Spotted knapweed (Centaurea maculosa L.) seed longevity, chemical control and seed morphology by Edward Stuart Davis A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Agronomy Montana State University © Copyright by Edward Stuart Davis (1990) Abstract: Spotted knapweed (Centaurea maculosa Lam.) is an introduced perennial plant that has spread rapidly to infest an estimated 4.7 million acres of grazeable rangeland, woodland and pastureland in Montana. Spotted knapweed is a primary invader that capitalizes on soil disturbances and once established is extremely competitive. Carrying capacity of rangeland is severely reduced as spotted knapweed displaces productive grasses and forbs. Seed longevity is an important survival characteristic of spotted knapweed. Over 50% of a population of spotted knapweed seeds remained viable after 5 years of burial. In a natural seedbank, spotted knapweed seed populations declined by 95% during the first 3 years following termination of seed production. However, approximately 390,000 viable seeds remained per hectare 7 years after seed production had been terminated. In laboratory tests 10 to 40% of freshly harvested spotted knapweed seed required light to germinate. Field trials were established to determine the long-term control of spotted knapweed achieved by a single application of low rates of picloram. All rates significantly reduced spotted knapweed densities 60 and 84 months after treatment at two locations. Grass forage production increased 200 to 700% when spotted knapweed was controlled. Spotted knapweed plants gradually reinfested the treated areas as picloram soil residues declined causing reduced grass production. Spotted knapweed seeds rapidly imbibe water and germinate within 18 hours. Water uptake occurs primarily through the hilum and to a lesser degree through the micropyle of the seed. A thick, durable seed coat provides effective protection which permits dormant seed to resist decomposition in soil. SPOTTED KNAPWEED (Centaurea maculosa L.> SEED LONGEVITY CHEMICAL CONTROL AND SEED MORPHOLOGY
by Edward Stuart Davis
A thesis submitted in partial fulfillment of the requirements for the degree of
Master of Science in
Agronomy
MONTANA STATE UNIVERSITY Bozeman, Montana September 1990 /VJ92 ii D ZcIZ 3
APPROVAL
of a thesis submitted by
Edward Stuart Davis
This thesis has been read by each member of the thesis committee and has been found to be satisfactory regarding content, English usage, format, citations, bibliographic style, and consistency, and is ready for submission to the College of Graduate Studies.
9 - /2- - 9£> Date Chairperson, Graduat Committee
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Approved for the College of Graduate Studies
Date Graduate Dean STATEMENT OF PERMISSION TO USE
In presenting this thesis in partial fulfillment of the requirements for ai master's degree at Montana State University, I agree that the Library shall make it available to borrowers under rules of the Library. Brief quotations from this thesis are allowable without special permission, provided that accurate acknowledgement of source is made. Permission for extensive quotation from or reproduction of this thesis may be granted by my major professor, or in his absence, by the Dean of Libraries when, in the opinion of either, the proposed use of the material is for scholarly purposes. Any copying or use of the material in this thesis for financial gain shall not be allowed without my written permission.
Signature Date ACKNOWLEDGEMENTS
I would like to thank my advisor and friend Dr. Pete Fay for his guidance, support, and encouragement throughout my. tenure at Montana State University. / I would also like to express my thanks to Dr. William Dyer and Dr. Sharon Eversman for their input and advice while serving on my committee.
A special thanks and acknowledgement goes out to Tim Chicoine and Celestine Lacey for the early research they conducted on spotted knapweed. Their dedication made the continuation of these valuable long-term studies possible.
The hours of assistance provided by Toni Moorman, Shaun Townsend and Jackie Stockwell made it possible to complete this research.
The scanning electron micrographs were made possible through the expertise of John Blixt, Veterinary Research EM
Laboratory Supervisor.
I would like to thank my parents for their guidance throughout my life, and my children for their love and understanding.
I would especially like to thank my wife Mett for her patience and sacrifice throughout this endeavor, and most of all for her love. vi
T&BLE OF COMTEMTS
Page APPROVAL...... ii STATEMENT OF PERMISSION TO USE...... iii
VITA...... iv
ACKNOWLEDGEMENTS ..... V TABLE OF CONTENTS...... vi LIST OF TABLES...... ;...... viii LIST OF FIGURES...... ix ABSTRACT...... xi
Chapter
I. LITERATURE REVIEW...... I
Introduction...... I Seed Germination and Seedling Establishment...... 2 Plant Development...... 10 Seed Production and Dissemination.... 12 Allelopathy...... 14 Cultural Control...... 19 Plowing...... 19 Mowing...... 19 Burning...... 19 Grazing...... 2 0 Producer Experiences...... 21 Economic Impact...... 23 Biological Control...... 26 Chemical Control...... 35 vii \
TABLE OF CONTENTS -.CONTINUED ^
Chapter Page
2. SPOTTED KNAPWEED SEED LONGEVITY...... 39 Introduction...... 40 Materials and Methods...... 40 Seed Burial Study...... 40 Decline of Viability from a Natural Seed Population in Soil...... 42 Seed Germination Study...... 44 Results and Discussion...... 45 Seed Burial Study...... 45 Viability of Natural Seed Populations...... 50 Germination Behavior...... 53 3. LONG-TERM CONTROL OF SPOTTED KNAPWEED WITH REDUCED RATES OF PICLORAM...... 58
Introduction...... 58 Materials and Methods...... 60 Results and Discussion...... 63
4. SEED MORPHOLOGY AND GERMINATION CHARACTERISTICS OF SPOTTED KNAPWEED...... 70 Introduction...... 70 Materials and Methods...... 71 Scanning Electron Micrographs...... 71 Imbibition Curves...... 71 Point of Water Uptake...... 72 Results and Discussion...... 73 Scanning Electron Micrographs..... 73 Water Imbibition..... 83 Point of Water Uptake.... . 84 5. SUMMARY...... 89 BIBLIOGRAPHY...... 91 viii
LIST OF TABLES
Table Page
1 Recovery of Buried Spotted Knapweed Seed.. 45 2 Viability of Intact, Ungerminated Spotted Knapweed Seed Recovered Over a 60 Month Period at Bozeman and Three Forks MT...... 48
3 Changes in the Seed Reserve in Soil 12 Months After Treatments were Applied in May, 1985 in an Attempt to Stimulate Germination of Spotted Knapweed Seed at Harlowton and Ovando...... 51 4 Number of Mature Spotted Knapweed Plants per m2 in July, 1983 Through 1989 Following Herbicide Application in June, 1982 at Ovando...... 63
5 Number of Mature Spotted Knapweed Plants per m2 in July, 1983 Through 1989 Following Herbicide Application in June, 1982 at Harlowton...... 63
6 Spotted Knapweed Herbage Production 2, 14, 26, and 84 Months After Herbicide Application at Harlowton...... 65 7 Spotted Knapweed Herbage Production 2, 14, 26, and 84 Months After Herbicide Application at Ovando...... 65 8 Perennial Grass Production 2, 14, 26 and 84 Months After Herbicide Application at Harlowton...... 66 9
9 Perennial Grass Production 2, 14, 26 and 84 Months After Herbicide Application at Ovando...... 66 ix
LIST OF FIGURES
Figure Page
I Percent Germination of in situ Spotted Knapweed Seeds Recovered Annually From Buried Containers at Bozeman and Three Forks...... 46
2 The Number of Viable Spotted Knapweed Seeds Recovered During a 60 Month Burial Period at Three Forks and Bozeman...... 49 3 The Decline in the Spotted Knapweed Seed Population Following Termination of Seed Production in 1982 at Ovando and Harlowton MT...... 52 4 Effect of Spotted Knapweed Seed Placement on the Soil Surface or Buried 0.6 cm Deep on Germination of Seed Collected at 7 Locations...... 54
5 Germination of Spotted Knapweed Seed in Light or Complete Darkness...... 55
6 Schematic Representation of a Spotted Knapweed Achene (Cypsela) in Cross- section...... 74 7 Proximal End of Spotted Knapweed Achene showing Hilum and Epidermal Hairs (XlOO).. 75
8 Cross Section of Hilum Showing "Honeycomb" Cellular Structure (X500)...... 76 9 Cross Section of Proximal End of Achene With Funicular Attachment (XlOO)...... 77 10 Hilum Surface Showing Waxy Texture with Numerous Pores (X800)...... 77
11 Distal End of Achene Showing Multiserial Bristles (X50)...... 78
12 Bristles on Outer Whorl of Pappus (X300) 79 X
LIST OF FIGURES - CONTINUED
Figure Page
13 Pappus Showing Inner and Outer Whorls of Bristles and Disc (X50)...... 80 14 Disc (Nectary) and Micropyle (X300)...... 80 15 Fractured Surface of a Fresh Achene Exposing the Testa and Pericarp (XlOO) .... 82 16 Fractured Surface of Achene After 5 Years of Burial Exposing the Testa and Pericarp (X200)...... 82 17 Spotted Knapweed Seed Imbibition Curve Measured over an 18 Hour Period Culminating with Germination...... 83 18 Germination of Spotted Knapweed Seeds with Paraffin Covering the Hilum, the pappus, the Entire Seed, and Non-coated Seeds... 84 19 Schematic Representation of Cross-section of Spotted Knapweed Seeds Showing Staining Patterns Following Imbibition in a 0.1% Methylene Blue Solution for 0, 2, 4, 6, 8, and 12 hours...... 86 xi
ABSTRACT
Spotted knapweed (Centaurea maculosa Lam.) is an introduced perennial plant that has spread rapidly to infest an estimated 4.7 million acres of grazeable rangeland, woodland and pastureland in Montana. Spotted knapweed is a primary invader that capitalizes on soil disturbances and once established is extremely competitive. Carrying capacity of rangeland is severely reduced as spotted knapweed displaces productive grasses and forbs. Seed longevity is an important survival characteristic of spotted knapweed. Over 50% of a population of spotted knapweed seeds remained viable after 5 years of burial. In a natural seedbank, spotted knapweed seed populations declined by 95% during the first 3 years following termination of seed production. However, approximately 390,000 viable seeds remained per hectare 7 years after seed production had been terminated. In laboratory tests 10 to 40% of freshly harvested spotted knapweed seed required light to germinate. Field trials were established to determine the long-term control of spotted knapweed achieved by a single application of low rates of picloram. All rates significantly reduced spotted knapweed densities 60 and 84 months after treatment at two locations. Grass forage production increased 200 to 700% when spotted knapweed was controlled. Spotted knapweed plants gradually reinfested the treated areas as picloram soil residues declined causing reduced grass production. Spotted knapweed seeds rapidly imbibe water and germinate within 18 hours. Water uptake occurs primarily through the hilum and to a lesser degree through the micropyle of the seed. A thick, durable seed coat provides effective protection which permits dormant seed to resist decomposition in soil. CHAPTER I
LITERATURE REVIEW
Introduction
Spotted knapweed (Centaurea maculosa Lam.) is a pernicious noxious weed in Montana infesting an estimated 4.7 million acres of range and grazeable woodland (Lacey,
1987). An additional 34 million acres may be vulnerable to spotted knapweed invasion (Chicoine, Fay, and Nielson, .1986).. The annual forage loss due to displacement of native forbs and grasses by spotted knapweed is estimated at $4.5 million (French and Lacey, 1983). The perennial growth habit, profuse seed production and aggressiveness of spotted knapweed result in rapid establishment and spread. Initial infestations occur in disturbed areas such as roadsides, trails, construction sites, overgrazed land and waterways (Watson and Renney,
1974). Once established it is very competitive, displacing native grasses and forbs, resulting in near-monoculture stands of spotted knapweed. Another factor contributing to its success in North America is the lack of natural enemies.
In Europe, its center of origin, spotted knapweed evolved with host-specific insects and pathogens which keep the 2 plant frequency and density at low levels. In North America most of these agents are not present.
Seed Germination and Seedling Establishment
Spotted and diffuse knapweed (Centaurea diffusa Lam.) seed is disseminated as a dry, indehiscent single seeded fruit called an achene. Members of the Compqsitae have epigynous flowers (flowers with an inferior ovary). The flower parts, calyx, corolla, and stamens originate above the inferior ovary. Achenes produced from epigynous flowers are termed cypselas. Throughout the text of this thesis, the term seed will be synonymous with achene or cypsela.
Knapweed seeds germinate in the fall and spring when soil moisture conditions are most favorable. Seedlings that germinate in the fall overwinter as rosettes and bolt the following June. Seedlings established in the spring usually flower the following season (Schirman, 1981).
