• IMPORTANCE TO LIVESTOCK AND WILDLIFE
• PALATABILITY
• NUTRITIONAL VALUE
• COVER VALUE
• VALUE FOR REHABILITATION OF DISTURBED SITES
• OTHER USES AND VALUES
• MANAGEMENT CONSIDERATIONS

IMPORTANCE TO LIVESTOCK AND WILDLIFE:

Several reports from western Montana indicate some use of spotted knapweed by livestock. In general, use of spotted knapweed by livestock is highest during spring and early summer when plants are green and actively growing in the rosette and bolt stages [96,160]. Use declines as spotted knapweed matures, and protein and digestibility decrease [89], although flowerbuds and seedheads may be grazed in the late summer [26,96,110]. Domestic sheep may also graze rosettes when present (from regrowth and late season germination) in the fall [26]. Cox [26,27] observed domestic sheep eating large quantities of spotted knapweed in the spring and early summer in preference to grasses and other forbs. Olson and Wallander [141,142] also report sheep readily grazing spotted knapweed in an infested pasture, although they grazed other plants as well and did not consistently graze one plant more than the others. Sheep grazed spotted knapweed leaves and avoided stems. In sagebrush steppe rangeland in southeastern Idaho, spotted knapweed was readily consumed by domestic sheep throughout the year. Grasses were consumed in amounts similar to spotted knapweed, but native forbs were most commonly used [108]. Sheep grazing has been proposed as a potential control method for spotted knapweed [27,53,108] (see "Management Considerations" below).

Reports on wildlife use of spotted knapweed are also varied. Bedunah [11] cites several studies suggesting large potential losses of elk range to spotted knapweed, though he indicates that quantifying the effects of infestation on elk populations is complicated by their mobility. Spotted knapweed infestation is considered more detrimental to elk than to deer because spotted knapweed replaces grasses that are preferred by elk, while deer have a diet of predominantly shrubs and conifers that spotted knapweed does not replace [109,229]. Elk use increased following removal of spotted knapweed with herbicides on sites in Montana [157,200]. In a study in western Montana on 2 game ranges in the Bitterroot Valley, Willard and others [229] report minimal use of spotted knapweed dominated sites by elk and mule deer. When used, mule deer consumed knapweed flowerheads from December through April, while elk consumed knapweed flowerheads only during winter. Spotted knapweed was preferred by deer and elk over other plant species on sites with dense knapweed cover [109,229]. It was suggested that deer and elk did not frequently use the spotted knapweed sites in this study because cervid densities were relatively low and other forage was available [232]. On spotted knapweed infested bunchgrass range in the Selway-Bitterroot Wilderness, Idaho, Wright and Kelsey [232] report that elk, mule deer, and white-tailed deer used spotted knapweed infested range as much as or more than uninfested bunchgrass range from 1 December through 25 April. All cervid species consumed both rosettes and seedheads of spotted knapweed, with seedhead consumption greatest during periods of snow cover. The authors suggest that when estimating carrying capacity of a cervid range, spotted knapweed can be considered a potential food source, because when animal densities are high and food choices are limited, elk and deer will consume spotted knapweed [232]. In British Columbia, deer and elk forage on diffuse and spotted knapweed rosettes in late fall and early winter, and again when snow cover recedes and spring green-up commences. Knapweed rosettes and bluegrass comprised 90% of mule and white-tail deer diets in February and early March [124].

In the Gilpin range, British Columbia, California bighorn sheep utilized diffuse and spotted knapweed seedheads as primary forage when snow depth was in excess of 8 inches (20 cm). As snow cover receded in January and February, knapweed basal rosettes were the largest component (80%) of their diet. Rocky Mountain bighorn sheep also utilized knapweed seedheads and basal rosettes throughout the year in the Robson/Syringa Park area, British Columbia [124].

Spotted knapweed is a nectar source for the endangered Karner blue butterfly in Wisconsin [52]. Rodent utilization of spotted knapweed seed has been suggested [221]. Deer mice have been observed eating larvae and seeds from spotted knapweed flowerheads infested with seedhead flies (introduced biocontrol agents) [145].

