Centaurea stoebe subsp. micranthos (spotted knapweed)
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IdentityTop of page
Preferred Scientific Name
- Centaurea stoebe subsp. micranthos (Gugler) Hayek 1931
Preferred Common Name
Other Scientific Names
- Centaurea biebersteinii DC.
- Centaurea maculosa Lam.
- Centaurea maculosa subsp. micranthos Gugler (1907)
International Common Names
- French: Centaurée maculée; Centaurée tachetée
Local Common Names
- Germany: Gefleckte Flockenblume; Kleinköpfige Rispen-Flockenblume; Rispen- Flockenblume
- USA: spotted knapweed
- CENBB (Centaurea biebersteinii)
Summary of InvasivenessTop of page
Spotted knapweed can thrive over a wide range of climate and soil conditions. It is a perennial invasive exotic that spreads quickly through successful dispersal and germination of numerous seeds. It has many negative economic and environmental impacts, and has been declared a noxious or restricted weed in 15 US states 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: Centaurea
- Species: Centaurea stoebe subsp. micranthos
Notes on Taxonomy and NomenclatureTop of page
Until recently, the accepted name of this invasive exotic spotted knapweed was Centaurea maculosa Lam. However, new research using molecular methods has led to revision. It is listed in the Plants database as Centaurea biebersteinii DC. with a synonym of Centaurea maculosa auct. non Lam. (USDA-NRCS, 2002). In the GRIN database the name is Centaurea stoebe L. subsp. micranthos (Gugler) Hayek with a synonym of Centaurea maculosa auct. Amer. (USDA-ARS, 2003). In the Flora Europaea it is called Centaurea biebersteinii DC. subsp. biebersteinii with synonyms C. maculosa Lam. subsp. biebersteinii DC. and C. micranthos S.G. Gmel. ex Hayek (Tutin et al., 1993).
The problem is that there are two kinds of spotted knapweed. One is a biennial diploid (2n = 18) that is monocarpic. It grows over two seasons, produces one stem, flowers once and dies. According to Ochsmann (2001a), the diploid plant is Centaurea maculosa Lam. This plant is native to western and central Europe. The accepted name is Centaurea stoebe L. subsp. stoebe and a synonym is C. rhenana Boreau.
The other is a perennial tetraploid (2n = 36) that is polycarpic. It lives and flowers for several years and produces more than one stem. The tetraploid plant is an invasive exotic that was introduced to North America and most of Europe from its native range in Russia and Eastern Europe. Molecular data confirm that the invasive North American and invasive European plants are the same taxon. The correct name is Centaurea stoebe L. subsp. micranthos (Gugler) Hayek, and accepted synonyms are C. biebersteinii and C. micranthos (Ochsmann, 2001a).
Due to inertia, most of the published papers dealing with invasive spotted knapweed continue to refer to Centaurea maculosa (Sheley et al., 1999b; Story and Piper, 2001).
DescriptionTop of page
Spotted knapweed is an aggressive perennial averaging 3-5 years that reproduces mainly by seeds. Germinating seeds form a stout taproot (>80 cm deep) and a basal rosette with leaves up to 20 cm long and 5 cm wide. Rosette leaves are deeply divided, lobed, oblanceolate to oblong (Muller, 1989; Whitson et al., 2001).
In the second year or later, rosettes bolt, forming 1-8 stems. Slender, erect stems are 0.3 m to 1.5 m tall and have wiry, rough-pubescent branches. Pale green leaves on stems and branches are 25-75 mm long. Leaves are alternate, pinnately divided, with narrow divisions, and have a rough-pubescent appearance; upper leaves are smaller, often linear. Most stems remain erect after drying, with leaves and flowerhead bracts attached.
Flowerheads are 6 mm in diameter and 16-20 mm high. They are solitary and terminal or occur in clusters of 2-3 at the end of branches. Floral bracts are mostly yellow-green to brown, smooth and strongly ribbed with a dark, comb-like fringe, 1-2 mm long. The central spine of the floral bracts is shorter than the lateral ones. Flowerheads contain 25-35 tubular pink or purple flowers.
Flowerheads open soon after they mature, releasing seeds when the plant is stirred by the wind. Fruits are achenes, oval, brownish about 2-3 mm long, with longitudal lines and a notch at one side of the base. At the tip is a pappus of persistent bristles shorter than the length of the fruit (USDA, 1970; Watson and Renney, 1974; Higgins and Schirman, 1977; Sheley et al., 1999b; Whitson et al., 2001).