Knapweed seeds imbibe water and germinate within 18 hours under optimum conditions (Chicoine, 1984). Spears et al. (1980) and Eddleman and Romo (1986) examined canopy cover, depth of emergence of seedlings, temperature and moisture and determined the optimum conditions for germination. Canopy cover had no effect on germination of buried seed. Maximum germination occured at soil moisture levels of 118 to 127% of field capacity in the soil mixture used at soil temperatures ranging from 10 to 28° C. 3 Seedlings did not emerge from depths of 3 and 5 cm for diffuse and spotted knapweed, respectively. The highest percent germination occurred when seeds were left on the soil surface.
Watson and Renney (1974) reported germination levels of freshly harvested diffuse and spotted knapweed seeds increased from 40 to 68% and from 20 to 80%, respectively, following 25 days of dry storage at room temperature. Their findings suggest that knapweed seeds possess a primary (innate) dormancy which is lost during dry after-ripening. Nolan and Upadhyaya (1988) describe three types of germination behavior in freshly harvested diffuse and spotted knapweed seed: I) nondormant seeds that germinate in darkness, 2) light-sensitive dormant seeds that germinate in response to red light (R), and 3) light-insensitive dormant seeds that fail to germinate after 5 days of continuous R. Seeds of all three germination types were found on individual plants of both species. Less than 5% were nondormant, 50 to 60% were light-sensitive primary dormant, and 35 to 45% were light-insensitive primary dormant for diffuse and spotted knapweed seeds. Seeds collected from different sites, and from individual plants within sites, had different germination levels in darkness and following exposure to two minutes of R. Exposure to R stimulated germination in each case. 4 The mean levels of primary dormancy over all diffuse and spotted knapweed sites were 78 and 84%, respectively. The germination behavior of seeds collected from individual plants was similar to samples collected from different sites for both species. Freshly harvested seeds from individual spotted knapweed plants generally possessed higher levels of primary dormancy (X = 93%) than those from individual diffuse knapweed plants (X = 71%).
Phytochrome is a cytoplasmic protein with a covalently attached linear tetrapyrole chromophore that regulates flowering and seed germination. The phytochrome molecule exists in two different conformations depending on the wavelength of light of the last exposure. The R absorbing form, Pr, has maximum absorbance in the 600-680 nm range and is the inactive form. Upon exposure to R phytochrome undergoes a conformational change to the active or Pfr form which absorbs light in the far-red (FR) region with maximum absorbance in the 720-740 nm range. Phytochrome in the Pfr form undergoes dark reversion to the inactive Pr form (Song, 1984).
The demonstration of R/FR reversibility implicates involvement of the phytochrome system in light-sensitive seed germination (Smith, 1982). Nolan and Upadhyaya (1988) showed that light-sensitive dormant diffuse and spotted knapweed seed germinated in response to R and that exposure to FR negated this effect. Exposure to FR did not establish 5 a light requirement in imbibed seeds that were initially non-dormant, as germination following FR was not significantly different from dark controls. The level of germination following any sequence of light exposures for light-sensitive knapweed seeds depends on the light quality of the last exposure. Exposure to R for just 2 minutes was adequate to stimulate the germination of all dormant seeds from some individual plants, however longer exposures were required for maximum germination of seeds collected from other individual plants. When the duration of R exposure was increased from 2 minutes to I day, germination increased from 15 to 53% for diffuse knapweed, and from 15 to 63% for spotted knapweed. Further increases in R duration from I to 5 days did not increase germination. Watson and Renney (1974) reported no difference in germination of diffuse and spotted knapweed seed when exposed to alternating light and dark versus dark incubation. In fact continuous light significantly reduced ) the germination of both species. Nolan and Upadhyaya explained this discrepancy by noting that they observed a loss of the light requirement in several samples of seeds used in preliminary studies following several months storage at room temperature. An increase in germination following dry after-ripening was reported by Watson and Renney (1974).
Storage of seeds at -200 C arrested after-ripening in the seeds.used by Nolan and Upadhyaya (1988) and maintained a 6 high level of primary dormancy. Alternatively, chance collection from plants bearing mostly nondormant seeds may explain the lack of light-sensitivity of spotted and diffuse knapweed seed reported by Watson and Renney (1974). The effect of gibberellic acid (GA3) and excision on germination of diffuse and spotted knapweed seed was also reported (Nolan and Upadhyaya, 1988). Germination in darkness increased with increasing GA3 concentration. There was no synergism between light and GA3 on knapweed seed germination. Germination of diffuse and spotted knapweed seed was 100% following excision of the distal end of the seed.
Polymorphic germination behavior is a common phenomenon among weed species which insures seed germination for an extended period of time (Bewley and Black, 1982).
Germination polymorphism occurs in both diffuse and spotted knapweed which exhibit three distinct types of germination behavior. Knapweed seeds germinate in the spring and fall (Watson and Renney, 1974). Nondormant and light-sensitive seeds that don't become buried or shaded by other plants would likely germinate immediately following dispersal in the fall if temperature and moisture levels were adequate.
Delay of germination until the spring would be facilitated by dormancy, retention of seeds in the capitula over the winter, or seed dispersal late in the season when low temperatures prevent germination. Seed dormancy would 7 further distribute germination over time by allowing seeds to become incorporated into a persistent seed bank in the soil (Fenner, 1985).
Differences in amounts of non-dormant seeds (seeds germinating in the dark) among individual plants within sites could be due to genetic variability between plants or differences in environmental conditions experienced by the maternal plant during seed development. The asynchronous flowering behavior of the knapweeds increases the possibility that variability in environmental conditions prior to seed dispersal leads to germination polymorphism. Variability in the rate or duration of the after-ripening period following seed maturation is one possible explanation for differences in dark germination (Fenner, 1985).
The higher average levels of primary dormancy in spotted knapweed compared to diffuse knapweed might be the 'result of the inherent differences in seed dispersal in the two species. Diffuse knapweed seeds are retained on the plant for prolonged periods because the capitula, except for a small distal opening, remain closed (Watson and Renney,
1974). Conversely, spotted knapweed seed heads open widely when mature and the seeds are loosely held in the capitula so they are dispersed quickly. Consequently, spotted knapweed seeds would undergo less after-ripening in the capitulum than seeds of diffuse knapweed. If seed germination behavior of the two species is comparable at the 8 time of maturation, and primary dormancy is lost during after-ripening, the capability of diffuse knapweed to retain seeds in the capitula for longer periods would favor after ripening resulting in lower levels of primary dormancy in field populations of this species.
Seed germination characteristics may influence the geographic range of adaptation of a species (Bewley and Black, 1982). Knapweed infestations are generally found in areas experiencing soil disturbance, overgrazing, or seasonal drought (Harris and Cranson, 1979; Popova, 1960; Watson and Renney., 1974) , and are uncommon in irrigated, shaded, or moist areas (Meyers and Berube, 1983; Watson and Renney, 1974). All of the above factors affect the abundance of vegetative cover, which in turn influences the quality of light incident upon seeds present on the underlying soil surface. The absorption of the red wavelengths of energy in sunlight by chlorophyll in a plant canopy causes a decrease in the RzFR (660:730 nm) ratio as the leaf index increases (Frankland, 1981). Rangelands dominated by bunchgrasses provide favorable conditions for knapweed seed germination due to bare areas between plants that allows R to reach the soil surface.
The effects of stratification, temperature, and water stress on spotted knapweed seed germination were investigated by Eddleman and Romo (1986). Optimum temperature range for germination was 15 to 25° C. 9 Temperatures above or below this range resulted in reduced germination suggesting an adaptation that minimizes germination in late summer or early fall when seedling mortality may be high under conditions of transient moisture. Low temperature-induced dormancy may be an adaptation that restricts germination in late fall, when slow growth rates could lead to seedling mortality. High mortality of spotted knapweed seedlings has been noted during severe winters (Watson and Renney, 1974) when extreme low temperatures and low snow accumulation occur simultaneously. Widespread winterkill of spotted knapweed was noted for the winter of 1988-89 in western Montana
(personal observation). However, the observed winterkill reduced plant density only slightly.
Immediately after harvest, stratification for 30 or 60 days alleviated a portion of the dormancy in spotted knapweed seeds incubated in the dark (Eddleman and Romo,
1988). Seeds of spotted knapweed under field conditions are subjected to cold-dry and cold-wet conditions during the fall and winter. Stratification increased spotted knapweed seed germination at both supraoptimal and suboptimaI temperatures (Eddleman and Romo, 1988). Loss of dormancy at colder temperatures would enhance early emergence of seedlings in the spring. This would maximize the period of available soil moisture for newly developing plants and avoid competition from later emerging species. Such 10 adaptations such as this that place seedlings in a favorable time-moisture relationship are critical for establishment of seedlings on grassland sites (Harris, 1967). Dry after ripening and stratification increase seed germination of diffuse and spotted knapweed seed.
Plant Development
Spotted and diffuse knapweed bolt in May after overwintering as rosettes. Diffuse knapweed produces a single branched stem, two year old spotted knapweed plants produce one to six stems per plant, and older plants typically produce more than a dozen branches (Watson and Renney, 1974). Rosettes begin to bolt and immature flowers are first observed in mid-June. Stems and branches elongate and flower heads continue to appear on the end of each branch throughout the summer. Flowering begins in mid-July, about 2 weeks earlier for spotted knapweed than for diffuse knapweed. Individual flowers remain open for 2 to 6 days. The Centaurea species are cross-pollinated by insects and mature seeds are produced 18 to 26 days after fertilization (Watson and Renney, 1974).
Diffuse and spotted knapweed are adapted to a wide range of soil types. Susceptibility to invasion is directly related to the degree of disturbance of the soil surface. However, these two species are not adapted to cultivated land or land under irrigation. Spotted and diffuse knapweed 11 thrive in areas where an arid period occurs during the summer months (Harris and Cranston, 1979). Open habitats are preferred, although spotted knapweed will invade disturbed forest soils at relatively high altitudes.
Spotted knapweed reproduces vegetatively and is classified as a short-lived perennial (Watson and Renney, 1974). Lateral shoots emerge 2.5 to 7.5 cm from the mother plant and form rosettes. Plants normally survive for 3 to 5 years which accounts in part for the dense stands formed by spotted knapweed. Diffuse knapweed is classified as a biennial or sometimes as a triannual (Watson and Renney, 1974) .
Boggs and Story (1987) studied the population age structure of spotted knapweed in Montana. Life span and mean age of spotted knapweed plants were determined by counting annual rings. Regardless of environmental conditions of the season, one ring of secondary xylem was added to the taproot of known-age spotted knapweed plants annually. Root ring counts at six study sites indicate spotted knapweed plants can live for at least 9 years. The average age of spotted knapweed plants, excluding plants less than I year old, ranged from 3.4 years in 1984 to 5.0 years in 1985. Based upon these data and the definition of a perennial as any plant living longer than 2 years
(Hitchcock and Cronquist, 1978), Boggs and Story suggest 12 that spotted knapweed should be classified as a perennial rather than either a biennial or short-lived perennial.
Seed Production and Dissemination
Massive seed production is a major competitive device of the knapweeds. Watson and Renney (1974) reported that diffuse knapweed produced 925 seeds per plant when grown on rangeland and 18,248 seeds when grown under irrigation. Spotted knapweed produced 436 and 25,263 seeds per plant when grown under dryland and irrigated conditions, respectively.
Schirman (1981) reported that diffuse knapweed is adapted to drier climates than spotted knapweed. In a 3- year study, which included prolonged drought conditions, diffuse knapweed seed production was more stable than spotted knapweed. Diffuse and spotted knapweed produced approximately 30,000 and 48,000 seeds/m2, respectively. The higher seed production by spotted knapweed was due to multiple flower stem production. While there was no correlation between spotted knapweed height and the number of flower shoots or flower heads produced per plant, there was a correlation found between the number of flower heads and shoot production per plant (Story, 1976).
Spotted knapweed plants produced 32 seeds per head and
29 heads per plant (approximately 1,000 seeds per plant) (Story, 1976). If 80% seed survival is assumed (Schirman,
/ 13 1981; Watson and Renney, 1974), the soil reservoir of spotted knapweed seed increases exponentially each year. The longevity of spotted knapweed seed viability has not been determined. However, preliminary results by Schirman (1984), Chicoine (1984), and Lacey (1985) indicate that, buried spotted knapweed seed remains viable in the soil for more than 5 years.
Each Centaurea species has a unique method of seed dispersal. Diffuse knapweed plants are commonly globe- shaped at maturity and have an abcission layer on the stem near the soil surface enabling them to break loose and tumble (Watson and Renney, 1974). The capitulum of diffuse knapweed is more constricted than spotted knapweed which favors seed retention prior to tumbling. Seeds are released as the plant rolls along the ground. Spotted knapweed does not disseminate its seeds over long distances since the flower stems do not tumble and the pappus is too small to permit wind dissemination of the seed. The bracts enclosing the flower heads begin to open approximately 3 weeks after maturity (Watson and Renney, 1974). As the relative humidity fluctuates the bracts open and close which loosens the seeds causing them to rise to the top of the capitulum (Watson and Renney, 1974). Seeds are disseminated by a flicking motion caused when plants are moved abruptly which scatters the seed for distances of up to I meter (Strang et al., 1979). 14 Spotted knapweed is moved from one locality to another by man and animals. The bristles of the pappus enable the seed to loosely adhere to animal hair or fur allowing transport over short distances (personal observation).