PALATABILITY:

NUTRITIONAL VALUE:

Values vary with season, plant part, age, and site. Nutritional value declines as summer progresses [142,232], with crude protein and nonstructural carbohydrates most concentrated during the spring. Spotted knapweed becomes more fibrous, with lower protein and carbohydrate levels, as stems mature over the summer [89]. Seedheads are less nutritious than rosettes, but may be available above the snow [232]. Willard and others [229] measured crude protein, fiber, and lignin content in spotted knapweed flowers and recorded average values of 6.6%, 45.6%, and 14.5%, respectively. Crude protein was similar for open and forested sites, while fiber and lignin values were higher on forested sites than on open sties [229]. Jones and others [80] provide a detailed study of the forage value of spotted knapweed from a sagebrush/grassland site in southeastern Idaho, comparing different parts of mature and immature plants from May through September.

Secondary compounds in spotted knapweed, such as cnicin, can negatively affect activity and growth of anaerobic rumen microorganisms in domestic sheep, reducing digestibility of spotted knapweed [138,140].

COVER VALUE:

VALUE FOR REHABILITATION OF DISTURBED SITES:

OTHER USES AND VALUES:

MANAGEMENT CONSIDERATIONS:

Experimental evidence suggests that spotted knapweed gains dominance in part by its ability to out-compete native grasses for nutrients such as nitrogen [139] and phosphorus [65]. Other evidence suggests that as succession proceeds and nutrients become less available, the competitive advantage shifts from spotted knapweed to native plants such as bluebunch wheatgrass [91]. Allelopathy, primarily from the compound cnicin, has been suggested as a growth inference mechanism in spotted knapweed [19,88]. However, because cnicin concentration in soil is lower than that found to be toxic in in-vitro experiments, allelopathy was not considered as important as resource competition in determining the ecological success of spotted knapweed [87,113]. Allelopathic activity of cnicin may be enhanced, however, when large quantities of stem and leaf tissue from live or dead spotted knapweed plants come in direct contact with the soil surface, as when plants are trampled or mowed. This allelopathic activity could be minimized by burning or removing plant material before it comes in contact with the soil surface [113]. More recent experimental evidence suggests that knapweed's advantage over its North American neighbors could be attributed to differences in the effects of its root exudates and how they, in turn, affect competition for resources [19], thus linking allelopathy and resource competition.

In many areas, eradication of spotted knapweed is no longer an option. Perhaps small patches can be eradicated with cultural practices or herbicides. Large infestations must be controlled or suppressed with cultural and biological methods, perhaps in conjunction with herbicides, to contain the weed and slow its spread. Before management plans for the control of spotted knapweed can be designed, land use objectives must be defined. A generalized objective may be to develop a plant community that is weed resistant and meets other land-use objectives. Desired plant communities can be designed to maximize niche occupation with desirable species that compete intensely, grow rapidly, and grow during much of the season [73,178]. It is important that the successional effects of spotted knapweed control are considered in this manner to avoid replacing spotted knapweed with another weedy species [69,178,230]. 

Spotted knapweed control requires a sustained, site-specific commitment over a period of several years [96,207]. Economic considerations for spotted knapweed control have been investigated [50,138], and Griffith [49] provides a procedure for performing an economic evaluation for noxious weed management on rangeland, with spotted knapweed as an example. 

Integrated weed management: Managers are encouraged to integrate different control methods that can complement one another in a given situation. Integrated management includes considerations of not only killing the target weed, but also of establishing desirable species and maintaining weed-free systems over the long-term. Factors to be addressed before a management decision is made include inventory and assessment to identify the target weed and determine the size of the infestation(s); assessment of non-target vegetation, soil types, climatic conditions and important water resources; and an evaluation of the benefits and limitations of control methods [131]. Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies [175].

Conceptual models can be developed to determine the probability that the weed management strategy will result in the desired plant community, based on the life histories and population dynamics of the species in the existing plant community [73,178]. A weed management strategy may include designed disturbance (e.g. cultural or chemical control), controlled colonization (e.g. planting competitive species), and controlled species performance (e.g. biological control) [178,230]. Management strategies may include several approaches designed to disrupt the stages in a weed's life cycle that are most vulnerable to stress or control [175]. Jacobs and Sheley [73] identified juvenile, the transition from juvenile to adult, and adult as critical phases in the life history of spotted knapweed. The key processes associated with these stages are competition, growth, and reproductive allocation. Successful control practices must target one or more of these processes. Sheley and others [178] provide examples of ecologically based spotted knapweed management systems.