Plant TypeTop of page
DistributionTop of page
Invasive spotted knapweed is native to Eastern Europe. The native range includes Bulgaria, Hungary, Romania, Ukraine and Russia (Ochsmann, 2001a; USDA-ARS, 2003). In North America, although it occurs in 45 of the 50 states, and most Canadian provinces, spotted knapweed is found primarily in the north-western states and south-western Canada (Watson and Renney, 1974; Duncan, 2001; USDA-NRCS, 2002). Spotted knapweed is considered a serious threat to rangelands in Montana, Washington, Idaho, Oregon, Wyoming and British Columbia. It occurs along the roadsides and in disturbed areas in New England, the north-east, Michigan, Illinois, Nebraska, the Great Plains, North Carolina, Tennessee, Virginia, West Virginia and Florida (Duncan, 2001; Zouhar, 2001).
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.
|Country||Distribution||Last Reported||Origin||First Reported||Invasive||References||Notes|
|-British Columbia||Widespread||Introduced||Invasive||Duncan, 2001|
|-Ontario||Present||Introduced||Invasive||Watson & Renney, 1974|
|-Quebec||Present||Introduced||Invasive||Watson & Renney, 1974|
|-Idaho||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Kansas||Present||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Michigan||Present||Introduced||Invasive||USDA-NRCS, 2002; Rice, 2003|
|-Montana||Widespread||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-Nebraska||Present||Introduced||Invasive||USDA-NRCS, 2002; USDA-NRCS, Rice|
|-Nevada||Present||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-New Hampshire||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-New Jersey||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-New Mexico||Present||Introduced||Invasive||Duncan, 2001; USDA-NRCS, 2002|
|-New York||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-North Carolina||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-North Dakota||Present, few occurrences||Introduced||Invasive||Duncan, 2001|
|-Rhode Island||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-South Carolina||Present||Introduced||Invasive||USDA-NRCS, 2002|
|-South Dakota||Present||Introduced||Invasive||Duncan, 2001|
|-Washington||Widespread||Introduced||1920||Invasive||Sheley et al., 1999b; Duncan, 2001|
|-West Virginia||Present||Introduced||Invasive||USDA-NRCS, 2002|
|Albania||Present||Native||Invasive||Tutin et al., 1976|
|Austria||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Bulgaria||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Czech Republic||Present||Introduced||Invasive||Tutin et al., 1976; Ochsmann, 2001a|
|Czechoslovakia (former)||Present||Introduced||Invasive||Tutin et al., 1976; Ochsmann, 2001a|
|Greece||Present||Kashefi & Sobhian, 1998|
|Hungary||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Moldova||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Romania||Present||Native||Invasive||Tutin et al., 1976; Ochsmann, 2001a|
|Russian Federation||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|-Central Russia||Present||Native||Invasive||Tutin et al., 1976|
|-Southern Russia||Present||Native||Invasive||Tutin et al., 1976|
|-Western Siberia||Present||Native||Invasive||Sheley et al., 1998|
|Serbia||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Slovenia||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Ukraine||Present||Native||Invasive||Ochsmann, 2001a; USDA-ARS, 2003|
|Yugoslavia (former)||Present||Native||Invasive||Ochsmann, 2001a|
|New Zealand||Present||Introduced||Invasive||Holm et al., 1979|
History of Introduction and SpreadTop of page
Spotted knapweed was introduced to North America as a contaminant in lucerne seed in the late 1800s. Canadian infestations started in Victoria, British Columbia in 1893. In 1920, the only infestation in the USA was at San Juan Islands, Washington, but it has spread quickly. In 1998, the known range included every county in Washington, Idaho, Montana and Wyoming (Sheley et al., 1998). The number of infested hectares in Oregon increased from about 1200 in 1988 to about 300,000 in 2000 (Duncan, 2001).
HabitatTop of page
Spotted knapweed infests roadsides, waste areas, plains and dry rangelands and prefers well-drained, light-textured soils. It is found along rivers and creeks in relatively dry disturbed sites. It tolerates a wide range of conditions and climate and can successfully invade non-disturbed areas. Non-disturbed, moist areas are more susceptible to invasion than non-disturbed dry areas (Sheley et al., 1997, 1999b; Zouhar, 2001).