Motorized vehicles are primarily responsible for the rapid long distance spread of spotted knapweed seed throughout North America (Mass, 1985; Watson and Renney, 1974). Spotted knapweed seed floats on water due to its waxy pericarp and bristly pappus which tends to trap air (personal observation) and so waterways also serve as transportation avenues for seed.
Allelopathy
There is some evidence that Centaurea species utilize allelopathy to maintain stand densities. Fletcher and
Renney (1963) partially characterized the suspected allelochemical as an indole. The highest concentration of allelochemical was found in the leaves. Russian knapweed (Centaurea repens L.). had the highest concentration of the allelochemical, followed by diffuse and spotted knapweed.
Soils from Russian knapweed infestations retarded growth of tomato (Lycopersicon esculentum) and barley (Hordeum vulgare
L.).
Kelsey and Locken (1987) used chloroform extraction to obtain compounds from spotted knapweed tissue. Column chromatography of extracts provided 17 fractions that were 15 tested for toxicity using a bioassay. All fractions showed some inhibition of lettuce germination and growth. A crystalline compound was isolated from the most inhibitory fraction and identified as cnicin, a sesquiterpene lactone.
CO — CHOH--CH OH
CNICIN
Seeds of several native plant species were incubated in varying concentrations of cnicin to determine the effect on germination and growth. Germination of crested wheatgrass (Agropyron cristatum (L.) Gaertn.), bluebunch wheatgrass (Agropyron spicatum (Pursh)Scribn. & Smith), and rough fescue (Festuca scabrella Torr.) was unaffected by up to 2 mg of cnicin per 9 cm diameter petri dish. Concentrations of 4 mg or greater reduced germination of crested and bluebunch wheatgrass 50 and 30% respectively. Germination of rough fescue seed was reduced 50% by concentrations of 8 and 10 mg per petri dish. Root growth was more sensitive to cnicin since there was some inhibition at nearly all levels tested. Of the two conifer species tested, western larch
(Larix occidentalis Nutt.), and lodgepole pine (Pinus contorta Dougl. ex Loud.), western larch was more sensitive to cnicin. Lodgepole pine germination was not affected by the highest cnicin concentrations tested. Spotted knapweed 16 seed germination was unaffected by cnicin, but seedling growth was strongly retarded at all concentrations. Under the conditions of these experiments, spotted knapweed is as sensitive or more sensitive to cnicin than the other species tested. While there is little published information on the mode of action of sesquiterpene lactones in plant tissue, there is substantial data from other organisms. In animal cells, the lactones inhibit DNA synthesis, RNA synthesis, respiration, and enzyme function resulting in impaired growth and development (Rodriguez, Towers, and Mitchell, 1976). Two types of inhibitors were isolated from diffuse knapweed. Polar and nonpolar inhibitors were extracted from both aboveground tissue and roots (Muir and Majak, 1983). A
ryegrass (LoH u m multiflorum L.) bioassay of fractionated chloroform extracts from leaves and stems led to the isolation of cnicin. Impure cnicin, and the remaining column fraction it was isolated from, inhibited ryegrass seedling growth. Purified cnicin at a concentration of 4 mg/ml did not inhibit germination or growth and therefore was rejected as the cause of inhibition. The remaining polar inhibitor was not identified. Although cnicin was the most abundant inhibitor found in knapweed tissue, numerous
other inhibitory compounds were reported (Muir and Majak,
1983). The alleged allelopathic properties of the knapweeds 17 may be a result of additive effects of cnicin and other uncharacterized inhibitory agents.
Germinating diffuse and spotted knapweed seeds did not produce or exude chemicals that inhibit germination of barley, crested wheatgrass, or ryegrass (Muir and Majak, 1983). Locken and Kelsey (1987) described the distribution of cnicin in spotted knapweed. Cnicin was found in the glandular trichomes that protrude from the epidermis. Knapweed trichome density and cnicin concentrations were highest in leaf tissue. Leaves attached to the branches of the inflorescence contain the most cnicin, followed by those on the main stem. Rosette leaves contained the least.
Cnicin was not detected in roots or flower heads of spotted knapweed. Cnicin concentrations were highest in September and October; it comprised 2.1% and 2.3% of the total dry weight of spotted knapweed tissue respectively. Only negligible quantities (0.7 ppm) of cnicin were found in soil infested with spotted knapweed (Locken and Kelsey, 1987).
In order for cnicin to function as an allelopathic compound it must enter the environment in sufficient concentrations to be toxic. Levels of cnicin found in soil were well below the concentrations needed to inhibit plant growth in bioassays (Kelsey and Locken, 1986).
Crystalline cnicin has limited water solubility. It is stored in trichomes surrounded by a hydrophobic cuticular 18 sac, therefore leaching of cnicin from leaves to the soil by rain is. unlikely. Even if cnicin leaches from spotted knapweed tissue, leaching would occur gradually and large quantities of cnicin would be required to accumulate in the soil to reach toxic levels.
The four modes of release of allelochemicals from plants to the environment include volatilization, exudation from roots, leaching, and decomposition of residues (Rice 1974, 1984). Since cnicin is not volatile, it does not occur in roots, and leaching from foliar tissue is slow (Locken and Kelsey, 1987), decomposition of litter is the most likely route for releasing cnicin (Kelsey and Bedunah, 1989). An important factor in decomposition is the rate of litter deposition on the soil surface, which is relatively slow and variable with spotted knapweed. Therefore, Locken and Kelsey (1989) concluded that cnicin probably does not routinely reach toxic concentrations in the soil.
In greenhouse studies, germination and growth of western larch and ponderosa pine (Pinus ponderosa Dougl.) were inhibited when dried spotted knapweed tissue was mixed in the rooting media, but only at levels 2.5, 5, and 10 times higher than levels normally found in spotted knapweed infested soil (Bedunah, 1988). Similar studies with lodgepole pine, Douglas fir (Pseudotsuga menziesii (Mirbel)
Grenco), and western larch showed no phytotoxic effect when spotted knapweed tissue was incorporated in soil (Bedunah, 19 1989). Under natural conditions, spotted knapweed litter is probably not phytotoxic to conifers or grasses since grasses were less sensitive than forbs to cnicin (Kelsey and Locken, 1987) .
Cultural Control
Plowing Plowing deeper than 5 cm will control diffuse and spotted knapweed (Spears, et al., 1980). Eddleman and Romo (1986) showed that seedlings of diffuse and spotted knapweed did not emerge from depths of 3 and 5 cm, respectively. The land should be reseeded immediately with a vigorous grass or legume to suppress reinfestation (Harris and Cranston, 1979) .
Mowing
Popova (1960) reported that diffuse knapweed density increased when mowed, contrary to the findings of Watson and Renney (1974). Watson and Renney (1974) measured a reduction in seed production when plants were mowed in the flowering stage. The continued production of low growing flowering branches permitted some flowers to "escape" mowing.
Burning
Knapweed plants retain their leaves, stems and flowerheads at senescence. These dry structures persist for 20 years and burn readily (Carpenter, 1986). However, the use of fire is generally discouraged since it can damage blue- bunch wheatgrass and is hard to control on open rangeland. Burning can increase competitiveness of some grasses. Popova (1960) reported that diffuse knapweed was almost entirely replaced by grass species 2 years after burning.
Burning eliminates seeds retained in the capitulum and may kill seeds on the soil surface (personal observation). Zednai (1979) found the germination of spotted knapweed was reduced from 68 to 3% after a burn. Strang et al. (1979) indicated that, in spite of its invasiveness, spotted knapweed rarely invades burned areas. However, the degree of soil disturbance associated with the burn and proximity of a seed source can influence establishment. Burning does not appear to reduce the soil seedbank of a natural population of spotted knapweed seed (Chicoine, 1984).
Controlled burning may be an economical method of reducing spotted and diffuse knapweed densities on low value land.
Grazing
Grazing of spotted knapweed in spring and early summer by sheep, goats and cattle has been reported in western
Montana (Cox, 1983 & 1989; Robertson, 1989; Spoon et al., 1979). Kelsey and Mihalovich (1987) determined that the nutrient content of spotted knapweed was comparable to native forage plants and adequate to meet livestock needs 21 during spring and early summer when stems were succulent and actively growing. Cnicin in spotted knapweed leaves imparts a bitter taste and may decrease palatability (Kelsey and Mihalovich, 1987; Watson and Renney, 1974). Developing seed heads contain only trace amounts of cnicin and are selectively eaten by livestock and wildlife (Bucher, 1984; Cox, 1989; Spoon et al., 1983; Watson and Renney, 1974 ) which could be exploited to reduce seed production of spotted knapweed. Spotted knapweed silage contained 50% less cnicin than fresh plant material and was comparable in chemical composition to alfalfa and hay silage (Kelsey and Mihalovich, 1987). ) Producer Experiences John Robbins manages the Double Fork Ranch in Ravalli county, MT which is heavily infested with spotted knapweed.
He has developed a grazing management program with cattle to utilize spotted knapweed after many attempts to eliminate it
(Robertson, 1989). He uses high intensity - short duration grazing and monitors cattle feeding habits and performance. After 3 years he still has knapweed and does not expect to eradicate it.
Robbins feels that knapweed is high in nutritive value, their cattle graze it readily, especially in spring, and the cattle have consistently performed well. Results from the monitoring program indicated mixed results but Robbins feels that the program will succeed in the long run. Robbins 22 states that when cattle stocking density is increased, they graze differently than under conventional stocking rates, including a significant change in grazing preference. Allan Savory, founder of the Center for Holistic Resource Management, visited,the Double Fork Ranch in
September, 1988 and concluded: "As I looked at the successional levels, water and mineral cycles and energy flow on the Robbins Ranch, I could only come to one conclusion - that the knapweed, far from being a problem, is actually an asset. In my opinion, attempts to eradicate it are unfortunate and unnecessary" (Robertson, 1989).
At the Knapweed Symposium held in Bozeman, MT in April, 1989, James Cox (1989) reported that sheep readily graze
spotted knapweed on his 80 acre river bottom property west of Missoula. Cox measured palatability of spotted knapweed versus annual brome grasses and forage legumes by counting
plants grazed along a transect within an enclosure. Measurements taken on July 2 and 3, 1987 indicate sheep selectively grazed spotted knapweed over the annual grasses, birdsfoot trefoil (Lotus corniculatus L.) and sanfoin
(Onobrychis viciaefolia). Grazing spotted knapweed on
July I was more effective in controlling flowering than
on June I. Intensive grazing of spotted knapweed by sheep would be most effective when spotted knapweed is in the flower stage, well after grasses have set seed. This approach would stress spotted knapweed when carbohydrate 23 levels are lowest and when grasses are summer dormant, thus reducing knapweed propagation without affecting grass reproduction.
Fred Mass, from Thompson Falls and Charles Hahnkamp, from Melstone have been instrumental in the formation of cooperative weed management groups involving ranchers, private land owners, weed boards, sportsman groups and county, state and federal personnel (Petersen, Hahnkamp and
Dodge, 1989). These and numerous other locally controlled weed cooperatives have successfully incorporated an integrated control approach to weed management including development of educational programs and the petitioning for legislative support. Fred Mass (1989) has been a strong proponent of increased public awareness of the "knapweed invasion" and an advocate of strict control of spotted knapweed along roadways and other transportation avenues.
Economic Impact
Spotted knapweed threatens the long-term productivity of native rangelands. Spotted knapweed has spread at an annual rate of 27% since 1920 to infest approximately 4.7 million acres in Montana (French and Lacey, 1983; Lacey, 1987). Chicoine, Fay and Nielsen (1985) matched the soil type, elevation, annual precipitation, evapotranspiration, frost- free season and maximum July temperature of 116 spotted knapweed infestations with maps developed from satellite 24 images. They estimate that about 50% of Montana (46.5 million acres) could support spotted knapweed infestations. When cultivated lands are subtracted from this total> nearly 34 million acres of range and grazeable woodland are vulnerable. The annual loss of income in Montana due to forage reduction caused by knapweed is estimated at $4.5 million (French and Lacey, 1983). If all vulnerable land became infested, loss estimates would be about $1.7 billion, almost three times the current annual gross income from cattle and sheep (Bucher, 1984). The impact of spotted knapweed on elk winter range is a growing concern in Montana since the plant is known to displace desirable forage species (Harris and Cranston,
1979; Maddox, 1979; Morris and Bedunah, 1984; Watson and Renney, 1974). Species diversity is inversely related to spotted knapweed stem density indicating that spotted knapweed invasion can alter plant community composition (Tyser and Key, 1988).