Prevention: Prevention of spotted knapweed establishment is the most cost-effective control strategy [31]. Prevention practices begin with the maintenance of healthy, desirable vegetation that is resistant to weed establishment. This includes minimizing soil disturbance in all activities and reestablishing desirable vegetation promptly whenever soil disturbance leaves areas of bare ground, with continued monitoring and immediate follow-up treatment of colonizing weeds, and/or revegetation with desirable species [31,207,208,210]. In areas where it is critical to maintain native vegetation (e.g. national parks, nature preserves, wilderness), avoid building new roads and trails, since this is the primary habitat for many invasive species including spotted knapweed [3,123,207,208]. Carefully monitor the intensity, frequency, and season of grass defoliation in grazing prescriptions so that grasses can tolerate grazing and resist weed invasion. One greenhouse study suggests that even moderate defoliation of competing grasses (30%) may allow greater spotted knapweed growth on Idaho fescue rangeland [72]. Another study found that spring defoliations increased spotted knapweed cover compared to summer defoliations; that grass defoliation greater than 60% caused an increase in spotted knapweed cover and density; and that more than one grass defoliation in a year increased spotted knapweed cover. The researchers suggest that a single, annual grass defoliation of 60% or less, regardless of the season, will not increase spotted knapweed invasion on rangeland [78]. Rangeland managers must also consider the potential for livestock to introduce spotted knapweed seed in their feces or fur. 

Regular removal of newly established spotted knapweed plants at trailheads, campsites and along road corridors is critical to prevent their spread into adjacent natural areas [95,117]. Monitor for weed emergence annually, especially in areas where there is vehicle or livestock movement, in riparian areas, areas of wildlife concentration, public use areas, and locations where sand, gravel, or fill materials have been imported, soil has been disturbed, or vegetation or overstory has been removed [210]. When spotted knapweed plants are found, remove them immediately. 

The introduction of spotted knapweed seeds from infested areas to recently disturbed and/or uninfested areas can be limited by monitoring vehicle, livestock, and wildlife movement [207,208,210]. Encourage public land users to avoid driving vehicles through or scheduling livestock use in existing spotted knapweed infestations when seeds are present, to inspect and clean vehicles of weeds and their seeds, to brush and clean animals, tack, and equipment before entering public lands, and to minimize soil disturbance by stock. Regulations promoting minimum impact camping and the use of weed-free feed, hay, straw, and mulch in natural areas may reduce spotted knapweed infestations [117,210]. Encourage the use of certified weed-free feed for several days before entering backcountry [131,208,210,220].

A key component of prevention practices is education and awareness of managers, land owners, and public land users [31,86,131,210]. In Montana, programs have been implemented such as a trust fund for weed research and weed management efforts, organized cooperative weed management programs for landowners [98,131], a curriculum for educating school children [133], and bounty programs [175]. Examples of successful cooperative spotted knapweed control efforts have been reported in Canada [3] and Wyoming [224].

Weed prevention and control can be incorporated into all types of management plans, including logging and site preparation, management of grazing allotments, recreation management, research projects, road building and maintenance, and fire management [210]. See the "Guide to noxious weed prevention practices" [210] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions. When prevention fails, or spotted knapweed populations already exist, several management approaches can be used to eradicate small populations or to control larger infestations including cultural, biological and chemical control methods, or some combination of methods timed in such a way as to be complementary. Tu and others [202] provide a comprehensive review of weed control methods that are applicable for use in natural areas. The information is also available online (Weed Control Methods Handbook).

Physical and mechanical control: Manual control techniques may be preferred in some areas with spotted knapweed infestations. For example, on the Salmon River watershed in the Klamath National Forest in California, it is recognized that manual control methods offer less risk to the high quality waters and high value fisheries than do chemical applications. A cooperative effort among the local residents and land management agencies, orchestrated by the Salmon River Restoration Council (SRRC), has successfully employed the use of several manual control approaches including propane torching of seedlings early in the season, hand digging with small tools, mulching with black plastic, and mowing with weed eaters [167]. For a detailed description of their knapweed control program see the SRRC website.