Habitat ListTop of page
|Disturbed areas||Present, no further details||Harmful (pest or invasive)|
|Managed grasslands (grazing systems)||Present, no further details||Harmful (pest or invasive)|
|Rail / roadsides||Present, no further details||Harmful (pest or invasive)|
|Natural grasslands||Present, no further details||Harmful (pest or invasive)|
|Riverbanks||Present, no further details||Harmful (pest or invasive)|
Biology and EcologyTop of page
Most of the North American plants called 'Centaurea maculosa' are perennial, polycarpic and tetraploid (2n = 36). However, there have been multiple introductions, and some infestations might be diploid. Diploids can be identified by genetic analysis and growth habit, because they flower once and die after 2 years (Smith, 2001). Ochsmann (2001b) cites evidence of hybridization between spotted knapweed and diffuse knapweed (Centaurea diffusa) in at least seven US states. The hybrid is named Centaurea x psammogena.
Physiology and Phenology
Spotted knapweed seeds are known for their longevity and durability. Most seeds germinate immediately, but some have a dormant period. For example, Watson and Renney (1974) observed that germination rates doubled, from 40 to 80%, after 25 days of dry storage. The period of dormancy may be broken by seed ageing, cool-moist stratification, freezing or exposure to red light (Eddleman and Romo, 1988; Nolan and Upadhyaya, 1988). Seeds can remain viable but dormant for at least 8 years (Davis et al., 1993).
Germination occurs over a range of temperatures (7-34°C), but 80% of seeds germinate between 10 and 28°C, and germination is optimal at 19°C (Watson and Renney, 1974). Germinating seeds require at least 55% soil moisture, but 65-70% is optimal. Seeds germinate equally well in open or closed canopy (Spears et al., 1980). Germination rate is greatest at the soil surface, decreases with depth, and nearly vanishes below 5 cm (Watson and Renney, 1974; Nolan and Upadhyaya, 1988).
Seeds germinate in the autumn or early spring and develop into rosettes, and most root growth occurs during this stage. Spotted knapweed overwinters as seeds; rosettes die back to root crowns to overwinter, but sometimes are killed by extreme conditions. Overwintering seeds germinate in early spring; overwintering rosettes bolt in early May (Watson and Renney, 1974; Sheley et al., 1999b).
Plants stay in the rosette stage for 1 to 4 years, producing flowering stems in the second year or later (Watson and Renney, 1974; Jacobs and Sheley, 1998). The percentage of flowering plants increases with age up to 5-7 years, with little or no flowering in the first and second years (Schirman, 1981; Boggs and Story, 1987).
Flower buds are formed in early June, and flowering occurs from July through September. Mature seeds are formed by mid-August. Flowerheads open about 2 to 3 weeks after the seeds mature, usually in late summer. Most seeds are shed upon maturity; very few overwinter in seedheads (Watson and Renney, 1974; Sheley et al., 1999b).
Spotted knapweed reproduces almost entirely from seed. Flowers are pollinated by insects, especially bees, and fertilization requires cross-pollination between different plants (Watson and Renney, 1974; Harrod and Taylor, 1995; Sheley et al., 1999b).
The number of seeds produced by one plant or a population is highly variable. The average number of seeds per plant ranges from about 65 in Montana (Jacobs and Sheley, 1998) to 400-900 in British Columbia (Watson and Renney, 1974) and to about 2000 in Washington and Idaho (Schirman, 1981). Stands of spotted knapweed can produce a seed rain of 5,000-40,000 seeds/m² (Sheley et al., 1998). Only 1500 seeds/m² are necessary to sustain existing stands (Muller-Schärer and Schroeder, 1993).
Plants are also able to form rosettes from buried lateral shoots near the parent. Multiple rosettes may surround a parent plant, but plant density in long-term stands usually stabilizes. In one instance, equilibrium level was 174/m² after 10 years (Jacobs and Sheley, 1998).
