Rhaponticum repens (Russian knapweed)
- Summary of Invasiveness
- Taxonomic Tree
- Notes on Taxonomy and Nomenclature
- Plant Type
- Distribution Table
- History of Introduction and Spread
- Risk of Introduction
- Habitat List
- Hosts/Species Affected
- Host Plants and Other Plants Affected
- Biology and Ecology
- Latitude/Altitude Ranges
- Air Temperature
- Rainfall Regime
- Soil Tolerances
- Natural enemies
- Notes on Natural Enemies
- Means of Movement and Dispersal
- Pathway Causes
- Pathway Vectors
- Plant Trade
- Impact Summary
- Economic Impact
- Environmental Impact
- Threatened Species
- Risk and Impact Factors
- Uses List
- Detection and Inspection
- Prevention and Control
- Distribution Maps
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PicturesTop of page
IdentityTop of page
Preferred Scientific Name
- Rhaponticum repens (L.) Hidalgo
Preferred Common Name
- Russian knapweed
Other Scientific Names
- Acroptilon picris (Pallas ex Willd.) C.A. Mey
- Acroptilon repens (L.) DC.
- Centaurea picris Pallas ex Willd.
- Centaurea repens L.
International Common Names
- English: creeping knapweed; hardhead thistle; hardheads; Turkestan thistle
- Spanish: centaurea rastrera
- French: centaurée aux amères
Local Common Names
- Argentina: centaurea rusa
- Germany: Federblume
- CENRE (Rhaponticum repens)
Summary of InvasivenessTop of page
R. repens is a deep rooted perennial that is native to Eurasia. It was accidentally introduced into North America as a contaminant of seed and spread rapidly. R. repens can be a serious crop pest in its native range and elsewhere. It forms large monotypic stands that reduce diversity and degrade forage quality on rangelands. As it is allelopathic and survives under a variety of conditions, it has been become an invasive exotic wherever it is imported. It has been declared a noxious weed in 18 US states (USDA-NRCS, 2016) and four Canadian provinces (Rice, 2003).
Taxonomic TreeTop of page
- Domain: Eukaryota
- Kingdom: Plantae
- Phylum: Spermatophyta
- Subphylum: Angiospermae
- Class: Dicotyledonae
- Order: Asterales
- Family: Asteraceae
- Genus: Rhaponticum
- Species: Rhaponticum repens
Notes on Taxonomy and NomenclatureTop of page
R. repens was named Centaurea repens by Linnaeus in 1763. However, it is distinguished from Centaurea by the attachment scar on the achene which is sub-basal rather than lateral. Also, the rust, Puccinia acroptili, which attacks R. repens is different from the rust species that attack Centaurea (Watson, 1980).
To distinguish R. repens from other Centaurea, the name was revised to Acroptilon repens (L.) DC (USDA-ARS, 2003). This was the name designated by de Candolle in 1838. Synonyms listed by Ochsmann (2003) are Centaurea picris Pallas ex Willd. (1803) and Acroptilon picris (Pallas ex Willd.) C.A. Mey. (1831). Based on phylogenetic analyses of the subtribe Centaureinae, the name was revised to Rhaponticum repens (L.) Hidalgo (Hidalgo et al., 2006).
DescriptionTop of page
R. repens is a long-lived perennial that forms dense stands by sprouting shoots from its creeping, horizontal roots. Adventitious buds on lateral roots produce erect stems that are thin, openly branched, and 0.5-0.7 m tall. Young stems are covered with woolly hairs, which rub off in time, giving older stems a dark-brown appearance.
Greyish-white leaves are alternate, simple and variable in shape. Basal leaves are deeply lobed or pinnatifid, 5-10 cm long and 1-2.5 cm broad, forming a quickly withering rosette. Lower stem leaves are lobed or sharply toothed. Upper leaves are entire, 1-3 cm long, linear to narrowly oblong with a sharp-pointed tip.