In forest clearcut sites spotted knapweed invasion was found to reduce tree seedling survival and wildlife forage (Willard et al., 1988). Many studies indicate that elk have a strong preference for grass during the winter and spring.
Mooers (1986) reported that knapweed grew on every habitat type in western Montana, but was best adapted to drier grassland types, many of which are important sites for overwintering elk. Tyser and Key (1988) found that spotted 25 knapweed was capable of expanding within Glacier National Park's fescue grasslands which serve as important elk winter range. Spoon et al., (.1983) referring to the noxious weed infestation problems of the Lolo National Forest in Montana stated:
nThe forage production lost on big game winter ranges could theoretically result in a loss of 220 elk annually by the year 1998. The resulting loss of big game populations has an adverse effect on hunter success and the incomes of outfitters and the state's tourist industry."
A study designed to measure the plant community response of three elk winter ranges to the control of spotted knapweed following herbicide application was conducted by Bedunah and Carpenter (1989). Two of the sites were dominated by bunchgrasses and the other site.by bluegrasses.
The level of spotted knapweed invasion varied by site. Two herbicides were investigated, picloram (TordonR) and clopyralid (Stinger*) at three rates of application, .14, .28 and .42 kg a.i./hectare. All treatments provided complete control of spotted knapweed the year of spraying. Two seasons after application, the two highest picloram treatments continued to provide excellent control of spotted knapweed. Grass production increased two to three-fold 24 months following application. Native perennial forb densities were not adversely affected by herbicide 26 application primarily because the spotted knapweed infested land had fewer perennial forks than the herbicide treated (knapweed-free) plots. The authors concluded that spotted knapweed had a greater impact on decreasing species diversity than the herbicides used to control spotted knapweed.
The low productivity of knapweed-infested winter range sites will not improve until the spotted knapweed is eliminated for a sufficient amount of time to allow the native grasses and forks to become reestablished. This recovery occurs very rapidly following treatment with picloram or clopyralid. Spotted knapweed will begin to reinfest herbicide treated areas within 2 to 3 years due to
the presence of dormant seeds that persist longer than the soil residual properties of the. herbicides (Chapters 2 and 3) . A retreatment of the area within 6 to 1,0 years may be necessary to maintain the vigor and competitive advantage of the grass community.
Biological Control
Spotted knapweed was introduced to North America around
1900 as a contaminate in Turkistan alfalfa seed (Groh, 1940). Since its introduction spotted knapweed has spread to infest over 7 million acres throughout at least 9 states and 2 Canadian provinces (Lacey, 1989). 27 A major factor contributing to the tremendous success of spotted knapweed in North America has been the lack of predators. In southeastern Europe, where knapweeds are native, a complex of insects exists which has evolved with Centaurea species and feed exclusively on them. As a result, knapweeds in Europe exist in small, widely scattered patches along disturbances and roadways. There are no vast infested acreages as there are in North America (Schroeder, 1985). Surveys of potential biological control agents started in 1961 and eventually developed into an aggressive biocontrol effort led by the Canadian and U.S. governments.
At the Knapweed Symposium held in Bozeman, Montana in June 1989, Story (1989) summarized the status of the biological control effort on spotted and diffuse knapweed. The first biological control agent to be employed against the knapweeds was a small European seed head fly, Urophora affinis, that was introduced to British Columbia in 1970
(Harris, 1980). In 1973 the same species was released in Montana and Oregon (Story and Anderson, 1978'; Maddox, 1982) .
A second seed head fly, Urophora quadrifasciata, was released in British Columbia in 1972 (Harris, 1980).
Although the second seed head fly was never released in the United States, it entered the U.S. in the early 1980's and is now widely self-distributed throughout the knapweed- infested areas of the Pacific Northwest. Both Tephritidae flies attack spotted and diffuse knapweed seed heads. The 28 larvae induce gall formation in the receptacle tissue of the flower buds. The galls act as metabolic sinks and sequester nutrients and sugars resulting in reduced seed production (Story, 1978). Harris (1980). reported up to 95% reduction of seed production in British Columbia where both species were present. In Montana seed reduction caused by the larvae ranges from 50% to 75% (Story, 1984).
Considerable research has been conducted on the Urophora species in the Pacific Northwest in the 20 years since their introduction. Important information has been obtained on the biology, life history, establishment, dispersal, impact on spotted and diffuse knapweed, interspecific competition, within-head distribution procedures, feeding strategy, and numerical sampling requirements of both flies (Berube, 1980; Gillespie, 1983; Harris, 1980a; Harris, 1980b; Harris, 1988; McCaffrey and Callihan, 1988; McCaffrey et al., 1987; Story and Anderson, 1978; Story, 1984; Story and Nowierski, 1984; Story et al., 1987; Story et al., 1988). A seed head moth, Metzneria paucipunctella, was released in British Columbia in 1973 and numerous collections of the moth have been made at the original release site for
subsequent release in the U.S. It is now established in
Montana, Idaho, Oregon, and Washington. Larvae of this moth feed on the seed, each larvae attacking about six seeds per seed head. Although several eggs may be laid, only one M. paucipunctella larva survives per seed head. The moth 29 larvae are predatory and can destroy up to 63% of U. affinis and U. quadrxfasciata larvae when they are encountered in infested seed heads (Story, et al., 1989) Despite this interspecific competition, the combined effect of the two seed head flies and the seed head moth significantly reduced seed production in attacked seed heads below the level caused by the two fly species alone (Story and Nowierski, 1984; Story, et al.).
Although the seed head insects have proven to be very effective at reducing seed production, their combined effect has not reduced population density of the knapweeds (Harris,
1980). As a result, a survey of potential rosette and root- feeders was conducted. Based on this survey four root feeding species, including three moths, Agapeta zoegana (L.)
(Lep.: Cochylidae), Pelochrista medullana Stgr. (Lep.:
Tortricidae) and Pterolonche inspersa Stgr.
(Lep.:Pterolonchidae) and one weevil, (Cyphocleonus achates (Fahr.) (Col.: Curculionidae) were selected for introduction into North America between 1979 and 1983 (Muller et al., 1988; Muller, 1989; Muller et al., 1989). The root weevil was released in British Columbia in 1987 where a small population is established. The larvae of this weevil feed on the core of large knapweed roots causing stress on the plant. The first release of the weevil in the
U.S. was made in Montana in 1988 (Story, 1989). 30 The root moth Pterolonche inspersa was introduced into British Columbia in 1986 where a small caged population is now established. The larvae of this moth feed on the core
of knapweed roots. While this moth has been released in Montana, Idaho, Oregon and Washington, its status is not known at this time.
The larvae of the root moth Pelochrlsta medullana feed in the cortex and epidermal tissues of knapweed roots. A
small population is established in British Columbia. Adults received in 1984 in Montana failed to produce fertile eggs. Inability to locate the moth in Eurasia since 1984 has prevented subsequent introduction during the past four years.
The root moth Agapeta zoegana is the most promising insect introduced on spotted knapweed to date. Larvae of the moth feed on the cortex and epidermal tissues of
knapweed roots. Heavy attack by the larvae can cause death to small plants (Muller et al., 1988). This moth is now established in British Columbia, Montana, and Washington.
The root beetle Sphenoptera jugoslavica was initially established in British Columbia and has been redistributed
to Idaho, Washington and Oregon. The larvae feed on the core of diffuse knapweed roots, and under favorable
conditions, can reduce diffuse knapweed density (Powell and Meyers, 1988). 31 Three additional insects are being screened for potential use on spotted knapweed: I) a seed head fly, Terellia virens; 2) a seed head fly, Chaetorellia sp.; and 3) a seed head weevil, Larinus obtusus. Larvae of T. virens and L. obtusus feed on the soft, developing seeds while Chaetorellia sp. larvae feed on the ovaries. These three species should complement the effects of the two established gall flies, Urophora spp., because the three insects attack the flower buds at a later date and, in the case of Chaetorellia sp., are more adept at attacking isolated plants (Harris, 1988).
Two insect species are currently undergoing host specificity screening for use on diffuse knapweed. The larvae of a seed head weevil, Bangasternus fausti, feed in the seed head receptacle. Recent concern has been expressed about this weevil since it reportedly destroys all other insects in the seed head, especially U. affinis. The second is the seed head weevil, Larinus minutus. Larvae of this insect feed on the soft, developing seeds. Of the remaining insects known to be associated with spotted knapweed in Eurasia, the insect with the most potential is the. small seed head gall wasp, Isocolus minutus. This wasp forms a woody gall in immature flower buds.
Harris and Cranston (1979) suggest that the establishment of a complex of at least six natural enemy 32 species would be necessary for biological control to be effective against diffuse and spotted knapweed. This complex must include significant feeders attacking different plant parts, such as seed head feeders and root-crown feeders. If screening tests proceed satisfactorily, a total of 11 and 10 insect species will have been introduced against spotted and diffuse knapweed respectively by 1995 (Story, 1989).
Other natural enemies such as plant pathogens have been investigated for their potential as biocontrol agents for the knapweeds. Puccinia spp. have been isolated from spotted and diffuse knapweed in Eurasia (Watson and Clement, 1986). Several of these rust pathogens are excellent candidates for biological control because they have restricted host ranges, are autoecious, and are capable of producing abundant quantities of uredospores which facilitate their spread. Unfortunately, host specificity tests conducted in controlled environment facilities indicate that safflower (Carthamus tinctorius L.) is a potential host of the rust pathogen Puccinia jaceae which attacks diffuse knapweed and Puccinia centaureae which attacks spotted knapweed (Watson and Clement, 1986). f Another fungus, Sclerotinia sclerotiorum (Lib) de Bary, was isolated on diffuse knapweed and identified by Watson,
Copeman and Renney (1974). Inoculation of spotted knapweed 33 with S. sclerotiorum resulted in the production of symptoms identical with those observed on diffuse knapweed.
Sclerotinia sclerotiorum is a well-known plant pathogen with a very broad host range (Purdy, 1979). In order for this naturally-occurring fungus to be utilized as a biocontrol agent against spotted and diffuse knapweed it would have to be contained so as to negate any threat to non-targeted crop plants. Research was begun in the early 19801s at Montana State University to produce stable clones of S. sclerotiorum with limited ability to spread or survive for more than one season (Ford, 1989). Two types of mutants were obtained through ultra-violet light mutagenesis. The first mutant is auxotrophic for cytosine. The second mutant
has limited ability to overwinter because it lacks the ability to produce sclerotia, survival structures comprised of dense, compacted aggregates of hyphae. They are
resistant to unfavorable environmental conditions and are capable of remaining dormant for long periods of time.
Also, sclerotia are precursors to ascospore production, the main means of dissemination of wild type S. sclerotiorum.
A critical requirement for infection is the presence of a readily available nutrient source. Numerous studies have shown that Sclerotinia ascospores are unable to infect
healthy hosts plants unless they first contact and colonize
a suitable organic nutrient source* Furthermore, cool wet weather or irrigation seems to be necessary for infection to
\ 34 occur. In field studies inoculum placed on grain, such as millet or oats, provided a nutrient base allowing the
Sclerotinia hyphae to grow and form appressoria that readily penetrate the host plant tissue, causing disease when adequate moisture was present. Stierle et. al. (1988) discovered the black leaf blight fungus Alternaria alternata Lam. on spotted knapweed. They isolated the phytotoxin maculosin from the fungus and synthesized the compound in the laboratory. Although the specific mode of action and field efficacy of maculosin is not known, it appears that both the naturally-occurring and synthetic forms of the compound are highly phytotoxic only to spotted knapweed. However, due to genetic diversity of spotted knapweed populations, some plants and seeds survive maculosin treatment.). Cuda, Sindelar ,and Cardellina (1989) proposed that established populations of Agapeta zoegana L., a root mining moth, would control these surviving plants. Overseeding the treated site with aggressive forage grasses would inhibit spotted knapweed seedling establishment and replace knapweed plants weakened from combined effects of maculosin and A. zoegana.
Maculosin has not been field tested and therefore performance outside of the laboratory has not been demonstrated. It appears that maculosin would need to be applied in a similar manner as conventional herbicides, but 35 uptake, translocation and persistence information is lacking. Biological control will contribute greatly to the longterm management of the knapweeds in North America; however, the ultimate benefits will not be evident for many decades. The implementation of effective biological control programs must include sound land resource management which favors desirable competing plant species. Muller and Schroeder (1989) concluded that establishment of a competitive grass cover is most effective for both the reduction of knapweed density and the long term stabilization of its population by refilling its niche once the knapweed population has declined.