In British Columbia, Canada, spotted knapweed is found in areas where mean annual temperatures range between 6.1 and 7.8°C and annual rainfall is 251-648 mm (Watson and Renney, 1974). In Montana, more than 70% of spotted knapweed stands are found in areas with rainfall of 310-760 mm, 50-130 frost free days, a maximum mean temperature of the hottest month between 26.6 and 30°C, and at altitudes between 610 and 1829 m (Chicoine et al., 1985). However, infestations have been found in other areas where mean annual temperatures range from 0.7 to 18.1°C and the mean maximum temperature of the hottest month ranges from 22 to 34.3°C. It can survive temperatures of -35°C (Strang et al., 1979; Miller, 1990; Sheley et al., 1999b; Rinella et al., 2001; Climate, 2003).
Spotted knapweed is found on soils with a wide range of chemical and physical properties. It does especially well in coarse-textured soils that are well drained with low water holding capacity (Zouhar, 2001). It grows in areas that are generally colder and wetter than diffuse knapweed (Centaurea diffusa) and where soils are in the pH range 6.4-7.4. It prefers loam to gravelly loam soil (Watson and Renney, 1974). It is poorly adapted to irrigated pastures where saturated soil is common, and does not compete well with vigorously growing grass in moist sites (Watson and Renney, 1974; Harris and Cranston, 1979).
In North America, spotted knapweed has been observed at altitudes ranging from 578 to 3040 m and in annual rainfall ranging from 200 to 2000 mm (Sheley et al., 1999b).
In Montana, USA, spotted knapweed is associated with ponderosa pine (Pinus ponderosa) and prairie grasses including bluebunch wheatgrass (Pseudoroegneria spicata), Idaho fescue (Festuca idahoensis), rough fescue (Festuca altaica) and needle-and-thread grass (Hesperostipa comata). On undisturbed sites additional associates may include slender wheatgrass (Elymus trachycaulus), prairie junegrass (Koeleria macrantha), timber oatgrass (Danthonia intermedia), Richardson needlegrass, western yarrow (Achillea millefolium), northern bedstraw (Galium boreale), field chickweed (Cerastium arvense) and silky lupine (Lupinus sericeus). On disturbed sites, old hayfields and pastures, common associates are Kentucky bluegrass (Poa pratensis), smooth brome (Bromus inermis), timothy (Phleum pratense) and wheatgrass/blue grama (Bouteloua gracilis). It grows along Montana roadsides with blue grama, wheatgrass, needlegrass and little bluestem (Sabal minor) (Tyser and Worley, 1992; Guenther et al., 1993; Zouhar, 2001).
Air TemperatureTop of page
|Parameter||Lower limit||Upper limit|
|Absolute minimum temperature (ºC)||-35|
|Mean annual temperature (ºC)||1||18|
|Mean maximum temperature of hottest month (ºC)||22||34|
|Mean minimum temperature of coldest month (ºC)||-23||7|
RainfallTop of page
|Parameter||Lower limit||Upper limit||Description|
|Dry season duration||8||number of consecutive months with <40 mm rainfall|
|Mean annual rainfall||200||2000||mm; lower/upper limits|
Soil TolerancesTop of page
Notes on Natural EnemiesTop of page
Extensive studies on the natural enemies were made in Europe prior to introductions into North America. A general account of this work was published by Schroeder (1986). There are also papers published on the biology and host specificity of some of the introduced species (see Julien and Griffiths, 1998; Bourchier et al., 2002).
Means of Movement and DispersalTop of page
A mature spotted knapweed plant can produce up to 25,000 seeds that are dispersed by infested hay, wind, water, vehicles and equipment, animals or shoes. Germination greatly decreases if seeds are buried deeper than 5 cm (Spears et al., 1980).
Natural Dispersal (non-biotic)
As soon as the seedheads open, any movement of the stem throws seeds within 0.9 to 1.2 m of the parent plant. In this way, spotted knapweed populations spread outward and downwind from the perimeter of existing stands. Seeds mixed with soil and mud may be carried by vehicles or other equipment that, in turn, create an ideal seedbed for spotted knapweed establishment (Sheley et al., 1999b). Spread of seeds on logging trucks, off-road vehicles and trail bikes has contributed greatly to the spread of knapweed into new areas of British Columbia (Strang et al., 1979).
Vector Transmission (biotic)
Animals and birds can carry seeds over long distances (Watson and Renney, 1974). Domestic sheep and mule deer excrete viable seeds of spotted knapweed in their faeces for 7 to 10 days after consumption (Wallander et al., 1995). Ants may also help to spread seeds (Pemberton and Irving, 1990).