Flower heads are urn-shaped, 0.8-1.3 cm in diameter just above the base, solitary, and found at the tips of branches. Silvery buds form tubular pink or purple flowers that turn straw-coloured at maturity. The involucre is slenderly ovoid, pale, and about 1 cm high. Green outer floral bracts are rounded, ovate with clear, entire margins. Inner bracts are oblong-acuminate and cut-margined with hairy tips.
The achene is oblong, 2-3 mm long and 0.6-0.7 mm wide, greyish or ivory coloured, smooth with inconspicuous lines. It has a whitish, thread-like pappus at the apex that drops off when the seed matures (USDA, 1970; Watson, 1980; Roche and Roche, 1991).
Plant TypeTop of page Herbaceous
DistributionTop of page
R. repens is native to southwestern and Central Asia, Russia, Mongolia and western China (USDA-ARS, 2003). It has been introduced in to North and South America, South Africa and Australia. In the USA, R. repens is widespread in Colorado, Idaho, Montana, Nevada, Oregon, Utah, Virginia, Washington and Wyoming. Most of the 0.6 million hectares in the USA are found west of the Rocky Mountains; however, it is found in disturbed areas and along roadsides in many other states (Duncan, 2001; USDA-NRCS, 2002).
Distribution TableTop of page
The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.
|Continent/Country/Region||Distribution||Last Reported||Origin||First Reported||Invasive||Reference||Notes|
|Afghanistan||Widespread||Native||Invasive||Maddox et al., 1985; EPPO, 2014|
|Armenia||Present||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|Azerbaijan||Present||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|Bahrain||Present||Introduced||Invasive||Maddox et al., 1985|
|Georgia (Republic of)||Present||Native||EPPO, 2014|
|India||Widespread||Introduced||Invasive||Maddox et al., 1985; EPPO, 2014|
|Iran||Widespread||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|Iraq||Present||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|Kazakhstan||Present||Native||Invasive||Zouhar, 2001; EPPO, 2014|
|Mongolia||Present||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|Pakistan||Present||Introduced||Invasive||Maddox et al., 1985|
|Turkey||Widespread||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|South Africa||Widespread||Introduced||Invasive||Maddox et al., 1985; EPPO, 2014|
|Canada||Widespread||Introduced||Invasive||USDA-NRCS, 2002; EPPO, 2014|
|-Alberta||Widespread||Introduced||Invasive||Carpenter and Murray, 2002; EPPO, 2014|
|-British Columbia||Widespread||Introduced||Invasive||Carpenter and Murray, 2002; EPPO, 2014|
|-Manitoba||Widespread||Introduced||Invasive||Carpenter and Murray, 2002; EPPO, 2014|
|-Ontario||Present||Introduced||Invasive||Carpenter and Murray, 2002; EPPO, 2014|
|-Saskatchewan||Widespread||Introduced||Invasive||Carpenter and Murray, 2002; EPPO, 2014|
|USA||Widespread||Introduced||Invasive||USDA-NRCS, 2002; EPPO, 2014|
|-Colorado||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Idaho||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Montana||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Nevada||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-New Mexico||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-North Dakota||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-Oregon||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-South Dakota||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-Utah||Widespread||Introduced||Invasive||Maddox et al., 1985|
|-Virginia||Widespread||Introduced||Invasive||Maddox et al., 1985|
|-Washington||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Wyoming||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|Argentina||Widespread||Introduced||Invasive||Maddox et al., 1985; EPPO, 2014|
|Russian Federation||Restricted distribution||Native||Invasive||USDA-ARS, 2003; EPPO, 2014|
|-Central Russia||Restricted distribution||EPPO, 2014|
|-Southern Russia||Widespread||EPPO, 2014|
|-Western Siberia||Present, few occurrences||EPPO, 2014|
|Ukraine||Restricted distribution||EPPO, 2014|
|Australia||Widespread||Introduced||Invasive||Maddox et al., 1985; EPPO, 2014|
|-New South Wales||Present||Introduced||Invasive||Maddox et al., 1985|
|-Queensland||Present||Introduced||Invasive||Maddox et al., 1985|
|-South Australia||Present||Introduced||Invasive||Maddox et al., 1985|
|-Victoria||Present||Introduced||Invasive||Maddox et al., 1985|
|-Western Australia||Present||Introduced||Invasive||Maddox et al., 1985|
History of Introduction and SpreadTop of page
R. repens was introduced to Canada as a contaminant of Turkestan lucerne seed in the early 1900s; it arrived in California, USA in about 1910 as a contaminant of Turkestan lucerne seed or possibly sugarbeet seed. Contaminated crop seeds are likely to be responsible for its spread worldwide (Groh, 1940; Watson, 1980; Maddox et al., 1985).