Spotted knapweed does not persist under annual cultivation or in irrigated alfalfa. This, however, does not present a widely applicable solution for infested wildland, rangeland or prairie [60]. Mowing, hand-pulling, planting competitive species, and good range management may reduce the spread of spotted knapweed, but may not eliminate well established stands [86]. Tillage can reportedly lead to the spread of spotted knapweed [31]. This is especially likely in mature stands since tillage creates an ideal weed seed bed from which individuals in the seed bank may emerge. Tillage may be more successful if followed by seeding with a strongly competitive grass-legume mixture [151].

Consistent hand pulling can control spotted knapweed, although it is time and labor intensive. Entire plants must be removed before they produce seeds each year, and flowering plants should be removed from the site so no seeds are dispersed [175]. In greenhouse studies, only severe defoliation reduced spotted knapweed root, root crown, and aboveground growth. Some spotted knapweed plants produced flowers even when clipped monthly from June through September [90]. Mowing diffuse and spotted knapweed in Canada at the bud stage, flowering stage, or once at bud stage and again at flowering, reduced the number of plants producing seed by 77, 99, and 96% compared to unmowed plants. The latter 2 treatments also reduced germination of the seeds by approximately 79% [221]. Rinella and others [159] found that a single mowing at the flowering or seed stage resulted in an 83-85% decrease in adult spotted knapweed density at 2 sites in western Montana. This reduction was as much as any treatment consisting of repeated mowing at both sites. The long-term effects on spotted knapweed densities are unknown, but Rinella and others [159] hypothesize that repeated annual mowing may shift the competitive balance in favor of desired grasses. Rolling plots with a pasture roller, burning, mowing, or harrowing treatments had no effect on the spotted knapweed seedbank at 2 sites in Montana [23].

Burning: For information on fire management considerations for spotted knapweed, please see the "Fire Management Considerations" segment of the "Fire Effects" section of this report.

Biological control: The aim of biological control is to stress spotted knapweed and shift the competitive advantage away from the weed to desirable grasses and forbs [31,130,175,230]. Biological control efforts for diffuse and spotted knapweed have been underway in North America since 1970. Wilson and McAffrey [230] provide a discussion of considerations and safety issues in developing and implementing a biological control program. The objective of biological control efforts is to propagate and redistribute sufficient insect populations to hold spotted knapweed to population levels similar to their populations in Europe [209]. It is believed that 4 agents are necessary to affect 1 plant species, and that 6 established agents would help control both diffuse and spotted knapweeds since most agents attack both plants [60]. To date, 13 Eurasian insects have been introduced for the control of these knapweeds, several of which are demonstrating some impact against 1 or both species. Of these, 8 of the insects attack the flower heads, while 5 attack the roots [195]. The idea is that these agents will work together to reduce viable seed production and stunt the overall growth and strength of the plants [209]. In this sense, there is an additional ecological niche to be filled, since no agents attack the rosette foliage or root crown. These structures are vulnerable to attack for at least 1 year before the plant can reproduce [182].

Story and Piper [195] provide a current assessment of the status of biocontrol agents on spotted knapweed. On sites in Montana where Urophora affinis and U. quadrifasciata coexist, spotted knapweed seed production is reduced by at least 50% [194,195], and Harris [59] reports a 92% reduction in spotted knapweed seed production at locations in British Columbia. He also notes that Agapeta zoenga and Cyphocleonus achates are causing noticeable reductions in density and vigor of spotted knapweed at several locations in western Montana [195]. Success has not been dramatic, and has not developed as expected [171]. The mistaken identity and origin of spotted knapweed (see the Taxonomy section of this report) may provide a partial explanation for the limited success of introduced biocontrol agents. Biocontrol agents introduced for control of Centaurea stoebe ssp. micranthos (native to eastern Europe) came from C. maculosa (native to central Europe) [182]. Climatic analyses indicate that the climate of western Montana is more similar to the area in eastern Europe, from which the plant originated, than the area from which the bulk (12 of the 13) of biocontrol agents came. It is therefore suggested that future explorations in the area of origin of the target plant are necessary to find agents that are adapted to colder climates and a more vigorous plant [171,182].