Spotted knapweed is not usually associated with crop production; cultivation and tillage tends to suppress it (Watson and Renney, 1974).
Spotted knapweed was introduced internationally through contaminated grain shipments (Zouhar, 2001).
Spotted knapweed has probably not been intentionally introduced, as it is not an ornamental.
Pathway VectorsTop of page
|Land vehicles||Logging, off road vehicles||Yes|
|Soil, sand, gravel etc.||Yes|
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||No|
|Plant parts not known to carry the pest in trade/transport|
|Bulbs, Tubers, Corms, Rhizomes|
|Flowers, Inflorescences, Cones, Calyx|
|Fruits (inc. pods)|
|Growing medium accompanying plants|
|Seedlings, Micropropagated plants|
|Stems (above ground), Shoots, Trunks, Branches|
Impact SummaryTop of page
|Fisheries / aquaculture||None|
ImpactTop of page
Millions of dollars are spent each year on knapweed management. Reduced forage quality can also lead to decreased profits for ranchers (Lacey and Olson, 1991; DiTomaso, 2000).
Environmental ImpactTop of page
Spotted knapweed infestations have been associated with reductions in forage production, soil fertility and wildlife habitat (Watson and Renney, 1974; Harris and Cranston, 1979). It increases surface water runoff, reduces water infiltration into soil and increases stream sedimentation rates (Lacey et al., 1989; Lacey and Olson, 1991). Spotted knapweed reduced seed germination and seedling establishment of a rare Montana plant, Mt. Sapphire rockcress (Arabis fecunda) (Zouhar, 2001).
Impact: BiodiversityTop of page
Wherever spotted knapweed establishes, it reduces plant species richness and diversity (Tyser and Key, 1988).
Social ImpactTop of page
Any social impact of spotted knapweed probably stems from its economic impact. It is not a human health problem, although high densities of pollen-producing plants might cause some allergies (Zouhar, 2001).
Risk and Impact FactorsTop of page
- Competition - monopolizing resources
- Produces spines, thorns or burrs
- Damaged ecosystem services
- Ecosystem change/ habitat alteration
- Negatively impacts agriculture
- Negatively impacts human health
- Negatively impacts tourism
- Reduced amenity values
- Reduced native biodiversity
- Has high reproductive potential
- Has propagules that can remain viable for more than one year
- Highly adaptable to different environments
- Highly mobile locally
- Invasive in its native range
- Proved invasive outside its native range
Likelihood of entry/control
- Difficult/costly to control
- Highly likely to be transported internationally accidentally
UsesTop of page
Spotted knapweed is nutritious and can be utilized by livestock especially during spring and early summer when plants are green and actively growing in the rosette and bolt stages (Kelsey and Mihalovich, 1987; Popay and Field, 1996). It is also grazed by deer, elk and bighorn sheep (Miller, 1990). Use declines as spotted knapweed matures, although flowerbuds and seedheads may be grazed in the late summer.
Spotted knapweed is a nectar source for the endangered Karner blue butterfly (Lycaeides melissa samuelis) in Wisconsin, USA (Zouhar, 2001). Seeds are source of food for wild rodents and flowers are utilized by bees (Watson and Renney, 1974). Deer mice have been observed eating the larvae and seeds from spotted knapweed flowerheads infested with seedhead flies (introduced biocontrol agents) (Pearson et al., 2000).
Prevention and ControlTop of page Introduction
As reproduction is mostly by seeds, the management strategy is the removal of plants before seeds are formed, and reseeding disturbed areas with competitive vegetation. Elements of an integrated vegetation management (IVM) plan include prevention, cultural, physical, chemical and biological controls (Woo et al., 2002).
Prevention is an important part of spotted knapweed management. Seeds and forage should be certified weed-free. Soil should be checked for weed seeds before it is moved to new areas. Because knapweed travels with automobiles, the undercarriage of vehicles or machinery exposed to knapweed should be constantly cleaned. Many construction companies have weed control measures and can steam clean the underside of machinery (Olliff et al., 2001; Woo et al., 2002).