Chemical Control
Herbicides are an effective means of controlling spotted and diffuse knapweed. Two,4-D ((2,4-dichlorophenoxy)acetic acid) applied from the bolt to early flowering stage of growth provides adequate control of established plants and excellent control of seedlings (Lacey et al., 1986). Annual applications of 2,4-D at rates of 1.12 to 2.24 kg a.i./ha halted seed production of spotted knapweed (Belles, et al.,
1980; Wattenbarger, et al., 1980). Reapplications are necessary to control regrowth of older established plants since translocation of 2,4-D to the root system of spotted 36 knapweed is minimal (Lacey, 1985). The ester formulations are more effective than amines (Belles, et al., 1978). Dicamba (3,6-dichloro-o-anisic acid) has soil residual properties that make it more effective than 2,4-D, giving 2 to 3 years of spotted knapweed control depending on the level of plant competition following treatment (Fay, et al., 1989). Application rates of 1.1 kg a.i./ha provided excellent control when applied in the rosette stage (Lacey et al., 1986).
Picloram (4-amino-3,5,6-trichloropicolinic acid) is the most effective herbicide for long term control of spotted and diffuse knapweed (Lacey, 1985; Penney and Hughes, 1969;
Wattenbarger et al., 1980) . Picloram applied at a rate of 0.28 kg a.i./ha has provided up to 5 years of complete control on certain sites (Belles et al., 1980; Fay et al., 1989). This extended period of control is due to soil persistence of picloram and plant competition after treatment which discourages reinfestation (Chicoine, 1984; Penney and Hughes, 1969).
Several factors influence the degradation and movement of picloram in soil. The decomposition rate increases in the presence.of plant roots, high soil organic matter levels, elevated moisture and soil temperatures, and acidic soils (Goring and Haymaker, 1971).
Picloram is susceptible to leaching because of its relatively high water solubility and low adsorption to soil 37 colloids (Bovey and Scifresz 1971). Resistance to leaching is correlated with adsorption. Leaching and sorption characteristics are influenced by soil properties and seasonal precipitation. Picloram has a pKa of 3.6, therefore the proportion of ionized to non-ionized picloram decreases with decreasing soil pH. Maximum soil adsorption occurs in low pH soils with high organic matter and hydrated iron and aluminum oxides. Adsorption is minimal in neutral or alkaline sandy loam soils low in organic matter. Spotted knapweed thrives on neutral to high pH, well-drained sandy or gravelly soils in Montana which may account for the shorter.duration of control achieved with picloram at these sites.
Picloram is susceptible to rapid photodecomposition on plant and soil surfaces (Hall et al.1968; Rice, 1989).
Rice (1989) applied picloram at 0.28 kg a.i./ha and measured 68 and 42% loss from vegetation and soil surfaces respectively after 7 days without precipitation during June in Montana. Although 90% of the picloram applied may intercept the vegetative canopy, only I to 2% of the foliar applied picloram is. actually absorbed into the plant (Bovey, Davis and Merkle, 1967). Therefore, picloram is vulnerable to photodecomposition until the first rainfall occurs and moves it into the soil.
Spotted knapweed seedlings begin to reestablish when picloram concentrations in soil drop below 0.012 ppm 38 (Watson, Rice and Monnig, 1989). Picloram provides long term control of spotted knapweed when site conditions and timing of precipitation following application are optimum (Cranston, 1984). Precipitation is necessary for incorporating most of the picloram into the soil where it is available for root uptake and protected from decomposition by sunlight.
Clopyralid (3,6-dichloro-2-pyridinecarboxylic acid) is a pyridine herbicide closely related to picloram. Clopyralid is more selective than picloram and has a shorter soil residual period. Rates of 0.28 kg a.i./ha provided 100% control of spotted knapweed I year following application at two sites in Montana without altering the native forb density or diversity (Lacey, McKone, and Bedunah, 1989).
Seedling reestablishment occurs by the second year after application so reapplication is necessary to prevent seed production. The narrow weed control spectrum of clopyralid makes it a valuable herbicide for spotted and diffuse knapweed control in environmentally sensitive areas where the broad spectrum herbicide picloram can't be used.
Spotted knapweed displaces native vegetation and often results in near monoculture infestations. In these situations chemical treatment with picloram or clopyralid at a rate of 0.28 kg a.i./ha results in greater plant community diversity than exists in untreated, knapweed dominated sites (Bedunah and Carpenter, 1989; Lacey, McKone and Bedunah, 39 1989). Reseeding infested areas following herbicide treatment increases forage production and augments control of spotted and diffuse knapweed by suppressing seedling reestablishment (Fagerlie, 1989; Hubbard, 1975). 40
CHAPTER 2
SPOTTED KNAPWEED SEED LONGEVITY
Introduction
Spotted knapweed currently infests over 4.7 million acres of grazeable woodland, rangeland, and pastureland in Montana (Lacey, 1987). The plant reproduces by seed, and individual plants in a natural' setting produce an average of 1,000 seeds (Story, 1976). Approximately 90% of the seed produced by spotted knapweed is viable (Schirman, 1981). The longevity of viability of spotted knapweed seed has not been determined. Previous research showed that 40% of buried spotted knapweed seed remained viable in soil for at least 5 years (Shirman, 1984). The objective of this study was to determine the long-term viability of seed in soil.
Materials and Methods
Seed Burial Study
This study was established at two locations in Montana by Chicoine (1982). Site One was located at the Research Farm near Bozeman in a 46 cm rainfall area on a Bozeman silty clay loam soil (fine-silty, mixed, Frigid Argic Pachic CryoborolI). Site Two was located.on rangeland at the Dale
Folkvord farm near Three Forks in a 31 cm rainfall area on a 41
Amesha loam soil (coarse-loamy, mixed, Borollic Calciorthid). Both soils are deep and well drained. Burial rings were constructed by cutting 7.6 cm diameter polyvinyl-chloride (PVC) pipe into 2.5 cm sections. Nylon window screen (16 mesh) was cemented to the bottom of each ring. The rings were half filled with dry soil obtained from the respective burial sites. One hundred spotted knapweed seeds were placed on the soil surface in each burial ring and covered with soil. Nylon window screen was cemented to the top of each ring. The PVC rings were buried flush with the soil surface at both locations in a randomized complete block design on September 22, 1982. The seed was collected on August 30, 1982 from spotted knapweed plants located 4 miles west of the Bozeman site. Initial percent germination of the freshly harvested seed was not determined. Following 2 months of storage at room temperature germination was 100% (Chicoine, 1984).
Five seed containers were recovered at each location on
November 11, 1982, April 7, 1983, May 16, 1983, June 2, 1983, and October 10, 1983. The number of seeds that had germinated while buried and the percentage of viable dormant seed was determined for each collection date. Because of low in situ germination and obvious seed dormancy, the study was revised so that data could be collected for a longer period of time. Three containers were recovered in October of each year at both locations from 1984 through 1987. Seed 42 was recovered by sifting the soil contained in each burial
ring through a 10 nun2 mesh screen followed by a 3 mm2 mesh
screen. The remaining seed and soil mixture was placed on a 0.6 mm2 screen and gently washed with running water for 3 minutes to remove fine soil particles. Seeds were removed from the remaining material by hand. The number of seeds that germinated during burial was determined by counting empty seed hulls. Germination was measured daily on 25 seeds by floating them on distilled water for 7 days at 21° C.
Decline of Viability from a Natural Seed Population in Soil The longevity of viability of spotted knapweed seed in a natural population in soil was studied at two locations. The first site was located on the Blackfoot Clearwater Game
Refuge near Ovando. The second site was located on the
Jones ranch 15 miles south of Harlowton on the east side of Highway 191. Both sites were heavily infested with spotted knapweed when the study was initiated in October, 1982. Five treatments were applied in 1982 by Chicoine (1984) to plots at each site to determine their effect on the longevity of seed viability. Each treatment was an attempt to'increase the rate of germination to hasten the
elimination of the spotted knapweed seed reserve in soil.
Treatments included rolling, harrowing, burning, mowing,
2,4-D application, and an untreated control. Plots were 43 arranged in a randomized complete block design with six 6.1 X 12.2 m plots per block, and three replications at each site. All plots except the untreated control received an annual application of 2.2 kg a.i./ha of 2,4-D amine in mid- June to prevent spotted knapweed seed production. One plot per replication was left as an unsprayed control treatment to permit annual seed production.
A second set of treatments was applied to the plots on May 22, 1985 to determine the germination response of spotted knapweed seed. Treatments performed on whole plots included fertilization with 86 kg 26-13-0 per ha, application of glyphosate at 2.2 kg a.i./ha, application of
2,4-D amine at 2.2 kg a.i./ha, rototilling 7.6 cm deep, and no treatment. Seed production was prevented with an annual application of 2.2 kg a.i./ha 2,4-D. An additional treatment, the original untreated control, was left untreated to permit annual seed production.
The seed population remaining in soil was measured by
taking six 5.6 cm diameter soil cores 7.6 cm deep within
each plot with a tapered tulip bulb planter in April and October from 1982 until 1989. Seed was separated from soil through a series of screening, washing, and air cleaning steps (Chicoine, 1984). Seed from each core was floated on distilled water at 21° C for 7 days and the number of germinated seed were counted daily. 44 Seed Germination Study
Mature spotted knapweed seed was hand collected from Drummond, Bozeman, Anaconda, Bonner, Butte, Hyalite and Florence, Montana in the fall of 1988 and stored at -260 C. A greenhouse study was established to determine the effect of burial on spotted knapweed seed germination. One hundred spotted knapweed seeds from each collection site were placed on the soil surface or buried 0.6 cm deep in 11 cm diameter by 8 cm deep plastic containers filled with a 1:1 mixture of sand and soil (Bozeman silty clay loam). The experiment was arranged in a randomized complete block design with five replications. Temperature was maintained at 24°.C .±.2° C day (14 hours light) and 18° C ± 2° C night. Uniform watering was maintained with an overhead automatic mist system which cycled every 3 hours delivering a total of 5.2 cm of water daily. Seedlings were counted and removed daily until germination ceased. The experiment was conducted twice.
A second germination study was conducted using seeds from the seven collection sites to determine the effect of light on spotted knapweed seed germination. One hundred spotted knapweed seeds from each seed source were placed on blotter paper in 10 cm diameter petri dishes. Ten ml of distilled water was added to each petri dish; half of the petri dishes received water in complete darkness and were then wrapped in aluminum foil to exclude light. Petri 45 dishes were placed in an incubator at 21° C. Seeds receiving light were exposed to 15 hours of light (combination of incandescent and fluorescent lights producing 660 ^mol s 1 m 2) per day. Percent germination was determined after 5 days. Each treatment was replicated five times. This experiment was also conducted twice.
Results and Discussion
Seed Burial Study
The PVC seed containers used for seed burial worked well since seed recovery was nearly 100% throughout the term of the study (Table I). Even when seeds had germinated during the first year, the remains of the original seeds were easily distinguishable after 5 years of burial. This indicates that the outer protective seedcoat is very durable and resistant to decomposition.
Table I. Recovery of Buried Spotted Knapweed Seed.
Months of ______Seed Recovered_____ Burial Bozeman Three Forks
%
12 OVab1 95b 24 99a 100a 36 99a 97ab 48 99a 9 Vab 60 94c 97ab
1Means within a column followed by the same letter are not significantly, different at the 5% level using the LSD test. 46 In situ germination of artificially buried spotted knapweed seeds averaged approximately 38 and 65% after 5 years of burial at Bozeman and Three Forks, respectively (Figure I). This relatively low amount of germination after 5 years indicates that conditions during burial induce and enforce dormancy in spotted knapweed seed since seeds germinated rapidly when removed from soil (Table 2).
Bozeman D Three Forks
MONTHS OF BURIAL
F i g u r e I. Percent Germination of in situ Spotted Knapweed Seeds Recovered Annually From Buried Containers at Bozeman and Three Forks.
Variability in germination occurred among replications from year to year, especially at the Three Forks site, due 47 to uneven settling of the rings after burial. Originally the top surface of each burial ring was flush with the soil surface. After settling, some rings were slightly exposed and some were covered with soil causing a large differential in germination response. Some of the rings that had settled below the soil surface wene saturated with standing water on occasion which stimulated higher rates of in situ germination. We have found that spotted knapweed seeds germinate in only 18 hours when floated on water, compared to 3 to 5 days when placed on wet filter paper (unpublished results). This may partially explain why spotted knapweed is adapted to extremely well drained, gravelly sites where standing water does not occur. The germination variability associated with small microsite changes in this experiment shows spotted knapweed seed germination response and longevity are strongly influenced by environmental conditions.