Risk of IntroductionTop of page
The risk of introduction is high if lucerne and other crop seeds are not screened for the presence of R. repens seeds (Zouhar, 2001).
HabitatTop of page
R. repens is found along roadsides, railways, riverbanks, irrigation ditches, pastures, waste places and clearcut forests. It is commonly found in areas near a water source, such as river bottoms, areas that are irrigated, or habitats with past or current soil disturbance. It interferes with grain crops and is found on cultivated land (Roche et al., 1986; Roche and Roche, 1988; Zouhar, 2001) and on rangeland.
In Washington State, USA, the indicator species for sites susceptible to R. repens invasion is basin wild rye, Elymus cinereus [Leymus cinereus]: basin wild rye occurs in 75% of sites invaded by R. repens (Watson, 1980; Roche and Roche, 1991).
Most of the Canadian infestations occur in the drier, southern regions of British Columbia, Alberta and Saskatchewan (Watson, 1980).
Habitat ListTop of page
|Terrestrial – Managed||Cultivated / agricultural land||Present, no further details||Harmful (pest or invasive)|
|Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Terrestrial ‑ Natural / Semi-natural||Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
Hosts/Species AffectedTop of page
R. repens is believed to be allelopathic and interferes with grain crops such as lucerne (Medicago sativa), wheat (Triticum species), barley (Hordeumvulgare) and oats (Avena sativa) (Heap and Mitchell, 1992). It spreads rapidly in good pastures and is a serious noxious weed of dryland crops in southern Russia (Watson, 1980).
Host Plants and Other Plants AffectedTop of page
Biology and EcologyTop of page
Many knapweeds are diploid or tetraploid, but R. repens is haploid, with 13 chromosomes (2n = 26). In Canada, different populations have different leaf shape and margin characteristics, but differences are not great enough to classify into two different forms (Watson, 1980).
Each plant contains both male and female flowers (monoecious). Flowers are pollinated mostly by insects, fertilisation occurs only with outcrossing (Watson, 1980; Harrod and Taylor, 1995). The seed-heads of R. repens generally remain closed at maturity. Seeds are relatively heavy, and long-range wind dispersal is not important. Seeds are thrown nearby as the plant is brushed by animals or shakes in the wind. The primary means of long-range seed dispersal is probably via livestock and wildlife (seeds remain viable after gut passage), contaminated hay and other seed (primarily lucerne), or by movement of farm machinery or other vehicles. A single plant can produce about 1200 seeds per year, but 100 seeds per plant per year is more typical along roadsides. Seeds are viable for 2-3 years, but longer with proper storage (Zouhar, 2001). Although it can reproduce by seed, vegetative reproduction predominates. Dense colonies form rapidly from adventitious buds on horizontally spreading roots. The root system consists of the taproot, one to many horizontal roots, and their vertical extensions. Tap roots reach a depth of 2 m in the first year, and 5-7 m in the second. Vegetative spread can be hastened by cultivation (Watson, 1980; Zouhar, 2001).
Physiology and Phenology
R. repens is a perennial, invasive forb that often forms dense, monotypic colonies from widely spreading horizontal roots. Roots are scaly and dark brown to black in colour, and form patches that can cover dozens of m² and penetrate up to 7 m deep within two growing seasons. Shoots emerge from perennial roots early in spring, shortly after soil temperatures remain above freezing. Emerging plants form rosettes and bolt in late May to mid-June. New shoots also emerge during the season after rainfall. R. repens flowers from June to October in the USA and from July to September in Canada (Watson, 1980). Seeds germinate over a wide range of temperatures, from 0.5 to 35°C, but optimum temperature is 20-30°C. Light is not necessary for germination, but alternating light and darkness improves germination, and white light stimulates germination. Seeds do not germinate easily, but an initial dormant period can be broken by alternating temperatures (Watson, 1980; Nolan and Upadhyaya, 1988).