Soil disturbance caused by water, livestock, vehicles or machinery should be minimized. Deferred or rotational grazing, water conservation, erosion control, proper fertilization, re-seeding to maintain dense, hardy grass cover, revegetation, and maintenance of competitive vegetation that can withstand weed invasion are part of prevention. Because most noxious weeds are pioneer species, a dense ground cover of desirable plants with a closed canopy will usually help prevent their establishment (Olliff et al., 2001).Cultural Control
Grazing by sheep and goats can suppress knapweeds. Seedheads and leaves of spotted knapweed are actually more nutritious than some native grasses (Olson and Wallander, 2001a, b). It is more palatable in late spring or early summer, and repeated grazing can reduce flower stem production, stop seed formation and gradually deplete root reserves (Popay and Field, 1996).
As animals usually prefer to eat nearby grasses in lieu of knapweeds, grazing is most effective against knapweeds when the livestock is enclosed in a fenced-off, weedy area (Beck, 1994; Olson and Lacey, 1994; Whitson, 1997). However, Olson et al. (1997) found that grazing sheep for three summers reduced seeds, seedlings, rosettes and mature spotted knapweed densities, but minimally affected native grasses. Animals should graze weeds only before they flower and set seed. If this is impossible, animals should be contained for 7 to 14 days before moving them to non-infested areas to prevent spread of seeds in animal manure.
Nitrogen fertilization may increase native grass competition in some cases (Sheley and Jacobs, 1997b). Unfortunately, where knapweed has already invaded, nitrogen fertilization with no other treatment may favour knapweed over natives (Prather and Callihan, 1991; Velagala et al., 1997; Sheley et al., 1998; Herron et al., 2001). For instance, Story et al. (1989) found that the addition of nitrogen to sites containing spotted knapweed, quackgrass (Agropyron repens) and crested wheatgrass (Agropyron cristatum) favoured knapweed and quackgrass but not crested wheatgrass. Also, Lindquist et al. (1996) did competition experiments between spotted knapweed, bromegrass (Bromus inermis), Idaho fescue (Festuca idahoensis) and the bluebunch wheatgrass (Pseudoroegneria spicatum). Additions of nitrogen fertilizer favoured bromegrass but did not help the native grasses compete with knapweed.
The effects of fertilization may be complicated by moisture conditions. According to Spears et al. (1980), knapgrass seeds germinate best close to the surface and at soil moisture of >55%. Fertilization in dry areas may remove moisture from the soil, leading to increased competition of drought tolerant grasses such as crested wheatgrass (Berube and Myers, 1982).
Soil analyses have shown that the perennial grasses that dominate climax populations on grazing lands are associated with low available nitrogen. Early colonizers such as knapweed do well in nitrogen-rich soil, but cannot compete with perennial grasses in a low nitrogen environment (Sheley et al., 1996a, b; Herron et al., 2001). To force plant succession towards the preferred climax vegetation, controlled colonization can be used. Intermediate and late colonizers are planted simultaneously. For instance, Herron et al. (2001) found that plantings of annual rye (Secale cereale) and bottlebrush squirreltail (Elymus elimoides) could deplete the soil of nutrients, especially nitrogen. As a result, late seral plants such as bluebunch wheatgrass (Pseudoroegneria spicata) will outcompete the early colonizer knapweed. Low nitrogen availability also makes knapweed more susceptible to root-feeding biological control agents (Steinger and Muller-Schärer, 1992).
As knapweeds thrive in direct sunlight, shading can suppress them (Kennett et al., 1992). Shading is an especially effective method against Russian knapweed (Acroptilon repens) because it cannot tolerate dense shade. Crops such as lucerne produce dense shade under irrigation and have been used to suppress Russian knapweed in crop areas (Roche and Roche, 1991).
Where knapweed is nearby, any patches of bare ground can sprout with knapweed (Lacey et al., 1990). Thus, reseeding bare areas with competitive perennial grasses is important. Crested wheatgrass (Agropyron cristatum) has been a successful competitor in dry areas of Canada (Berube and Myers, 1982). Crested wheatgrass is more competitive than Russian wildrye (Psathyrostachys juncea) or lucerne (Maxwell et al. 1992), but orchardgrass (Dactylis glomerata) and thickspike wheatgrass (Agropyron dasystachyum) are more competitive than crested wheatgrass (Larson and McInnis, 1989). Meadow fescue (Festuca pratensis) and rough fescue (Festuca altaica) are more competitive than bluebunch wheatgrass (Pseudoroegneria spicata) (Lacey et al., 1990; Steinger and Muller-Schärer, 1992).