Biological control efforts may also be hindered by indirect effects of herbivory on nontarget species. Callaway and others [20] found that herbivory on spotted knapweed had substantial negative, indirect effects on Idaho fescue under 2 very different sets of experimental conditions. They hypothesized that moderate herbivory may have stimulated compensatory growth in spotted knapweed, induced the production of defense chemicals that also had allelopathic effects, or stimulated root exudates that altered the relationship between knapweed and Idaho fescue via soil microbes [20].

Additional indirect affects of biological control of spotted knapweed have been observed in west-central Montana [145]. Gall fly larvae (Urophora spp.) released as biological control agents for spotted knapweed are the primary food item in native deer mouse diets for most of the year and made up 84-86% of their winter diet. The implications of these findings include the possibility that deer mice and other predators may reduce Urophora populations below a threshold to effectively control knapweed; the unknown effects on deer mouse population dynamics and subsequent effects on food chains; and the effects on seed dispersal, since 9% of deer mouse stomachs also contained knapweed seeds during the period following seed dispersal [146].

The following table shows insects that have been established in North America for the control of spotted knapweed, and the states or provinces in which they have been established or recovered, and additional references pertaining to each [105,175,191,195,203]:

In addition to insect control agents, several microorganisms have been considered for potential control of spotted knapweed including the fungi Sclerotinia sclerotiorum [76,175], Fusarium avenaceum [28] and F. oxysporum [79], and the bacteria Pseudomonas syringae [84]. A phytotoxin isolated from the black leaf blight fungus (Alternaria alternata), maculosin, was found to be the active ingredient in this host-specific pathogen of spotted knapweed, and was synthesized in the laboratory [187,198]. Maculosin appears to be highly toxic only to spotted knapweed and was being researched for potential field efficacy in 1993 [130], though no work has been completed to date [33].

Grazing: Low to moderate levels of grazing of spotted knapweed by cattle, domestic sheep, and domestic goats have been observed in Montana [175]. Of these, sheep seem to have the most promise for control of spotted knapweed through prescribed grazing [27,53,108,142,143]. Others suggest that livestock grazing is not likely to seriously reduce spotted knapweed populations in native bunchgrass communities because of its many other competitive attributes [90,101].

Two consecutive years of sheep grazing in May to early June and again in late summer on 40 acres (16 ha) in western Montana heavily infested with spotted knapweed completely eliminated spotted knapweed seed production, and the sheep were healthy [26,86]. Olson and others [143] observed that 1- and 2-year-old spotted knapweed plants were effectively controlled by sheep grazing in Montana. Sheep grazing when grasses are dormant can reduce potential negative impacts on associated grass species [143] and reduces density of very young spotted knapweed seedlings, thereby limiting seedling recruitment [31,175]. Timing of grazing is important, as sheep were observed to pass viable seed of spotted knapweed up to 7 days after consumption [220].

Chemical: Herbicides are effective in gaining initial control of a new invasion or a severe infestation, but are rarely a complete or long-term solution to weed management. If chemical control is used it must be incorporated into long-term management plans that include replacement of weeds with desirable species, careful land use management, and prevention of new infestations [18]. Use of herbicides may be limited in natural areas. See the Weed Control Methods Handbook for considerations on the use of herbicides in natural areas and detailed information on specific chemicals.

Chemical control of spotted knapweed can be effective in some cases, but may be cost prohibitive because repeat applications are usually necessary to exhaust the seedbank, and because spotted knapweed often infests large tracts of marginal rangeland and rough terrain [50,168,209]. It is important to consider not only the efficacy of the herbicide for control of spotted knapweed, but also its effects on non-target organisms (plants, animals, aquatics, and invertebrates), the environmental persistence of the chemicals used, their decomposition products, and the environmental effects of chemical contaminants and other additives found in herbicide preparations. Chemical control methods are the focus of considerable research. The following summary is current as of July 2001: Clopyralid, dicamba, picloram, and 2,4-D, alone and in combination, can effectively control spotted knapweed on rangeland [175]. Each chemical or combination of chemicals provides different degrees of control for varying periods of time [12,13,14,22,37,42,97,106,114,151,154,173,227], has different effects on non-target organisms [10,12,13,107,152,153,154,155,156,173] (including biocontrol agents [60,77,121,193,215]), and has different degrees of environmental persistence [222]. Rate and timing of application and site conditions (e.g. soil texture and precipitation) will affect the degree of control, the impacts on non-target organisms, and the persistence of the chemicals in the environment.

