By Celestine Duncan and Melissa Brown Munson
The following comprehensive review was originally prepared in 2001 and revised in 2019.
Introduction and Biology
Spotted knapweed and diffuse knapweed are closely related noxious weeds infesting over 7 million acres in 17 western states (Duncan and Jachetta 2005). These weeds are well adapted to a wide range of habitats including open forests, rangeland, roadsides, pastureland, riparian areas, and ditch banks.
Spotted knapweed is indigenous to southcentral and southeastern Europe and northwestern Asia (Story et al. 2004). The weed was introduced to North America from Eurasia as a contaminant in alfalfa (Muller et al. 1988). It was first recorded in the northwest in Victoria, British Columbia in 1893 (Groh 1944), and in Ravalli County, Montana in 1920 (Rice 2007). Currently spotted knapweed infests all states in the U.S. except Alaska, Texas, Oklahoma and Mississippi (USDA, NRCS 2016), infesting 6.9 million acres in the United States with 75 percent of infestations occurring in Montana, Idaho, Oregon, and Washington (Duncan 2005b). Spotted knapweed occurs in higher rainfall regimes than those commonly associated with diffuse knapweed (Harris and Cranston 1979).
Spotted knapweed is a tap-rooted perennial forb that spreads by seed. Seedlings and mature plants over-winter in a rosette stage and resume growth in early April. Spotted knapweed blooms from mid to late July through early September. The weed is a prolific seed producer with 1,000 or more seeds per plant that are spread easily by water, animals, humans, and vehicles (Duncan et al. 2011). Seeds remain viable in soil for more than seven years (Davis et al.1993). Wallander and others (1995) reported that viable spotted knapweed seeds can pass through the digestive tract of sheep and mule deer for up to three days. Individual spotted knapweed plants can live for as long as nine years (Boggs and Story 1987). Disturbed sites are most susceptible to initial establishment of the weed.
Diffuse knapweed is native to grassland and shrub steppes of eastern Mediterranean and western Asia, occurring from southern former USSR to western Germany (Müller and Schroeder 1989). It is most common in the Ukraine and Crimea (Popova 1960). Diffuse knapweed was first reported in western Northern America at Bingen, Washington in 1907 (Roche and Talbott 1986). Duncan (2005a) reported approximately 1.8 million acres in 17 western U.S. states infested with diffuse knapweed.
Diffuse knapweed is normally a biennial, but may behave as an annual or short-lived perennial (Watson and Renney 1974). Seeds germinate in fall or spring, develop into rosettes the first year and bloom either the first or second year following germination. The weed spreads by vehicles and by tumbling in the wind. Disturbance greatly increases the rate of invasion for diffuse knapweed.
Environmental and economic impacts of spotted and diffuse knapweed are described in Duncan (2005 a, b) and briefly summarized as follows. Spotted and diffuse knapweed threaten long-term productivity of grazing lands by reducing bio-diversity (Harris and Cranston 1979; Tyser and Key 1988; Lesica 1991) and increasing soil erosion (Lacey et al. 1989). These weeds out-compete native species, change plant community structure, degrade or eliminate habitat for native animals, reduce forage for livestock and wildlife, and provide food and cover for undesirable non-native animals (as cited in Duncan 2005 a,b). Pollination of native plants may also be compromised by dense infestations of spotted knapweed (Herron-Sweet 2014). Economic impact to agriculture and wildlands from these weeds is substantial. In 1996, the potential annual loss from knapweeds just to the state of Montana was estimated to be $42 million (Hirsch and Leitch 1996).
Management of spotted and diffuse knapweed is similar. Although there are more data regarding management of spotted knapweed, results can be shared between species.
Several herbicides provide good to excellent control of spotted and diffuse knapweed and are often the most cost-effective treatment for newly established infestations. Herbicide treatments on large infestations are most effective when combined with other management methods that impact knapweed (e.g. biological control agents and grazing livestock) and enhance the competitive ability of desirable forage species. Effective herbicides, application rate, and optimum timing of herbicide application for controlling knapweed are shown in Table 1.
Each herbicide has special characteristics that make them useful in specific situations. Milestone® and GrazonNext® HL herbicides can be applied in both riparian and upland sites, and provide significantly better knapweed control than either 2,4-D or dicamba (Banvel and others). Knapweed occurring in forest habitat types can be effectively treated with Transline® herbicide without injury to conifers.
TABLE 1. HERBICIDE OPTIONS FOR SPOTTED AND DIFFUSE KNAPWEED MANAGEMENT (GRAZING LANDS AND GRAZED WILDLANDS). HERBICIDES INCLUDED IN THIS TABLE ARE SELECTIVE FOR BROADLEAF PLANTS WHEN APPLIED AT RECOMMENDED RATES.