Viability of ungerminated spotted knapweed seeds recovered from burial rings averaged approximately 90%, which indicates that dormant seeds are well protected from decomposition (Table 2). Over 50% of the original buried seeds were dormant 60 months after burial at both locations (Figure 2). Recovered seeds germinated within 24 hours of the start of imbibition under laboratory conditions suggesting that light or some other rapidly reversible 48 factor related to burial is responsible for maintaining dormancy.
Table 2. Viability of Intact, Ungerminated Spotted Knapweed Seed Recovered Over a 60 Month Period at Bozeman and Three Forks MT.
Months of Germination Burial Bozeman Three Forks --- % ------12 91a1 96a 24 83b 92a 36 78b 50b 48 84b 91a 60 83b 97a 1Means within a column followed by the same letter are not significantly different at the 5% level using the LSD test.
Lewis (1973) investigated the survival characteristics of crop and weed seeds in undisturbed soil in a 20 year seed burial study. He concluded that the survival potential of seeds is revealed during the first 4 years of burial. After four years seed viability decreased approximately 5% per year. At this rate, in the spotted knapweed study, it would take 77 years to reduce the current number of approximately 50 viable spotted knapweed seeds per ring to one seed per ring. Lewis (1973) reported that viability of creeping buttercup (Ranunculus repens L.) seed was 51 percent 20 years after burial in undisturbed soil, a figure almost identical to the viability he measured after 13 years. The long term survivability of this species was attributed to 49
THREE FORKS, MT
BOZEMAN, MT
MONTHS AFTER BURIAL Figure 2. The Number of Viable Spotted Knapweed Seeds Recovered During a 60 Month Burial Period at Three Forks and Bozeman. 50 the durable achene which protects the seed from degenerativesoiI conditions. Additionally, important enzymatic biological repair processes would remain active during burial since the seeds were imbibed intermittently during dormancy. Spotted knapweed achenes, like creeping buttercup, have a thick, durable pericarp that protects the seed but permits water imbibition (Chapter 4).
A second burial study was established by the author in October, 1989 that was designed to last as long as 100 years if necessary to determine the extent of longevity of buried seed.
Viability of Natural Seed Populations
When the soil seed reserve in spotted knapweed infested soil was measured in June, 1982, there were 501 and 647 viable spotted knapweed seeds recovered per m? to a depth of
7.6 cm at Ovando and Harlowton, respectively. The number of viable seed decreased 87 and 72%, respectively, at these locations in the 10 month period from June, 1982 through
April, 1983 (Chicoine 1984, Lacey 1985). The seed reserve had declined 96 and 92% at Ovando and Harlowton, respectively, by April, 1985, 34 months after termination of seed production. One month later Lacey (1985) applied treatments, designed to stimulate germination of the remaining seeds. None of these treatments were effective in accelerating the reduction of seed populations (Table 3). 51 The seed population remaining in soil did not decline measurably from October, 1985 to October, 1989 (Figure 3). In October 1989, approximately 22 and 17 viable seeds remained.per 0.5 m2 (440,000 and 340,000 seeds per ha) at Ovando and Harlowton respectively, 88 months after seed production was halted.
Table 3. Changes in the Seed Reserve in Soil 12 Months After Treatments were Applied in May, 1985 in an Attempt to Stimulate Germination of Spotted Knapweed Seed at Harlowton and Ovando. Viable iseeds ner 0.5 m2 Harlowton Ovando Treatment May, '85 May, '86 May, •85 May, '86
Fertilization 25 ± 18 32 ± 33 32 ± 32 31 ± 10 Glyphosate 22 ± 36 21 ± 18 21 ± 18 10 ± 18 Rototilling 22 ± 36 32 ± 33 42 ± 19 31 ± 10 2,4-D amine 10 ± 18 10 ± 18 10 ± 18 21 ± 18 No treatment 54 ± 68 21 ± 18 10 ± 18 21 ± 37
The rapid decline in the spotted knapweed seed reserve during the first 34 months of the experiment is probably due to germination of seeds located in microsites suitable for germination, on or near the soil surface. I propose that the small but vital population of seed, remaining 3 years after seed production was halted, represents dormant buried seed that has acquired a light requirement for germination.
This will be discussed below. Harlowton MT. Harlowton 3. e r u g i F Following Termination of Seed Production in 1982at Ovando and CM NUMBER OF VIABLE SPOTTED KNAPWEED SEEDS PER 0.5 HS T SE PRODUCTION SEED T U O ITH W S TH N O M The Declinein the Spotted KnapweedPopulationSeed 2 8 4 0 6 2 8 64 58 52 46 40 34 28 22 2 8 4 0 46 40 34 28 22 52 2 8 64 58 52 ALWTON HARLOW OVANDO 6 88 76 53 Germination Behavior
The germination of freshly harvested spotted knapweed seed from.seven locations was 21% higher when placed on the soil surface than when buried 0.6 cm deep (Figure 4). It is unlikely that the difference in germination of buried and surface placed seeds was due to inadequate moisture, temperature or soil atmosphere surrounding the buried seed since I) frequent, uniform watering was provided by the automated misting system, 2) temperature conditions were quite constant, and 3) the shallow seeding depth and well drained soil allowed for adequate gas exchange. Spotted knapweed seed from 7 locations exposed to light during imbibition germinated 24% higher than seed kept in the dark (Figure 5). Approximately 13% of the seed from each location did not germinate initially in either the light or dark treatment. Several months after collection, germination was nearly 100% indicating a portion of the seeds were innately dormant at the time of collection and required a period of after-ripening. If individual seeds with a light requirement become buried before conditions favorable for germination occur, they will remain in a dormant state until the light requirement is either satisfied or terminated.
Polymorphic germination behavior, where seeds produced on the same plant or within a population of plants possess 54 different germination requirements, is a common phenomenon among plant species (Bewley and Black, 1982). It permits
F i g u r e 4 Effect of Spotted Knapweed Seed Placement on the Soil Surface or Buried 0.6 cm Deep |g| on Germination of Seed Collected at 7 Locations. seed germination over time insuring long-term survival of at
least a portion of each seed crop (Popay and Roberts, 1970).
In addition plants such as spotted knapweed that exhibit asynchronous flowering behavior are more likely to display germination polymorphism due to the influence of variable environmental conditions during seed ontogeny (Fenner, 1985). 55 Since the soil surface is continually changing due to climatic factors and biotic activity, microsites (and their conditions) are likewise changing (Harper, Williams and
100 Z 90 LSD ■ O 0.05 I h- 80 < Z 70 2 60 DC LU 50 0
LU 3° CC 20 S 10
Drummond Bozeman Anaconda Bonner Butte Hyalite Florence COLLECTION LOCATION
Figure 5. Germination of Spotted Knapweed Seed in Light or Complete Darkness ■ § .
Sagar, 1964). Seeds that are dispersed into microsites where conditions are unfavorable for germination may remain in a state of enforced dormancy for many years (Harper,
1959). Wesson and Wareing (1969) demonstrated that burial induces light sensitivity in seeds of some weed species that were previously light insensitive. 56 Spotted knapweed seeds may fail to germinate immediately after being shed because of environmental factors such as supra or suboptimaI temperature and insufficient moisture, or innate dormancy factors such as after-ripening.
Subsequent germination of these after-ripened seeds is stimulated by exposure to light caused by a soil disturbance (Wesson and Wareing, 1969). Furthermore, Nolan and Upadhyaya (1988) showed that 50-60% of freshly harvested spotted knapweed seed displayed a light requirement.for germination. If these seeds became buried a portion might remain dormant for long periods of time prior to exposure to light. This supports the hypothesis that the small population of ungerminated spotted knapweed seed remaining in the natural decline study, and the seeds remaining in ttie burial study, represent dormant seed which require light to germinate.
Nolan and Upadhyaya (1988) reported three forms of polymorphic germination behavior in populations of spotted and diffuse knapweed seed: I) non-dormant seed which germinated readily under suitable conditions, 2) primary dormant seed which required light to germinate, and 3) primary dormant seed which remained dormant in spite of light exposure. The evidence presented here suggests that the light-sensitive and light-insensitive primary dormant spotted knapweed seed that becomes buried in soil shortly f
57 after dehiscence may be the small but extremely important portion of the seed produced each year that remains dormant to permit long-term survival of spotted knapweed. 58
CHAPTER 3
LONG TERM CONTROL OF SPOTTED KNAPWEED WITH REDUCED RATES OF PICLORAM
Introduction
Spotted knapweed is the most troublesome weed in Montana. Its invasive nature and the absence of most of its natural enemies permits establishment of dense, near monoculture stands. Infestations of this perennial weed can reduce forage production 90% (Watson and Renney, 1974).
Herbicides are an effective means of controlling spotted, knapweed. Piclofam provided I to 5 years of complete control of spotted knapweed when applied at 0.28 kg a.i./ha (Fay et. al., 1989). The exact longevity of complete
control depends on site conditions, especially the presence of significant amounts of soil organic matter (Cranston,
1984). Grover (1971) showed a positive correlation of picloram adsorption with soil organic matter content. Leaching of picloram is minimized in high organic matter
soils (Keys and Friesen, 1968; Scifres et al., 1969), providing longer periods of control than in light textured soils low in organic matter. The length of time between application of picloram and the occurrence of precipitation greatly influences the loss of picloram through 59 photodecomposition (Hall et al., 1968? Johnsen and Martin, 1983; Rice, 1989). The period of control of spotted knapweed provided by 0.28 kg a.i./ha of picloram often extends well beyond the predicted period of residual activity for the herbicide. Haymaker et ad. (1967) reported that 90% of picloram applied at 0.28 kg a.i./ha will dissipate in 12 months; therefore, no more than 0.03 kg a.i./ha should remain in the soil during the second year following application. In contrast, Lacey (1985) determined through a bioassay that spotted knapweed seedlings are sensitive to 0.02 kg a.i./ha picloram. Thirty months was required for picloram residues to degrade to this level following application of 0.28 kg a.i./ha at Harolwton and Ovando. The longevity of spotted knapweed control measured at Harlowton and Ovando was partially dependent upon the presence of phytotoxic levels of picloram in soil, therefore the residual period of picloram in Montana was 40 to 50% longer than the period predicted by Haymaker. Since the viability of spotted knapweed seed exceeds 7 years (Chapter 2), factors other than picloram residues, such as plant competition, influence the rate of reinfestation of spotted knapweed. The objective of this study was to determine the longevity of control of spotted knapweed using quantities of picloram at and below the present recommendation of 0.28 kg a.i. picloram per hectare. 60
Materials and Methods
Field trials were established in June, 1982 by T . K. Chicoine, former graduate research assistant. Plant and Soil Science Department, Montana State University (Chicoine, 1984) . Site one is located near Ovando on the Blackfoot- Clearwater Game Refuge. The study was established on gently rolling foothills characterized by a grassland community type dominated by timothy (Phleum pratense), smooth bromegrass (Bromus inermis), and Kentucky bluegrass (Poa pratensis). The dominant forb was spotted knapweed. The site had been cultivated in 1954 and seeded to timothy.
There was no grazing use by domestic livestock or wildlife for the duration of the trial, however the area was inadvertently cut for hay by Fish, Wildlife, and Parks
Department personnel during the summer of 1986.
The elevation of the study site is 1837 m and average annual precipitation during the 7 year study period was 32.6 cm. measured at Ovando approximately 10 miles east of study site. The soils are classified as loamy-skeletal, mixed,
Frigid Typic Haplustols.
Site two is located on the Bill Jones ranch approximately 15 miles south of Harlowton on a floodplain. The site consisted of a grassland vegetation community dominated by western wheatgrass (Agropyron smithii), Kentucky bluegrass needle-and-thread (Stipa comata), and blue grama (Bouteloua gracilis). Spotted knapweed was the dominant forb. The study area is located within a 245 hectare pasture that was heavily stocked (700 animal units) with cattle each year from November through May. Although supplemental feed was provided, the area received heavy grazing pressure during the early spring months of 1982 through 1986. The experimental area was fenced in the fall of 1986 to exclude livestock.
The elevation at the site is 2041 m and average annual precipitation measured at Harlowton was 34.4 cm during the 7 year study period. The soil is classified as a loamy- skeletal, mixed, Frigid Ustic Torrifluvent. Further details of the plant community and soil characteristics of both sites were described by Lacey (1985).
Herbicide treatments were applied by Chicoine in June,
1982 with a C02-pressurized backpack sprayer delivering 137
1/ha spray solution. Plots (3.4 X 12.2 m) were arranged in a split block design with 3 replications. Picloram was applied at 0.07, 0.11, 0.14, 0.22, 0.25, and 0.28 kg a.i./ha as main plot treatments, and 2,4-D amine was applied at 2.24 kg a.i./ha to half of each plot as the subplot treatment.