R. repens is often found with big sagebrush (Artemisia species) and basin wild rye (Elymus cinereus [Leymus cinereus]) in Oregon. In Montana, it grows in lucerne and grain fields, sagebrush areas and grasslands. Grasses include western wheatgrass (Pascopyrum smithii [Elymus smithii]), smooth brome (Bromus inermis) and Canada wild rye (Elymus canadensis). Other associations include barley (Hordeum spp.), witchgrass (Panicum capillare), green bristle grass (Setaria viridis [Setaria italica subsp. viridis]), pigweed (Amaranthus spp.), lamb's-quarters (Chenopodium spp.), summer-cypress (Kochia scoparia [Bassia scoparia]) and common dandelion (Taraxacum spp.). In Wyoming and Colorado, R. repens occurs on perennial grasslands dominated by blue grama (Bouteloua gracilis [Chondrosum gracile]). Other associations are cottonwood (Populus spp.), willow (Salix spp.) and shrublands (Zouhar, 2001).
R. repens can grow in bottomlands or as high as 2800 m. It tolerates a wide range of moisture. It is more competitive under drier conditions, but occurs in irrigated land. It does well in arid areas with only 250 mm of annual rainfall (Watson, 1980). In the USA, it has also been found in Virginia, where the average annual rainfall is 1180 mm (Maddox et al., 1985). It grows in Minnesota, where average annual temperatures are 4.4°C and in Virginia, where the average is 13.6°C (Maddox et al., 1985; Climate, 2003). In Washington, it thrives in areas with relatively high soil moisture near rivers, creeks, canals, coulees and draws; on toeslopes; in deep, fine-textured soils in high precipitation zones and valley bottoms; and in poorly drained soils (Zouhar, 2001).
R. repens tolerates saline soils and a wide range of soil conditions, but it prefers fine-textured, alkaline soils, high in clay content. It occurs mostly in alkaline, seasonally wet habitats in Montana. In its native range, it grows on clayey, sandy or rocky steppes and sunny meadows, on saline soils, clayey, rocky, or sandy shores of lakes and rivers, on rocky and clayey slopes of hills, and on bottomlands' (Zouhar, 2001).
Latitude/Altitude RangesTop of page
|Latitude North (°N)||Latitude South (°S)||Altitude Lower (m)||Altitude Upper (m)|
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Mean annual temperature (ºC)||4||14|
|Mean maximum temperature of hottest month (ºC)||20||30|
|Mean minimum temperature of coldest month (ºC)||-21||-4|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||0||5||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||250||1180||mm; lower/upper limits|
Rainfall RegimeTop of page Summer
Soil TolerancesTop of page
Special soil tolerances
Natural enemiesTop of page
|Natural enemy||Type||Life stages||Specificity||References||Biological control in||Biological control on|
|Boeremia exigua var. rhapontica||Pathogen||Leaves|
Notes on Natural EnemiesTop of page
R. repens is relatively free of natural enemies in North America. In its native range it is attacked by various beetles (species of Chrysolina and Lixus strangulatus) and flies (Urophora, xanthippe, U. kasachstanica and Jaapiella ivannikovi), and a number of other insects, nematodes and fungi (Watson, 1980).
Arthropods evaluated for biocontrol include a mite (Aceria acroptiloni) that forms galls in flower heads, a wasp (Aulacida acroptilonica) that forms stem galls and a midge (Jaapiella ivannikovi) that forms pseudo-galls at the shoot tips. Various pathogens such as a species of Alternaria, Sclerotinia sclerotiorum, Boeremia exigua and the rust fungus Puccinia acroptili have been considered as biocontrol agents. The nematode, Subanguina picridis, has been the most successful biocontrol agent (Rees et al., 1996).
Means of Movement and DispersalTop of page
Seeds are dispersed locally by the wind. Vegetative propagation via reprouting from root fragments is also responsible for the spread of this species.