The following section summarizes results of herbicides labeled for spotted and diffuse knapweed control on grazing lands and wildlands. The duration of knapweed control on any site is dependent on herbicide properties, number and type of biological control agents present, and environmental conditions such as soil type, rainfall, and presence of competitive desirable vegetation. In general, control with herbicides tends to be shorter in duration on coarse textured soils and on sites with annual rather than perennial grasses.
MILESTONE® herbicide at 5 to 7 fluid ounces of product per acre (fl oz/A) (1.25 to 1.75 ounces acid equivalent per acre) provides excellent spotted and diffuse knapweed control (Duncan et al. 2005, 2009, 2011) for at least two years following treatment. Timing of application is not critical for controlling spotted or diffuse knapweed; Milestone may be applied any time during the growing season or fall. Applications must be made prior to mid-bud growth stage to stop or reduce seed production the year of treatment. Milestone at 5 to 7 fl oz/A applied to spotted knapweed at the bolting growth stage gave excellent control (>90%) of both established plants and seedlings up to one year after treatment (YAT). Spotted knapweed control 2 YAT at four sites in western Montana averaged 87 and 99 percent respectively with Milestone at 5 to 7 fl oz/A. Milestone at 5 to 7 fl oz/A applied at rosette to pre-bud growth stages provided excellent control (>95%) of diffuse knapweed for at least 1 YAT. Field trials have been conducted to determine the tolerance of desirable forbs to application of Milestone herbicide. Data on individual forb tolerance is available at Report: Native Forb & Shrub Tolerance to Milestone Herbicide.
Benefits of Milestone compared to historical standard herbicide treatments include reduced application rates, treatment up to waters edge, reduced damage to desirable non-target vegetation, and low environmental and mammalian toxicity.
FIGURE 1. PRODUCTION OF DESIRABLE NATIVE GRASSES ONE YEAR AFTER TREATMENT OF SPOTTED KNAPWEED WITH MILESTONE® HERBICIDE AT 5 FLUID OUNCES PER ACRE COMPARED TO NON-TREATED PLOTS AT TWO SITES IN WESTERN MONTANA. (Duncan et al. 2005)
GRAZONNEXT® HL herbicide at 1.5 to 2.1 pints per acre (pt/A) provides similar control of spotted and diffuse knapweed as Milestone at 5 to 7 fl oz/A. Timing of application is not critical for controlling spotted or diffuse knapweed, GrazonNext HL may be applied any time during the growing season or fall (DiTomaso et al. 2013). The combination of 2,4-D with aminopyralid in GrazonNext HL increases the weed control spectrum compared to Milestone alone, and may increase impact on desirable broadleaf plants.
TRANSLINE® herbicide at 2/3 pt/A provides good to excellent control of spotted and diffuse knapweed for up to 2 to 3 years following treatment depending on site conditions and time of application (Duncan et al. 2009, 2011; Sheley et al 2000; Mercier et al. 1997; Rice et al. 1997). This herbicide is most effective when applied during bolt or bud stages, and least effective when applied in fall (Duncan et al. 2011). Foliar activity of Transline on spotted knapweed was not improved with the addition of 2,4-D. Transline can be applied in conifer forest habitat types. A forb diversity study conducted in Montana found no large declines in forb diversity caused by application of Transline at 2/3 pt/A, and small depressions were transitory (Rice et al. 1997; Rice and Toney 1998). Plant diversity increased to pre-treatment levels by the third year.
DICAMBA (Banvel) at 1 quart per acre (1 lb ai/A) and 2,4-D at 2 quarts per acre (2 lbs ai/A) provide inconsistent results. These herbicides must be applied annually until the soil seed bank is depleted. Banvel in combination with 2,4-D provides better control than Banvel alone (Duncan et al. 2011).
FIGURE 2. SPOTTED KNAPWEED CONTROL WITH MILESTONE® HERBICIDE AT 5 FL OZ/A COMPARED TO TRANSLINE® HERBICIDE, AND 2,4-D. DATA COLLECTED 1, 2 AND 3 YEARS AFTER TREATMENT (YAT). DATA FROM MULTIPLE LOCATIONS IN MT, ID, AND WA.
Livestock and wildlife will graze spotted and diffuse knapweed when plants are present at low to moderate levels. Domestic sheep and goats have been shown to readily graze spotted knapweed and can be used as a management tool to control the weed (Olson and Lacey 1994; Olson et al. 1997; Benzel 2009; Sheley et al. 2004). Although rosettes of first year knapweed plants are nutritious and edible, they are difficult for cattle to eat because they grow close to the ground (Popova 1960). Controlled, repeated grazing of spotted knapweed by sheep can reduce the number of one and two year old spotted knapweed plants within an infestation (Olson et al. 1997). However, grazing should be timed so that palatability of associated grasses is reduced. Spotted knapweed can tolerate defoliation; however, severe defoliation will reduce root, crown, and aboveground growth of the weed (Kennett et al. 1992). Cattle and sheep grazed sequentially will eat more spotted knapweed and less grass when spotted knapweed is grazed in late bud to early flowering stage (Henderson et al. 2012).