Individual plots were spatially separated from each other by a 2.1 m buffer zone treated with a tank mix of picloram and 2,4-D amine at 0.37 and 2.24 kg a.i./ha respectively to prevent contamination from spotted knapweed seed production in neighboring plots. The perimeter of the experimental 62 area was further separated by a buffer zone 3.4 m wide which was sprayed with the same mixture of picloram and 2,4-D amine. The buffer zones were retreated in June, 1986 with the same treatment described above. Spotted knapweed plant counts were taken in June, 1982 prior to herbicide application, and in July from 1983 through 1989. Four I m2 frames were placed at random within each plot and mature spotted knapweed plants were counted. Biomass production was measured 2, 14, 26, and 86 months following herbicide application. Two 0.5 m2 frames were placed at random locations within each plot. The standing crop was clipped at ground level and the current year's growth separated into grasses or spotted knapweed, the only forb present in significant frequency. Vegetation was oven- dried at 50° C for 48 hours to determine herbage dry weight production.
Results and Discussion
All rates of picloram reduced spotted knapweed density 60 and 84 months after application at Harlowton and Ovando, respectively. Complete control of mature spotted knapweed plants 36 months following treatment was obtained with rates ranging from 0.14 to 0.28 kg a.i./ha at Ovando (Table 4).
Rates of 0.11 to 0.28 kg a.i./ha provided 100% control of mature spotted knapweed 24 months after herbicide application at Harlowton; however, only the 0.28 kg a.i./ha 63 rate provided complete control after 36 months (Table 5). The addition of 2,4-D amine as a subplot treatment did not significantly improve control over picloram alone (Lacey, 1985).
Table 4. Number of Mature Spotted Knapweed Plants per m2 in July, 1983 Through 1989 Following Herbicide Application in June, 1982 at Ovando. Picloram ______Year______(kg/ha) 1983 1984 1985 1986 1987 1988 1989
0.28 0. Oa1 0.0a 0.0a 0.0a 0.6a 0.3a 0.2a 0.25 0.0a 0.0a 0.0a 0.0a 0.4a 0.2a 0.1a 0.22 0.0a 0.0a 0.0a 0.1a 0.5a 0.5a 0.6a 0.14 0.0a 0.0a 0.0a 0.2a 1.7a 0.8a 0.9a 0.11 3.2a 4.6a 2.2a 0.2a 0.2a 1.3a 0. 5a 0.07 41.2b 21.1b 4.1b 1.4b 4.7b 2.0b 1. la 0.00 65.8c 58.4c 57.2C 54.2c 41.7c 52. Oc 46.5b
1Means in a column followed by the same letter are not significantly different at the 5% level using the LSD test.
Table 5. Number of Mature Spotted Knapweed Plants per m2 in July, 1983 Through 1989 Following Herbicide Application in June, 1982 at Harlowton.
Picloram ______Year (kg/ha) 1983 1984 1985 1986 1987 1988 1989
0.28 0.0a1 0.0a 0.0a 1.7a 8.8a 21.0 a 15.2a 0.25 0.0a 0.0a 0.2a 1.5a 12.3a 15.8a 15.3a 0.22 0.0a 0.0a 1.1a 2.5a 6.5a 17.5a 9.4a 0.14 0.0a 0.0a 1.2a 3.2a 8.4a 25.4a 16.4a 0.11 0.0a 0.0a 1.2a 2.7a 10.8a 22.8a 11.8a 0.07 2. Ia 2.1a 1.8a 2.7a 10.2a 12.3a 15.2a 0.00 126.7a 84.1b 75.7b 62.2b 61.9b 30.8a 15.3a
1Means in a column followed by the same letter are not significantly different at the 5% level using the LSD test.
Picloram soil residues dropped below the sensitivity level of spotted knapweed seedlings by 36 months after 64 application of 0.28 kg a.i./ha picloram at Ovando and Harlowton (Lacey, 1984). Reinvasion by spotted knapweed seedlings started 2 to 4 years after application depending upon picloram rate in treated plots at Harlowton. By July, 1988, the density of mature spotted knapweed plants within picloram treated plots was not significantly different than in untreated control plots (Table 5). Reinvasion also occurred at the Ovando site. Low densities of mature spotted knapweed plants were present in plots treated with picloram doses lower than 0.25 kg a.i./ha 48 months after treatment. Picloram ranging from 0.11 to 0.28 kg a.i./ha resulted in less than one mature spotted knapweed plant per m2 in 1989, 84 months following application, compared to 47 plants per m2 for the untreated control (Table 4). Picloram at 0.14 to 0.28 kg a.i./ha provided 100% control of spotted knapweed 26 months after application, decreasing spotted knapweed herbage from 1990 and 2316 kg/ha to zero kg/ha at Harlowton and Ovando respectively (Tables 6 and 7). Grass production increased 200% at Harlowton and 700% at Ovando 26 months after treatment in response to the elimination of spotted knapweed (Tables 8 and 9).
? 6 5
Table 6. Spotted Knapweed Herbage Production 2, 14, 26, and 84 Months After Herbicide Application at Harlowton.
Picloram Months After Herbicide Application Rate 2 14 26 84 (kcf/ha) Spotted Knapweed Herbage Production fka/ha) 0.28 659.Vab1 0.0a O.Oa 1797.0a 0.25 466.5a O.Oa O.Oa 1740.0a 0.22 589.9ab 0.0a O.Oa 1390.0a 0.14 763.9ab 0.0a 0.0a 1533.0a 0.11 963.6b 3.7a O.Oa 2097.0a 0.07 1022.7b 52.2a 78.3a 1687.0a 0.00 1607.5c 2106.6b 1990.0b 1510.0a
1Means in a column followed by the same letter are not significantly different at the 5% level using the LSD test.
Table 7. Spotted Knapweed Herbage Production 2, 14, 26, and 84 Months After Herbicide Application at Ovando.
Picloram Months jAfter Herbicide Application Rate 2 14 26 84
fkg/ha) Spotted Knapweed Herbage Production fkq/ha)
0.28 1267.5a1 0.0a 0.0 163.3a 0.25 1242.8a O.Oa 0. Oa 226.7a 0.22 1573.1a 0.0a O.Oa 110.0a 0.14 972.7a 1.7a 0.0a 393.3a 0.11 1413.9a 17.9a 23.0a 490.Oab 0.07 1563.4a 210.4b 597.3b 566.7ab 0.00 3991.9b 1446.6c 2316.0c 1113.0b
1Means in a column followed by the same letter are not significantly different at the 5% level using the LSD test.
Spotted knapweed and grass herbage production within all picloram treated plots was not significantly different than the untreated plot 84 months following application at Harlowton (Tables 6 and 8). Heavy grazing pressure by cattle during the spring of 1983, 1984, and 1985 provided 66 enough soil disturbance to permit significant germination of dormant spotted knapweed seed to occur. With the picloram soil residues depleted by 1983, and grass competitiveness reduced, spotted knapweed quickly reinfested the treated plots.
Table 8. Perennial Grass Production 2, 14, 26 and 84 Months After Herbicide Application at Harlowton.
Picloram Months .After Herbicide Aoolication Rate 2 14 26 84
fka/ha) Perennial Grass Herbaae Production fka/ha) 0.28 898.Sab1 1700.8a 807.2a 196.7a 0.25 727.9ab 1747.4a 795.2a 230.0a 0.22 516.4ab 1287.6a 841.6a 121.0a 0.14 693.lab 1466.2a 702.Oa 130.0a 0.11 698.9ab 1472.2a 640.4a 280.0a 0.07 704.4ab 1437.4a 659.6a 286.7a 0.00 429.5b 321.2b 256.7b 63.3a
1Means in a column followed by the same letter are not significantly different at the 5% level using the LSD test.
Table 9. Perennial Grass Production 2, 14, 26 and 84 Months After Herbicide Application at Ovando.
Picloram Months After Herbicide Aoolication Rate 2 14 26 84
fka/ha) Perennial Grass Herbaae Production fka/ha) 0.28 560.5a1 1630.8ab 2612.8b 803.3ab 0.25 307.1a 1474.Obc 2686.4b 1200.Oab 0.22 682.3a 1655.4ab 2739.2b 1330.0a 0.14 440.5a 1433.8b 2896.8a 1050.Oab 0.11 316.5a 1275.0b 2517.6b 910.Oab 0.07 350.2a 1021.2bc 1636.4c 766.7b 0.00 360.7a 332.Od 418.4d 93.3c
1Means in a column followed by the same letter are not significantly different at the 5% level using the LSD test. 67 While the recolonizing spotted knapweed plants were in low density, they were much larger than individual plants within the control plots due to reduced interspecific competition. The large plants produced a heavy seed crop, leading to increased spotted knapweed density. . Powell, Risley and Myers (1989) measured the biomass, population density and seed production of recolonizing diffuse knapweed plants 3 years following treatment with picloram. They found that while the density of knapweed was less than one tenth of the control population, individual plants were much larger. Plant biomass and seed production per mz was greater than from plants in unsprayed areas. These results suggest that recolonization occurs rapidly once individual knapweed plants become established following dissipation of picloram residues on sites devoid of plant competition.
The suspected allelopathic influence of spotted and diffuse knapweed may also contribute to their ability to invade and displace native vegetation (Harris and Cranston, 1979; Kelsey and Locken, 1987; Muir and Majak, 1983).
Perennial grass herbage production increased 700% at Ovando by 26 months after herbicide application of picloram at 0.14 to 0.28 kg a.i./ha (Table 9). Spotted knapweed herbage production averaged 178 kg/ha in plots treated with picloram at rates of 0.14 to 0.28 kg a.i./ha compared to 1113 kg/ha for the untreated control (Table 7). 68 Grass production at Ovando 84 months after treatment was 2.5 times lower than it was 26 months following treatment. This reduction was not due to reinvasion from spotted knapweed as at Harlowton. While the grass herbage production in picloram treated plots was 11 times greater than in the untreated plot 84 months after treatment, the reduction in grass production in treated plots from 1984 to 1989 is substantial (Table 9). The overall decrease in grass production observed at Ovando may be due to a sodbound condition of the smooth brome dominated site following years of heavy grass production without utilization as evidenced by the heavy thatch of dead grass in treated plots. In addition to reducing grass productivity, the heavy thatch may inhibit spotted knapweed- seed germination by filtering out red light (Nolan and Upadhyaya, 1988).
Long-term control of spotted knapweed with reduced rates of picloram appears to be more dependent on site conditions and management practices than on the rate of picloram applied. Picloram applied at 0.14 to 0.28 kg a.i./ha provided the same level of spotted knapweed control for 84 months at Ovando, a highly productive, low disturbance site. The same concentrations of picloram provided similar control for 36 months at Harlowton, followed by a uniform rate of reinfestation throughout all 69 treated plots. In addition, the Harlowton site represents a case of extreme disturbance from high density animal grazing. 7 0
CHAPTER 4
SEED MORPHOLOGY AND GERMINATION CHARACTERISTICS OF SPOTTED KNAPWEED
Introduction
Spotted knapweed belongs to the Compositae (Asteraceae) family which contains about 1310 genera and 13,000 species. The knapweeds are classified in the genus Centaurea, tribe Cynareae and subtribe Centaureinae Durmort. There are 500 to 600 members of the genus
Centaurea, mostly originating in the Mediterranean area and Southwest Asia (Dittrich, 1977). Grouping plants in tribes and subtribes is commonly based on differences in flower and fruit morphology. Fruit characteristics serve major functional roles in seed dissemination, water uptake, seed positioning on a soil surface, speed of seed germination and longevity of seed survival in soil (Fenner, 1985; Lewis, 1973; Sheldon, 1974). Scanning electron micrographs, speed of imbibition, and point of water uptake were investigated to determine their relationship to long-term survival of spotted knapweed seed. 71 Materials and Methods
Scanning Electron Micrographs
Spotted knapweed achenes collected at Bozeman in 1987 were sputter-coated with a 60 nm thick layer of gold and viewed through a JEOL IOOCX electron microscope utilizing a ASID-4D scanning attachment located at the Veterinary Research Laboratory, Montana State University. The achenes were scanned at magnifications ranging from 20 to 80OX. Polaroid micrographs were taken of the achene pericarp, pappus, hilum, epidermal hairs, nectary and micropyle. Longitudal cross section micrographs of the embryo, testa, radicle, cotyledons, and internal air space were taken. The protective pericarp and testa of unburied seeds and seeds recovered after 5 years of burial (obtained from the burial study in chapter 2) were fractured and compared.