Birds and rodents eat the seeds of R. repens and may transport them to new locations (Roche et al., 1986).
Accidental introductions of R. repens have occurred in contaminated grain seed (Zouhar, 2001) and are likely to be responsible for the spread of this species around the world.
Pathway CausesTop of page
|Crop production||Accidental introduction with other species||Yes||Yes|
Pathway VectorsTop of page
Plant TradeTop of page
|Plant parts liable to carry the pest in trade/transport||Pest stages||Borne internally||Borne externally||Visibility of pest or symptoms|
|True seeds (inc. grain)||seeds|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
Economic ImpactTop of page
R. repens can have an economic impact. Allelopathic and competitive interactions with crop plants can reduce yields. The weed is so bitter that contamination of grain by as little as 0.01% can reduce the quality of flour produced (Watson, 1980; Heap and Mitchell, 1992). The quality of forage is decreased if contaminated with R. repens and attempts to control it can be expensive (DiTomaso, 2000).
Environmental ImpactTop of page
R. repens can grow rapidly and produces allelopathic chemicals which can suppress the growth of other plant species enabling it to form monocultures. Infestations reduce yields of desired plants and decrease the production quality of rangelands. In Idaho, USA,R. repens has threatened a number of rare species such as Spalding's silene (Silene spaldingii), Smallhead goldenweed (Pyrrocoma liatriformis), sagebrush Mariposa lily (Calochortus macrocarpus var. maculosus) and Idaho hawksbeard (Crepis bakeri ssp. idahoensis) (Zouhar, 2001). The impact of R. repens on co-occurring native plant species is greater in North America than in the native range, suggesting biogeographic differences in the way it interacts with other species (Callaway et al., 2012)
Horses with prolonged consumption of R. repens can develop 'chewing disease' or equine nigropallidal encephalomalacia (ENE). ENE is a permanent disease caused by lesions in the brain. Symptoms include the inability to eat or drink, aimless or awkward movement, and spontaneous activity. Symptoms may occur after ingestion of 60-200% of its body weight for at least 30 days (Lacey and Olson, 1991; Panter, 1991). However, R. repens is not toxic to sheep and cattle have grazed infested pastures with no evidence of toxicity (Watson, 1980; Lacey and Olson, 1991; Panter, 1991; Olson, 1999).
Threatened SpeciesTop of page
|Threatened Species||Conservation Status||Where Threatened||Mechanism||References||Notes|
|Centrocercus minimus (Gunnison sage-grouse)||USA ESA listing as threatened species USA ESA listing as threatened species||Colorado; Utah||Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2013|
|Cirsium wrightii (Wright's marsh thistle)||NatureServe NatureServe; USA ESA candidate species USA ESA candidate species||Arizona; New Mexico||Competition (unspecified); Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2015|
|Enceliopsis nudicaulis var. corrugata (Ash Meadows sunray)||USA ESA listing as threatened species USA ESA listing as threatened species||California; Nevada||Competition - monopolizing resources; Ecosystem change / habitat alteration||US Fish and Wildlife Service, 2011|
|Grindelia fraxinipratensis (ash meadows gumplant)||NatureServe NatureServe; USA ESA listing as threatened species USA ESA listing as threatened species||California; Nevada||Competition - monopolizing resources||US Fish and Wildlife Service, 2007a|
|Silene spaldingii (Spalding's catchfly)||USA ESA listing as threatened species USA ESA listing as threatened species||Idaho; Montana; Oregon; Washington||Competition - monopolizing resources; Competition - smothering||US Fish and Wildlife Service, 2007b|
|Zeltnera namophila||No Details||California; Nevada||Competition - monopolizing resources||US Fish and Wildlife Service, 2009|
Risk and Impact FactorsTop of page Invasiveness
- Invasive in its native range
- Proved invasive outside its native range
- Highly adaptable to different environments
- Tolerates, or benefits from, cultivation, browsing pressure, mutilation, fire etc
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Negatively impacts agriculture
- Negatively impacts animal health
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Competition - monopolizing resources
- Competition - smothering
- Produces spines, thorns or burrs
- Highly likely to be transported internationally accidentally
- Difficult/costly to control
UsesTop of page
Uses ListTop of page
Animal feed, fodder, forage
Detection and InspectionTop of page
Monitoring is also important for control of R. repens. If possible, monitor three times a year: in the spring when plants have bolted, in the summer when flowering plants are easy to see, and in the autumn to find plants regrowing from roots. Disturbed areas such as roadsides are good targets (Woo et al., 1999; Zouhar, 2001).