Establishment of competitive desirable perennial grasses may promote long-term suppression of knapweeds. However, Sheley and others (1997) reported that regardless of vigor of native or desirable introduced grasses, knapweed seedlings may establish. Results from 9 to 15 year old revegetation sites show that seeded desirable species sometimes persist and suppress spotted knapweed for long periods, but short-term data cannot predict if, when, or where this will occur (Rinella et al. 2012). Jacobs (2017) reported that ecological restoration did not effect spotted knapweed density unless combined with an herbicide application.
Desirable grass species also differ in their competitive ability to keep weeds out of an area. Diffuse knapweed density was greater in areas seeded to Russian wildrye (Psathyrostachys junea) compared to areas seeded to crested wheatgrass (Agropyron cristatum) (Berube and Myers 1982; Maxwell et al. 1992). Herbicide applications were more important than seeding competitive grasses for controlling knapweed (Maxwell et al. 1992). Durar hard fescue (Festuca trachyphylla) limited diffuse knapweed reinvasion more effectively than smooth brome (Bromus inermis) or orchardgrass (Dactylis glomerata) (Roche and Roche 1999). Lindquist and others (1996) found that smooth brome was capable of suppressing the growth of spotted knapweed, with the degree of suppression increasing with increasing nitrogen levels.
There are few studies on the effect of burning or burning in combination with herbicides on spotted or diffuse knapweed. In general, fire alone has been observed to increase knapweed density on some sites. Fires may create the type of disturbance that promotes colonization of knapweeds by creating areas of bare soil and increasing sunlight that reaches the soil surface (Sheley et al. 1999). A single, low-intensity fire does not control knapweed, and may increase knapweed cover and density (Sheley and Roche 1982; Arno unpub.). Under wildfire situations, spotted knapweed has been observed to increase (Noste 1982; Toth 1991) probably as a result of increased niches which promote establishment and spread of the knapweeds (Pokorny 2010). In prairie regions, a combination of prescribed fire and spot-burning reduced spotted knapweed populations (McGowan-Stinski 2003), with spring (McDonald et al. 2007) or summer burns suggested as an effective management tool (Emery and Gross 2005).
Fire followed by herbicide treatments may increase effectiveness of herbicide treatments on knapweed (Sheley and Roche 1982; Dewey 2000). Spot spraying with herbicide may help maintain desired species richness while managing spotted knapweed following wildfire (Pokorny et al. 2010).
Manual and Mechanical
Spotted and diffuse knapweed are tap-rooted plants that can be controlled with tillage. Cultivation to depths of seven inches or more have been shown to control spotted knapweed (Popova 1960). However, even under intensive cultivation knapweed can regenerate from seed remaining in soil. Cultivation in combination with seeding perennial grass species may enhance establishment of grasses (Velagala et al. 1997) and decrease re-establishment of spotted knapweed.
Persistent and careful hand pulling can control spotted knapweed. Since regrowth can occur from root crowns, the entire crown portion of the plant (from soil surface to a depth of about 3 inches) must be removed. Plants can be pulled most effectively following a rain or when the soil is moist. Disturbance caused by hand pulling may increase susceptibility of the site to reinvasion by knapweed seed present in soil (Lacey et al. 1992). While this control method is effective on single plants or relatively small infestations, it is not economically feasible on large, well-established knapweed infestations (Brown et al. 1999).
Data regarding mowing is inconsistent. Roche and Roche (1990) reported that 22 percent of diffuse knapweed plants mowed to a two-inch height each month of the growing season of April through October were still growing after four years. However, mowing will reduce the number of viable seed produced by diffuse knapweed if mowed at early flowering stages.
Davis and others (1993) reported that a single mowing at late bud growth stage can reduce the number of seed produced on spotted knapweed. A single fall mowing when spotted knapweed was flowering or producing seed reduced mature knapweed density 85 and 83 percent below non-mowed sites (Rinella et al. 2001). Mowing treatments may also reduce subsequent seedling density. Another study indicated that spotted knapweed mowed at bolt and late bud stage for 2 consecutive years did not reduce spotted knapweed cover (Brown et al. 1999).
The addition of nitrogen fertilizer has been reported to enhance both spotted knapweed and diffuse knapweed growth (Popova 1960; Story et al. 1989; Roche 1988). Nitrogen plus phosphorous applied at 28 plus 35 pounds per acre (lb/A) increased grass yield but did not reduce knapweed on sites with a substantial grass understory (Sheley and Jacobs 1997).