Imbibition Curves
Spotted knapweed seed was collected at Bozeman, MT in August, 1988 and stored at 20° C. The seeds contained 5% moisture by weight at the initiation of the study. Ninety- six lots of 50 seeds were weighed and placed between two sheets of moist blotter paper in 100 mm diameter petri dishes. Seeds were incubated in a growth chamber at 25° C with 14 hours of light. At I hour intervals seeds in two petri dishes were blotted dry on 72 absorbent tissue papper for 3 minutes and weighed. This procedure was repeated until germination (protrusion of the radicle by 0.5 mm) occurred. Percent water imbibition was determined by dividing the initial seed weight by the imbibed seed weight. This value was subtracted from one and multiplied by 100. An imbibition curve for spotted knapweed seed was constructed by plotting the percent increase in seed weight against the duration of imbibition.
Point of Water Uptake The location of water imbibition of the spotted knapweed achene was determined by coating portions of achenes with paraffin and recording germination over time. Four paraffin treatments were compared: I) pappus end coated, 2) hilum end coated, 3) entire achene coated and
4) no paraffin treatment. Four replications of 100 seeds for each treatment were placed in 100 mm diameter petri dishes containing moist blotter paper. Percent germination was recorded every 6 hours until germination of the uncoated achenes reached 100% (48 hours).
Viability of seeds that had not germinated after 48 hours was determined by a tetrazolium test. This experiment was conducted twice and data were analyzed by analysis of variance.
In a second study 100 spotted knapweed seeds were imbibed in a solution of 0.1% methylene blue which stains 73 tannin and pectic substances blue (Sheldon, 1974). Every two hours, 10 seeds were randomly selected and the outer pericarp and testa were removed by hand to observe the degree and location of staining by viewing through a dissecting microscope at 20X magnification. Schematic drawings of the embryo were used to demonstrate the pattern of staining during imbibition.
Results and Discussion
Scanning Electron Micrographs The achene of spotted knapweed is 2 to 3 mm long, 1.5 mm wide and olive green in color with 4 prominent light colored stripes evident to the base and with finer lines between. The seed is dull or slightly lustrous, obovately shaped, somewhat bossed on the abaxial side in the upper third and always flattened at the sides. The pappus is white with the longest bristles 1.5 to 2 mm long (U.S. Dept. Agr. Handbook 219, 1958).
Scanning electron micrographs provided clear, high- magnification images of spotted knapweed achenes. From these SEM's a schematic diagram of a spotted knapweed achene (cypsela) was drawn and morphological structures identified (Figure 6). The hilum is a scar from the original point of attachment of a seed to the maternal plant. The hilum is a 74
Inner Style P a p p u s .C o ro lla
O u te r P a p p u s
M ic ro p y le
C o ty le d o n s
E m b ry o
P e ric a rp
T e s ta R a d ic le
H ilum
F u n ic u lu s
Figure 6. Schematic Representation of a Spotted Knapweed 75 distinctive means of identifying the point of insertion of achenes to maternal tissue and helps to distinguish among subtribes of Cynareae. The fruit insertion of the Centaurea spp. is described as lateral-adaxial, concave (Dittrich, 1977). A portion of the maternal receptacle tissue remains attached to the achene at the hilum (Figure 7) and consists of maternal tissue cell walls arranged in a "honeycomb" like matrix (Figure 8). The developing ovule receives nourishment from the maternal plant through the funicular attachment between the ovule (seed) and the receptacle.
Figure 7. Proximal End . of Spotted Knapweed Achene Showing Hilum (a) and Epidermal Hairs (b) (XlOO) .
The chalaza is that portion of an ovule where the integuments (testa) originate. The orthotropous ovules of Centaurea species are the simplest type of ovule arrangement 76 in which the ovule is erect, with the micropyle at one end and the funiculus at the other. The chalaza of orthotropous
Figure8 . Cross Section of Hilum Showing "Honeycomb" Cellular Structure (X500). ovules is located directly below the funicular attachment. In this area the testa apppears to be thinner than in other parts of the seed (Figure 9). Upon germination this is the area of the seed coat that splits, allowing emergence of the radicle, perhaps indicating a weak spot in the testa. The surface of the dome-shaped receptacle tissue remaining attached to the achene appears waxy with numerous pores which may serve as ports of entry for water (Figure 10). These observations suggest that water uptake by spotted knapweed achenes would likely occur at the hilum end of the achene. 77
Figure 9. Cross Section of Proximal End of Achene With Funicular Attachment (arrow) (XlOO) .
Figure 10. Hilum Surface Showing Waxy Texture with Numerous Pores (arrows) (X800). 78
The pappus and micropyle are located at the apical end of the achene (Figure 13). In the Centaureinae the pappus is characterized by multiserial bristles defined as a double pappus containing two distinct whorls of bristles which display different morphological features. The bristles in the outer whorl are longer on the inside than the outside (Figure 11) and have deeply serated margins with barbs arranged in a herringbone pattern (Figure 12). The inner whorl of bristles are shorter and flatter than the bristles of the outer whorl and have thickened, spoon-like bases (Figure 13).
Located within the inner pappus is the disc (nectary) to which the style is attached (Figure 14). The micropyle, the 79 integumentary opening of the ovule through which the pollen tube enters prior to fertilization, is located within the nectary. This small rudimentary opening may also serve as a port of entry for water during imbibition.
Figure 12. Bristles on Outer Whorl of Pappus (X300) .
The protective tissues surounding the embryo are comprised of two layers of integuments making up the testa
(seed coat) and the outer pericarp (fruit wall) (Figure 15). The inner integument is relatively thin and made up of parenchyma tissue. The thick outer integument consists of sclerenchyma tissue, having cells arranged in a palisade layer. These palisade cells have radial walls that are thickened and lignified. The pericarp surrounds the testa and is thinner than the outer integument. The pericarp is 80
Figure 14. Disc (Nectary) (a) and micropyle (b) (X300). 8 1 olive green and mostly glabrous with sparse monocellular epidermal hairs of different length on both ends of the achene (Figure 7). The presence of chlorophyll in the pericarp may influence the light sensitivity of the seed contained within the achene. Cresswell and Grime (1981) showed that if the structures enveloping the seeds (ovary walls, calyx, bracts) remain green throughout the maturation period of the seeds, a light requirement for germination will be induced in the seeds before they are shed. Spotted knapweed achenes acquire.a dark green color prior to dispersal and are contained within the green bracts of the involucre during seed development; therefore, they have a light requirement. Spotted knapweed seeds germinate readily under favorable conditions in a laboratory; however, 50% of seeds buried for
5 years remain viable (Chapter 2). In the natural decline of a spotted knapweed seed bank, 3-5% of the original seed bank of viable seeds were recovered after 80 months without seed production. Seeds recovered after 5 years of burial (Chapter 2) were compared to fresh seeds by examining the outer protective structures of the seed. The pericarp and testa appeared to be weathered but fully intact in spotted knapweed seeds recovered after 5 years of burial (Figure 16). The ability of buried spotted knapweed seed to remain viable for long periods of time can be largely attributed to the durable seedcoat. 82
Figure 15. Fractured Surface of a Fresh Achene Exposing the Testa (a) and Pericarp (b) (XlOO).
Figure 16. Fractured Surface of Achene After 5 Years of Burial Exposing the Testa (a) and Pericarp (b) (X200). 83 Water Imbibition Spotted knapweed seeds were fully imbibed after just 13 hours and germination started within 18 hours (Figure 17). The ability of seeds to germinate quickly when conditions are favorable for seedling establishment is an important survival mechanism for plants of arid or mesic environments (Fenner, 1985).
R -.98 Z O
Z Z CC LU O h- Z LU O CC LU 0.
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 HOURS
Figure 17. Spotted Knapweed Seed Imbibition Curve Measured Over an 18 Hour Period Culminating with Germination.
Spotted knapweed is well adapted to droughty, gravelly sites. Rapid germination of seeds lying on the soil surface in response to precipitation enables seedlings to become 84 established quickly thus allowing spotted knapweed to capture ephemeral niches.
Point of Water Uptake Achenes uncoated with paraffin germinated 100% after 48 hours of imbibition (Figure 18). Achenes that had the pappus end sealed with paraffin but the hilum exposed germinated at a slightly slower rate than the control group but reached 100% germination by 48 hours. The achenes which
— UNCOATED PAPPUS COATED HILUM COATED COMPLETELY COATED
0.05
HOURS Figure 18. Germination of Spotted Knapweed Seeds with Paraffin Covering the Hilum, the Pappus, the Entire Seed, and Non-coated Seeds. 85 had the hilum end sealed with paraffin germinated only 32% after 48 hours. Only 4% of the achenes which were completely coated with paraffin germinated which indicates that the paraffin was an effective deterrant to imbibition. Spotted knapweed seeds imbibed in a 0.1% solution of methylene blue for varying lengths of time showed a distinct progression of staining of the embryo (Figure 19). There was intense staining of the receptacle tissue surrounding 4V the hilum and the nectary tissue surrounding the micropyle after 2 hours of imbibition. A^fter 4 hours the radicle tissue of the embryo at the hilum end of the seed was lightly stained, and by 6 hours almost one-half of the embryo at the hilum end was stained. By 8 hours some staining of the cotyledons was apparent at the micropyle end of the seed. Two thirds of the embryo, proceeding from the
hilum end of the achene, was stained after 10 hours and by 12 hours the entire embryo showed some degree of staining. Sheldon (1974) reported similar results with the achenes of
Taraxacum officinale when imbibed in a methylene blue solution for 4, 16 and 24 hour periods.
These two experiments demonstrate the importance of the hilum, and to a lesser degree the micropyle, in water imbibition of spotted knapweed seed. By restricting water zuptake at only two specific locations in the testa, the
overall integrity of the hard seed coat can be maintained. Additionally, the two water-permeable pores may serve as 86 semipermeable membranes which prevent or inhibit introduction of detrimental microorganisms to the seed which would serve to extend seed longevity in soil.
6 hr 8 hr 12 hr
Figure 19. Schematic Representation of Cross-sections of Spotted Knapweed Seeds Showing Staining Patterns Following Imbibition in a 0.1% Methylene Blue Solution for 0, 2, 4, 6, 8, and 12 hours. 87 The orientation of achenes on the ground surface can markedly affect the probability of germination. Sheldon (1974) reported the highest germination rate for T. officinale was obtained when the seeds were placed with the long axis at about a 45 degree angle to the horizontal.
This is the orientation that dandelion seeds would normally assume on a flat surface if the pappus was still attached.
This orientation optimizes contact between the hilum, through which water is initially absorbed, and the soil surface.
The rigid bristles of the spotted knapweed pappus may serve a similar function in positioning the seed for optimum water uptake. As the spotted knapweed achene matures and is . exposed to large changes in humidity, the pappus extends to an angle approximately 45 to 600 in relation to the long axis of the achene (unpublished data). Once the pappus is fully extended it maintains this orientation permanently regardless of humidity, similar to the pappus of Leontodon autumnalIs (Sheldon, 1974). The special morphological features of spotted knapweed
achenes promote rapid water imbibition optimizing timely seed germination and seedling establishment. The thick lignified palisade layer of the outer integument protects the inner seed, prolonging longevity in the soil. The light sensitivity observed in portions of spotted knapweed seed 88 populations (Chapter 2) may be associated with the green color of the fruit wall surrounding the seed. 89
CHAPTER 5
SUMMARY
Spotted knapweed is an aggressive perennial weed infesting millions of acres in Montana. This extremely competitive weed displaces native plant species resulting in near monoculture stands which severely reduce the carrying capacity of rangeland.
Spotted knapweed seeds buried in soil can remain dormant for many years. Over 50% of the seeds recovered after 5 years of burial were viable. In a natural setting, over 90% of the spotted knapweed seeds germinate or are lost from the indigenous seedbank within the first 3 years following the termination of seed,production. After 7 years approximately 20 viable but dormant seeds remained per 0.5 m2 (390,000 seeds per ha). A significantly greater number of freshly harvested spotted knapweed seeds germinate in the light than in the dark. If individual seeds displaying this light requirement become buried before conditions favorable for germination occur, they will remain in a dormant state until the light requirement is either satisfied or terminated.
Picloram is a very effective herbicide for long-term control of spotted knapweed. All rates tested provided 7 years of control at Ovando with a 700% increase in grass 90 production. Spotted knapweed was controlled at Harlowton 4 years following treatment, resulting in a 200% increase in grass production, but control rapidly diminished as. soil residues of picloram dissipated and grass competition was reduced by grazing cattle. Site conditions and land management practices greatly influence the longevity of control achieved with picloram.
Spotted knapweed seeds rapidly imbibe water, enabling germination to occur when environmental conditions favor seedling establishment. Imbibition occurs primarily through the hiIum and to a lesser degree through the micropyle. The thick outer layer of the testa provides a durable protective shield against degenerative soil microbes, thus allowing the seed to remain in the soil for many years. 9 1
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