Prevention and ControlTop of page
The control strategy for R. repens and many perennials involves stressing the plant by mowing or defoliation, causing depletion of nutrients in the root system. Plants should not be allowed to produce seed and root fragments should not be spread to other locations following treatment (Zouhar, 2001). R. repens 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 (Bottoms et al., 2001). The number of shoots produced increases with light intensity (Roche et al., 1986).
Any treatment that provides control of R. repens must either release competitive species present in the understory or be combined with reseeding before long-term sustainable control can be achieved (Whitson, 1999; Whitson, 2001). Plants that compete well include smooth brome (Bromus inermis), thickspike wheatgrass (Elymus lanceolatus), crested wheatgrass (Agropyron cristatum) and Russian wild rye (Psathyrostachys juncea). Sod-forming perennials such as r thickspike wheatgrasses compete better than bunch grasses (Benz et al., 1999; Beck, 2003).
Animal grazing is not very effective. Animals will not graze on it when other vegetation is available because of its bitter taste (Olson and Lacey, 1994; TD Whitson, Extension weed specialist, University of Wyoming, Laramie, WY 82071, USA, personal communication, 1997). The weed is also poisonous to horses, producing a neurological condition called equine nigropallidal encephalomalacia. Symptoms include the inability to eat or drink and aimless or awkward movement. R. repens is not toxic to sheep and dogs and cattle have grazed infested pastures with no evidence of toxicity (Lacey and Olson, 1991; Panter, 1991).
R. repens often invades riparian areas, where its ability to recover from treatment is enhanced by moist soils. R. repens is a perennial with an extensive root system that can resprout from root fragments. Therefore, many physical or mechanical methods are not highly effective. Hand pulling is generally ineffective against mature stands because of its deep roots and its ability to resprout from root fragments. Hand pulling or cutting the plants to kill the tops will starve the roots if done repeatedly, but this might not be a practical solution. Hand pulling might be successful for the early rosette stage (TD Whitson, Extension weed specialist, University of Wyoming, Laramie, WY 82071, USA, personal communication, 1997).
Cutting, mowing or disking infestations several times annually will control the existing top-growth and prevent seed production. Cutting or mowing three times a year depletes nutrients in the roots, but unless mowing is continued, plants will recover. Mowing may also not be possible in environmentally sensitive areas (Zouhar, 2001). Cutting is slightly less effective than pulling since cutting does not remove any portion of the root. Pulling plants two to three times annually contained, but did not eliminate, an infestation in Washington (Carpenter and Murray, 2002). Cutting roots for 3 years to a depth of 30 cm can destroy the root system in the top metre of soil. Root fragments up to 40 cm long are killed by burial below 30 cm, indicating some control by deep ploughing (Watson, 1980).
Thick mulches of straw and manure have suppressed small patches of the weed. Sheet metal and paper have also been used successfully to mulch small patches, and black plastic might be effective (Zouhar, 2001).
Unlike other knapweeds, R. repens can survive and grow in tilled soils. In fact, tilling breaks up the roots into fragments and facilitates the spread of R. repens. As root fragments are killed if buried below 25 cm, repeated deep ploughing and cutting of the roots over a period of 3 years may destroy the top layers of the root system. Even when dead R. repens can leave allelopathic residues in the soil and prevent the growth of other plants for two growing seasons. To alleviate the effect of residual plant chemicals, remove or plough the treated R. repens plants and later re-seed to facilitate grass establishment (Watson, 1980; TD Whitson, Extension weed specialist, University of Wyoming, Laramie, WY 82071, USA, personal communication, 1997).