Biological control is the use of native or foreign insects, pathogens or other living organisms to attack weeds. As a weed management method, biological control offers another tool to compliment conventional methods. Thirteen Eurasian natural enemies (all insects) have been introduced for biological control of spotted and diffuse knapweed. Most of the insects attack both plant species. Status of the insects follows (from Duncan et al. 2011 unless otherwise noted):
Two flower head flies (Urophora affinis and U. quadrifasciata) were introduced into the Pacific Northwest in 1973 and 1980, respectively, and are now well established. The larvae induce galls in the flower heads which reduce seed production. Seed reductions in excess of 50 percent are occurring in areas where the two fly species coexist (Story et al. 1989).
A flower head moth (Metzneria paucipunctella), released in 1980, is established in small numbers in western Montana. The larvae feed on the florets and seeds of spotted knapweed. Each larva destroys about eight seeds per flower head (Story et al. 1991).
A root moth (Agapeta zoegana) and a root weevil (Cyphocleonus achates), released in 1984 and 1988, respectively, are both established at numerous locations. Larvae of the moth girdle knapweed roots, while the weevil larvae feed in the center of the roots. The two insects are causing measurable reductions in knapweed biomass at several locations. In western Montana, C. achates reduced spotted knapweed density by up to 99% (Story et al. 2006), and reduced vigor of the weed (Corn et al. 2006).
A root beetle (Sphenoptera jugoslavica), released in 1983 is established on diffuse knapweed. The larvae feed in the center of the root. The beetle primarily attacks the roots of diffuse knapweed, but will also attack spotted knapweed. The insect appears to be causing reductions in diffuse knapweed biomass in selected areas.
Larinus minutus, a flower head weevil released against diffuse knapweed, is well established, and is causing significant reductions in the biomass and density of that plant. Larinus obtusus, a flower head weevil released against spotted knapweed, is established but is increasing at a much slower rate than L. minutus. The larvae of both weevils feed on knapweed seeds, and the adults feed on knapweed leaves.
Three insects, a root moth (Pelochrista medullana) released in 1984, and two seed head flies (Chaetorellia acrolophi and Terellia virens) are established on spotted knapweed, but in very small numbers. The life history and behavior of P. medullana is very similar to A. zoegana, but for unknown reasons, P. medullana has had great difficulty establishing in some states. Similarly, C. acrolophi and T. virens are not establishing nearly as easily as the seed head gall flies (Urophora spp.) In contrast to the Urophora species, the larvae of C. acrolophi and T. virens feed directly on the seeds and do not form galls.
Two insects, a root moth (Pterolonche inspersa) and a flower head weevil (Bangasternus fausti) have failed to establish on spotted knapweed in some areas of the Pacific Northwest.
The effect of herbicides on specific biological control agents has been studied. Resultssuggest that application of the herbicide 2,4-D is compatible with both Urophora affinis and U. quadrifasciata for management of spotted knapweed (McCaffrey and Callihan 1988; Story et al. 1988). Similar results were found with Transline® herbicide on S. jugoslavica. Transline was applied in June in combination with S. jugoslavica, which improved diffuse knapweed control without harming insect density (Wilson 2001). Reduced rates ofpicloram (Tordon® 22K) were not found to limit establishment of Cyphocleonus achates (Jacobs et al. 2000). Complete removal of spotted knapweed from a plant community will cause biological control insects tomove to other knapweed-infested sites.
Integrated Weed Management
Management of large-scale knapweed infestations is more effective with a combination of treatments rather than using any one treatment method alone. Several studies have reported knapweed control with herbicides combined with other treatment methods. Maxwell and others (1992) reported that diffuse knapweed treated with herbicides re-established more rapidly when the treatment was followed by late-season grazing compared to non-grazed plots. Sheley and others (2004) reported that application of herbicides released perennial grasses from spotted knapweed competition and changed the weed population from mature, less palatable plants to juvenile plants that were preferred by sheep. Sheley and Jacobs (1997) found that fertilizer in combination with an herbicide treatment did not increase effectiveness or longevity of spotted knapweed control or increase grass production. Application of selective herbicide in combination with reseeding desirable grasses can reduce spotted knapweed establishment (Sheley et al. 2001; Mangold et al. 2015; ). Field studies that combine hand-pulling, mowing, and insects in combination with various herbicide treatments have been conducted Brown and others (1999) reported that mowing at the late bud stage in combination with an herbicide treatment may be more effective than the herbicide alone. Hand pulling following mowing and herbicide treatments increased spotted knapweed control (Brown et al. 1999; McDonald et al. 2013) and is more effective than hand-pulling alone, but cost is greater than herbicide treatment alone.
RELATED: Spotted Knapweed Management with Herbicides >
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