar beet. Early-season cultivation shovel cultivator (figure 3). The brush when compared to the non-cultivated caused severe crop damage. While hoe provided some residual and in- check. These results suggest that culti - weed control was greater in onions row weed control in all crops. Weed vation of small-seeded crops such as with the flex-tine harrow, yield reduc - management with cultivation alone carrot can be difficult and variable. tions were also greater, suggesting a lack of selectivity. In-row weeding Figure 3. Differences in weed growth and snap bean yield due to reduced labor requirements by 50%. use of different cultivation tools. Adapted from Colquhoun et al.

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20 INTEGRATED WEED MANAGEMENT

Integrated weed management

he most successful weed control lowed by timely in-season control programs combine multiple man - strategies such as cultivation or Tagement strategies. There are sev - flame weeding can delay weed eral benefits to an integrated emergence and development to program: the point where no mature weed seed is present by crop harvest. I The risk of complete weed control Timely mowing after harvest will failure drops when using multiple then prevent weed seed produc - strategies, since the chance that all tion, and winter cover crops will strategies would simultaneously fail suppress future weeds. is minimal. And finally, be sure to consider the I Multiple strategies minimize the when planning inte - chance of selecting for weeds that grated weed management programs. tolerate or resist a single control By controlling weeds well during the strategy. For example, perennial part of the crop rotation that allows grass weeds such as quackgrass are the most weed management strate - often not controlled by flex-tine gies and greatest weed suppression cultivation, but may be suppressed by the crop, you’ll reduce weed pres - by a competitive cover crop and sure during more difficult crops in the timely mowing. rotation. A little bit of planning can I Integrated weed management pro - make a world of difference when grams provide season-long weed growing less-competitive crops in the control, minimizing competition rotation! between the crop and weeds and limiting production of weed seeds. For example, beginning the season using the stale seedbed system, fol -

21 Copyright © 2007 University of Wisconsin-System Board of Regents and University of Wisconsin-Extension, Cooperative Extension. Author: Jed Colquhoun is associate professor of horticulture, College of Agricultural and Life Sciences, University of Wisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension. Credits: Produced by Cooperative Extension Publishing, University of Wisconsin- Extension. Edited and designed by Linda Deith. Photography courtesy Robin Bellinder (interseeded cover crop, finger weeder, torsion weeder, flex-tine harrow, and flame weeder), Bigham Brothers, Inc. (spider gang), Beat Hauenstein (brush hoe), Robert R. Kowal/Wisconsin State Herbarium (common mullein), iStockphoto (vegetables). University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture and Wisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; and provides equal oppor - tunities and affirmative action in employment and programming. If you need this infor - mation in an alternative format, contact Cooperative Extension Publishing at 608-262-2655 or the UWEX Affirmative Action Office. This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publishing. To order, call toll free 877-WIS-PUBS (947-7827) or visit our web site at learningstore.uwex.edu.

Weed Management in Organic Vegetable Production: A Practical Review of Applied Research (A3845) I-11-2007-1.5M