In general, herbicides should only be used as a last resort and as part of an integrated vegetation management programme. Herbicides are expensive and do not provide lasting control. They can also cause negative impacts on water quality, and select for resistant weeds that may be worse than the original problem (Powles and Shaner, 2001). The persistent herbicides clopyralid and picloram often recommended for control are not metabolised by grazing animals and are not destroyed by composting (Bezdicek et al., 2001; Houck and Burkhart, 2001).
If herbicides are used, they should be applied when R. repens is most susceptible and before seeds are produced; perennials are most susceptible to herbicides in the autumn (Whitson, 1999). The most effective herbicides are picloram and chlopyralid (Benz et al., 1999; Whitson, 2001).
The biological control agent that has received the most study is the Russian knapweed gall nematode (Subanguina picridis). Larval and adult stages of the nematode form galls on plant stems, leaves and root crowns. Infective larvae form galls in the spring and multiply until August; the larvae then disperse into the soil until the next year. S. picridis was first introduced into Canada from Kazakhstan in 1976 (Julien and Griffiths, 1998) but only became established in British Columbia (see also Watson and Harris, 1981). It was sent from Canada to the USA in 1984 but did not establish. However, it was introduced again from Turkey and Uzbekistan in 1990 and became established. In both countries, it has had little impact but has had most potential in wetter areas. In the USA, it is now established in Colorado, Montana, Oregon, Utah, Washington and Wyoming (Rees et al., 1996). However, S. picridis has not been very effective in the field (Schaffner et al., 2001). S. picridis from Central Asia has been successfully applied as a spray in trials carried out in Ukraine and Uzbekistan. The suppression of seed production by Aceria acroptiloni was also claimed in Ukraine and Uzbekistan (OV Kovalev, cited in Julien and Griffiths, 1998).
The rust fungus Puccinia acroptili, which was accidentally introduced into North America (Canada) before 1970 is widespread, but it does not control R. repens. Isolates of P. acroptili collected in Turkey appear to have a higher potential as a candidate for biological control (Bruckart et al., 2005). The facultative saprophytic fungus Boeremia exigua has also been assessed for its suitability as a biological control candidate of R. repens (Berner et al., 2015).
Two insect herbivores found in Central Asia, the gall wasp Aulacidea acroptilonica and the gall midge Jaapiella ivannikovi were approved for field release in the USA and Canada (USDA APHIS, 2008; 2009). Both species have established in several states in the USA and in Alberta, Canada (Schaffner et al., 2015).
Integrated management involves combining monitoring and prevention with physical, cultural, chemical or biological controls. A combination of methods is always more successful for a difficult weed. A very important part of integrated management is revegetation and competitive plantings. Decisions to be made are based on the conditions at the site, and whether the goal is containment or eradication (Sheley et al., 1996; 1999b; Woo et al., 2002).
Best results for control of R. repens have combined mowing or herbicide treatment with revegetation. When perennial grasses such as western wheatgrass (Pascopyrum smithii [Elymus smithii]) and blue grama (Bouteloua gracilis [Chondrosum gracile]) were present in the understorey, single applications of picloram resulted in 85% control of R. repens 8 years following treatment. Herbicides plus seeding with wheatgrass and Russian wildrye (Elymus junceus [Psathyrostachys juncea]) provided control up to 5 years after seeding (Whitson, 2001). 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 R. repens lateral roots (Bottoms et al., 1995; Bottoms and Whitson, 1998; Bottoms et al., 2001).
Some experts believe that herbicide treatment of R. repens is more effective than mowing before establishing perennial grasses (Benz et al., 1999; Whitson, 1999). Tillage of surface residue to hasten decomposition of allelochemicals may be necessary before planting competitive vegetation (Bottoms and Whitson, 1998; Whitson, 1999; Beck, 2003). Tilling, however, is not possible or appropriate for most natural areas (Carpenter and Murray, 2002). The key to revegetation is finding the proper plant.
ReferencesTop of page
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ContributorsTop of page
10/03/2017 Updated by:
Urs Schaffner, CABI-CH, Switzerland
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