Acroptilon repens (L.) DC.

Rounded Global Status Rank: GNR - Not Yet Ranked

More info for the terms: allelopathy, competition, cover, density, fire management, fresh, invasive species, natural, presence

Impacts: Russian knapweed is considered a serious habitat invader [86]. A single patch or infestation of Russian knapweed can grow quite rapidly. Once established, it can form dense infestations that reduce desirable vegetation through a combination of competition and allelopathy [45,86]. Both roots and leaves of Russian knapweed have been found to produce chemicals inhibitory to the germination and growth of other species [9,29,41,71,72]. The presence of Russian knapweed can thereby reduce forage for livestock and biodiversity for wildlife habitat [86]. Russian knapweed plants, ingested in sufficient quantities as fresh or dried forage, are toxic to horses causing the neurological disorder nigropallidal encephalomalacia [52,57]. The presence of Russian knapweed in hay decreases its feed and market value [61,83].

Whitson [86] claims an 11% average annual increase of infestations in Wyoming. See Distribution and Occurrence for current estimations of infested acres for several western states. Olson [51] and Griffith [34] provide guidelines and methods for estimating the ecologic and economic impact of noxious weeds on rangelands.

Control: Russian knapweed has a competitive advantage in a variety of environments, and will continue to persist unless it is suppressed long enough to allow the introduction and establishment of desirable species [13,17]. Continued monitoring and follow-up treatments must be conducted at least annually to eliminate reinfestation of Russian knapweed [17,61].

Keys to controlling Russian knapweed are 1) stressing the plant and causing it to expend nutrient reserves in its root system [8], 2) eliminating new seed production, and 3) controlling its vegetative spread by planting competitive species and/or isolating the infestation so as not to spread root fragments to other locations during treatment. If sufficient human resources are available, mechanical control is a good place to start. Hand pulling or cutting the plants to kill the tops will starve the roots if done repeatedly, every time they sprout [61]. Continued vigilance is necessary since removed aerial parts are rapidly regenerated from vegetative buds on the roots [83].

Integrated weed management: Managers are encouraged to integrate control methods that complement one another. Integrated management includes considerations of not only killing the target plant, 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(s) and determine the size of the infestation(s); assessment of nontarget vegetation, soil types, climatic conditions, and important water resources; and an evaluation of the benefits and limitations of each control method [48]. Components of any integrated weed management program are sustained effort, constant evaluation, and the adoption of improved strategies [67].

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 [68]. A weed management strategy may include designed disturbance (e.g., mechanical or chemical control), controlled colonization (e.g., planting competitive species), and controlled species performance (e.g., biological control) [10,68,88]. An integrated management plan for Russian knapweed includes efforts to place continual stress on the plant [8].

Prevention: The most efficient and effective method of managing invasive species is to prevent their invasion and spread [69]. The fact that Russian knapweed became established wherever Turkestan alfalfa seed was planted suggests that only seed that has been cleaned and tested for purity should be used [61]. Preventing the establishment of Russian knapweed in natural areas is achieved by maintaining healthy natural communities and by conducting aggressive monitoring at least 3 times each year [17]. Begin monitoring in spring (late May to mid-June) when the plants have recently bolted; next in the summer (July) to find any missed plants that have flowered and are easily recognizable; and finally in the fall (late August or early September) to find any late-blooming plants that might have regrown from the root system of plants that had been pulled during an earlier search. Monitoring efforts are best concentrated on the most disturbed areas in a site, particularly along roadsides, parking lots, fencelines, and waterways. When an infestation is found, the location can be recorded and the surrounding area surveyed to determine the size and extent of the infestation, so these sites can be revisited on follow-up surveys [17]. For more on monitoring see Johnson [38]. It is important to kill whatever plants are found, followed by some combination of mechanical, chemical and/or biological control [17].

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 [76]. See the Guide to noxious weed prevention practices [76] for specific guidelines in preventing the spread of weed seeds and propagules under different management conditions.

Physical or mechanical control: Removal of the aboveground portion of Russian knapweed plants reduces the current year's growth and will eliminate seed production for that year, but it will not kill the roots of Russian knapweed. Cutting, mowing or disking Russian knapweed infestations several times annually will control the existing topgrowth. Often, the plants that reemerge are smaller in size and lower in vigor. This is a good indication that the plants are under stress and that their nutrient reserves are declining [17]. Cutting (or mowing) 3 times a year (spring, summer, fall) stresses Russian knapweed plants and forces them to use nutrient reserves stored in the root system. Unless repeated frequently, the cut plants recover vigorously the following year [8]. Mowing can also damage surrounding plants and is not likely to be feasible in natural areas [17]. Mowing twice during the growing season in Wyoming was not effective in controlling Russian knapweed [10]. Mowing may stimulate regeneration of aerial parts and prevent dehydration of underground organs (Tarshis 1967, as cited by [83]).