To compete successfully, intermediate wheatgrass (Elytrigia intermedia) may need much higher seeding rates (6000/m²) than currently recommended (Velagala et al., 1997; Sheley et al., 1999a). Bluebunch wheatgrass (Pseudoroegneria spicatum) is a poor competitor, as 90% reduction of spotted knapweed populations are necessary before it can successfully compete (Sheley and Jacobs, 1997a). Bromegrass (Bromus inermis) is a more aggressive competitor than wheatgrasses, especially as nitrogen levels increase (Lindquist et al., 1996).Mechanical Control
Controlled burns can potentially help control knapweeds. However, knapweeds are not very flammable, resulting in low temperature fires and patchy, discontinuous burns. Even when the surface area of the plant is killed, knapweed can often resprout from root reserves. Fires might actually be detrimental to knapweed control, creating disturbances favourable to knapweed establishment and growth (Sheley and Roche, 1982; Whitson, 1997; Sheley et al., 1998).
Flaming with a propane torch can kill young knapweed seedlings or rosettes. The flame sears the plant, causing cells to rupture and the plant to dehydrate and die. Flaming is done on green plants; not on dead foliage. Plants can be seared at any time before flowering. Flaming is useful for clearing small areas (Whitson, 1997).
Pulling or digging out knapweeds is best done in the spring during the rosette or early bud stage, before flowers appear, and when soils are moist. As roots can extend a few feet into the soil, the entire taproot should be pulled out, otherwise the plant may resprout (Payton et al., 1986; Sheley et al., 1999b).
Uprooted plants should be disposed of properly. They can be piled and burned, buried in a landfill, buried in a deep, covered pit, or hot composted to kill the seeds. The first and second years of a hand-pulling programme will involve intensive removal because first year rosettes will then mature. During the third and fourth years, knapweed numbers decrease and pulling efforts are noticeably easier (Payton et al., 1986).
Mowing decreases flower and seed production, and in the long-term it may effect knapweed densities. Timing is critical. Rinella et al. (2001) found that best results for spotted knapweed were obtained with one mowing in August, when the plants were in the flowering stage.Chemical Control
In general, herbicides should only be used as a last resort, because they 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, which are often recommended for knapweed control, are not metabolized by grazing animals or destroyed by composting (Fay et al., 1991; Bezdicek et al., 2001; Houck and Burkhart, 2001). If herbicides are used, they should be applied before seeds are produced, and should be combined with a revegetation programme (Woo et al., 2002).Biological Control
To date, 13 beneficial insect species from Europe have been released in the USA and Canada as biocontrol agents of spotted and diffuse knapweeds. The biocontrol complex includes four moths, four weevils, one beetle and four flies. Eleven species have established, and at least seven are well established and spreading (Julien and Griffiths, 1998; Smith, 2001). Larval forms do most of the damage and attack either the roots or seedheads (Muller-Schärer and Schroeder, 1993; Rees et al., 1996; Smith, 2001). Knapweed densities are lower in areas where biocontrols have been released (Clark et al., 2001a). So far, the most successful biocontrol agents are the root moth Agapeta zoegana, the root weevil Cyphocleonus achates, and the seedhead flies Urophora affinis and U. quadrifasciata (Story and Piper, 2001). The 13 beneficial insects that have been released in the USA and Canada as biocontrol agents of spotted and diffuse knapweeds are:
Agapeta zoegana, a small yellow root-boring moth works best where knapweeds are abundant but not yet a monoculture (Rees et al., 1996; Lang, 1997). The moth may preferentially attack older plants and is more successful in combination with competitive plantings of grasses (Story et al., 2000).
Metzneria paucipunctella, a seedhead moth was released to control spotted knapweed. By 1988 it spread onto diffuse knapweed. Larvae can destroy up to 90% of the seeds. However, low temperatures restrict the geographical range (Rosenthal et al., 1991; Rees et al., 1996; Good et al., 1997; Lang, 1997). Two other moths, Pterolonche inspersa and Pelochrista medullana were released but did not establish (Rees et al., 1996).