Germination of spotted knapweed seeds was not affected by the application of wheat gluten meal at any rate [47]. Fertilization trials in western Montana using variable rates of nitrogen fertilizer suggest that N fertilization alone is an impractical control approach, and would likely contribute to an increase in spotted knapweed [194].

Prescribed burning in combination with herbicide application may increase the efficacy of the herbicide and stimulate growth from competitive species. See the "Fire Management Considerations" segment of the "Fire Effects" section of this report for more information. A study examining the effect of the timing and frequency of grass defoliation on spotted knapweed reinvasion in areas treated with picloram and fertilizer combinations found increased densities of spotted knapweed at intermediate fertilizer applications rates, and better grass growth at higher rates. Also, alternating spring/fall defoliation resulted in higher spotted knapweed density and biomass than did annual spring or fall defoliation, while fall defoliation alone appeared to be the best for minimizing spotted knapweed. The site with residual understory of smooth brome and timothy was more responsive to picloram treatments than the site which had a residual understory of Kentucky bluegrass, which was more affected by fertilizer and clipping treatments [75].

Competition: Reducing spotted knapweed density on wildlife ranges without filling the empty niches with more desirable forage and/or native species may encourage the proliferation of other exotics that are less palatable and more toxic, spiny, or otherwise noxious than spotted knapweed [130,232]. Temporary control of spotted knapweed is an attainable objective; however, the subsequent establishment of a healthy community of desirable plants is required for a more permanent solution [69,174,178]. In a container study, competition with grass had a more negative effect on spotted knapweed growth than did either root herbivory or nitrogen shortage [184]. A healthy, weed-resistant plant community consists of a diverse group of species that occupy diverse niches [74,178]. Establishing desirable species with diverse above- and belowground growth can enhance resource capture by desirables on the site and may limit exotic invasion. In a growth chamber study of competition between spotted knapweed, bluebunch wheatgrass and northern sweetvetch (Hedysarum boreale), Jacobs and Sheley [74] found evidence to support the idea that maintaining taprooted forbs along with grasses increases niche occupation and may be more effective in minimizing invasion of taprooted weeds than grasses alone.

Due to the limited availability of seed from native species that are capable of rapid development under stress conditions [69], natural revegetation of spotted knapweed infested rangelands often fails [216]. The initial level of spotted knapweed reduction necessary to shift the competitive relationship between spotted knapweed and bluebunch wheatgrass varies with location, plant densities, and the initial composition and density of the suppressed grasses [76,174]. Jacobs and others [76] provide evidence that establishment of bluebunch wheatgrass on spotted knapweed infested rangeland may be improved by combining the fungus Sclerotinia sclerotiorum with dense grass seeding, although the fungus has not been approved for use as a biocontrol agent to date.

In areas without a residual understory of desired plants, artificial revegetation is required for effective control of spotted knapweed [175,178,216]. Revegetation of aggressive species has been shown to inhibit reinvasion by spotted knapweed [71,175,216]. In a controlled environment experiment, Lindquist and others [112] found that smooth brome (an invasive species itself) is capable of suppressing the growth of spotted knapweed, with the degree of suppression increasing with increasing nitrogen levels. Bluebunch wheatgrass and Idaho fescue had no impact on spotted knapweed growth [184]. Vigorous grass (especially crested wheatgrass) slow knapweed invasion; however, the drought tolerance and high seed production of spotted knapweed enable it to rapidly occupy dryland range, especially if grass vigor is reduced by grazing [60]. Both 'Durar' hard fescue (Festuca trachyphylla) and 'Covar' sheep fescue (F. ovina) are fairly aggressive competitors with spotted knapweed in the Columbia River Basin, although both are introduced grass species [63]. Reseeding may be limited by the challenge of achieving adequate seed coverage, which is usually achieved by drilling and not feasible on most rangelands [69]. Some specific approaches to controlling spotted knapweed by planting competitive species are given by Sheley and others [175] and Velegala and others [216]. Combining herbicide treatments with reseeding and/or fertilizer applications has met with some success [71,162,177].

Related categories for SPECIES: Centaurea stoebe ssp. micranthos | Spotted Knapweed