Cutting is slightly less effective than pulling since cutting does not remove any portion of the Russian knapweed root. Pulling plants 2 to 3 times annually contained, but did not eliminate, a Russian knapweed infestation in Washington (Youtie 1998, as cited by [17]). Cutting roots for 3 years to a depth of 12 inches (30 cm) can destroy the root system in the top meter of soil. Root fragments up to 16 inches (40 cm) long are killed by burial below 12 inches (30 cm), indicating some control by deep plowing ([83], and sources therein).

Early control attempts involved starving the roots by smothering with straw and manure, which worked on small patches when materials were thick enough to prevent stems from penetrating through to sunlight. Sheet metal and paper have also been used successfully to control small patches [61], indicating that solarization with black plastic may be effective for controlling Russian knapweed, especially since its growth form causes it to occur in patches.

Fire: See the Fire Management Considerations section of this summary.

Biological: Subanguina picridis, a gall-forming nematode, is the first biological control agent approved for release on Russian knapweed and is established at limited sites in Colorado, Montana, New Mexico, Oregon, Utah, Washington, Wyoming, Alberta, and British Columbia [25]. Subanguina picridis has not proven to be an effective agent under field conditions. Additional agents are being sought and tested [44,62,64]. The biology, gall morphology and physiology, and effectiveness in weed control of S. picridis are discussed by Harris and Shorthouse [35] and Rosenthal [62].

Grazing is not a viable method of control for Russian knapweed since the plant is generally avoided by grazing animals and can be poisonous to horses. When Russian knapweed is present on horse pasture, it must be removed or fenced off to prevent horses from eating it.

Chemical: Before using herbicides for control of invasive plants, managers must consider the effectiveness of the herbicide on the target plant, appropriate timing and rates of application, the potential impacts on nontarget organisms, and residual activity and toxicity of the herbicide. 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 [16]. 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.

In most situations, Russian knapweed cannot be effectively managed by herbicide alone [8]. Chemical control of Russian knapweed has proven more difficult than that of other knapweed species [58]. However, herbicide treatments may stress the plant enough to give desirable plants a competitive advantage [8,10]. Herbicides evaluated for control of Russian knapweed include 2,4-D, picloram, dicamba, clopyralid, glyphosate, and combinations thereof. Comparative trials designed to evaluate chemicals, combinations, rates, and/or timings are available [23,28,80]. Carpenter and Murray [17] give a synopsis of the effects of picloram, clopyralid, and glyphosate on Russian knapweed and associated species. Benz and others [10] evaluated several chemicals with and without seeding of "improved" grass species. Best results were obtained with clopyralid + 2,4-D and 'Sodar' thickspike wheatgrass (Elymus lanceolatus) [10]. Carpenter [18] includes extensive data on the effects of different rates of picloram, clopyralid, and metsulfuron on the density of nontarget plants from several families.

Cultural: Russian knapweed is sensitive to light competition [22], and crops that produce dense shade under irrigation (e.g., alfalfa) have been successfully used to suppress Russian knapweed [59]. Any treatment that provides control of Russian knapweed must either release competitive species present in the understory or be combined with reseeding before long-term sustainable control can be achieved [86,87]. When perennial grasses such as western wheatgrass and blue grama were present in the understory, single applications of picloram resulted in 85% control of Russian knapweed 8 years following treatment. When sites that lacked a desirable understory were similarly treated, annual weeds replaced Russian knapweed. Two applications of picloram and clopyralid before seeding and 1 application 1 year after seeding with wheatgrass (Triticaceae) species and Russian wildrye provided control up to 5 years after seeding [87]. Herbicide application and planting of Russian wildrye provided a competitive monoculture 7 to 9 years after treatment in Wyoming. The dense, fibrous root system of Russian wildrye may give it a competitive advantage in the capture of moisture and nutrients and physically inhibit the entry of Russian knapweed lateral roots [12,13].

Whitson [86] suggests that the use of herbicides to control Russian knapweed before establishing perennial grasses can be an important part of a management system. Benz and others [10] found that seeded grass species needed an initial control treatment of Russian knapweed in Wyoming old fields before they could establish. It has also been suggested that tillage of Russian knapweed surface residue is necessary to hasten the decomposition of allelochemicals before planting competitive species [8,12,86], although no experimental evidence was given to support this assertion. Furthermore, experimental evidence provided by Bottoms and Whitson [12] suggests that grass cover is similar between tilled and untilled plots, and that "the only significant treatment providing both yield and control was nontilled Russian wildrye treated with picloram." Tilling is not possible or appropriate for most natural areas [17]. The nonnative smooth brome competes well with young Russian knapweed shoots and is somewhat tolerant to the growth inhibitors in the soil [8]. Unfortunately, smooth brome is itself invasive in many grassland and meadow habitats across North America [17].

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