The four weevil and one beetle species released for biocontrol of knapweed include three that attack seedheads and two that attack roots. Bangasternus fausti, a seedhead weevil, attacks diffuse, spotted and squarrose knapweeds, and to a lesser extent, purple and yellow starthistle. Larvae feed inside seedheads, destroying up to 100% of its contents. B. fausti requires undisturbed release sites with dry summers (Rees et al., 1996; Lang, 1997).
Larinus minutus and L. obtusus are seedhead weevils. L. minutus prefers diffuse knapweed, and L. obtusus prefers spotted knapweed (Story and Piper, 2001). Adults are strong flyers and can easily disperse to new patches. The larvae feed on flowers and seeds, reducing seed production by up to 100% in infested seedheads. The weevils require dry areas with some bare ground, such as the outer edges of knapweed patches (Jordan, 1995; Lang et al., 1996; Rees et al., 1996; Lang ,1997; Kashefi and Sobhian, 1998).
Cyphocleonus achates, a root-boring weevil, prefers spotted knapweed, but will attack diffuse knapweed (Story and Piper, 2001). The weevil does not fly. Adults feed on knapweed rosettes. Larvae destroy the interior of the taproot and increase the plants susceptibility to pathogen attack. The insect requires high soil temperatures (Rees et al., 1996; Story et al., 1996, 1997; Lang, 1997). Weevil numbers are highest when knapweed cover is 30-70%. Largest establishment rates in Montana, Idaho and Washington occurred at about 900-1500 m (Clark et al., 2001b).
Sphenoptera jugoslavica, a root borer, is a flat, blue-black, copper-coloured beetle that attacks roots of diffuse knapweed. The larvae feed on taproots, causing gall formation near the root crown. Although larval root feeding can kill rosettes, mature plants survive (Rees et al., 1996; Lang, 1997; Lang et al., 1998).
Urophora affinis and U. quadrifasciata, two seedhead flies, attack both spotted and diffuse knapweeds. Eggs are laid between floral bracts and hatching larvae induce gall formation on the seedhead, causing new flowers to abort. Together, the seedhead flies can cause up to a 95% reduction in seed production. Both species prefer open areas with full sun (Maddox, 1979; Rosenthal et al., 1991; Rees et al., 1996; Lang, 1997; Lang et al., 1997). Although galls are eaten by deer mice, extremely cold temperature is the most important factor affecting overwintering survival of the flies (Nowierski et al., 2000; Pearson et al., 2000).
Chaetorellia acrolophi, a seedhead fly, feeds on both diffuse and spotted knapweed. Larvae feed on the flower buds, reducing seed production. However, the effects of larval feeding on diffuse knapweed are unclear (Rees et al., 1996; Lang, 1997).
Terellia virens, a seedhead fly, primarily attacks spotted knapweed. It does not form galls and has two generations a year (Rees et al., 1996; Lang, 1997).
A number of pathogens attack knapweeds. Maculosin, a host-specific toxin produced by the black leaf blight fungus Alternaria alternata can destroy two-thirds of spotted knapweed foliage; however, younger leaves and buds are not affected and plants are able to resprout (Bobylev et al., 1996). Sclerotinia sclerotiorum, a common soil fungus with a broad host range, is effective in killing juvenile spotted knapweed (Rosenthal et al., 1991; Jacobs et al., 1996).
Fusarium avenaceum [Gibberella avenacea], a stem blight fungus, causes stunted growth, yellowing, and stem decay on spotted knapweed. Originally isolated from diseased knapweeds in Montana, a strain of the fungus can prevent seedling germination and plant growth (Czembor and Strobel, 1997).Integrated Control
Combination treatments of fertilizer and herbicides have given inconsistent results. Sheley and Roche (1982) found that knapweed suppression and grass growth was greater at sites receiving both treatments. Hubbard (1975) got similar results in dry areas, but not where rainfall was greater. Sheley and Jacobs (1997b) found that the addition of nitrogen fertilizer had no effect on the growth of knapweed or native grasses in areas treated with the herbicide picloram for knapweed suppression.
Bounty programmes, where rewards are offered for plant removal, have been successful in Montana as part of IPM programmes (Lacey et al., 1988).
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- = Present, no further details
- = Evidence of pathogen
- = Widespread
- = Last reported
- = Localised
- = Presence unconfirmed
- = Confined and subject to quarantine
- = See regional map for distribution within the country
- = Occasional or few reports