Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


Centaurea diffusa
(diffuse knapweed)



Centaurea diffusa (diffuse knapweed)


  • Last modified
  • 08 November 2018
  • Datasheet Type(s)
  • Invasive Species
  • Pest
  • Preferred Scientific Name
  • Centaurea diffusa
  • Preferred Common Name
  • diffuse knapweed
  • Taxonomic Tree
  • Domain: Eukaryota
  •   Kingdom: Plantae
  •     Phylum: Spermatophyta
  •       Subphylum: Angiospermae
  •         Class: Dicotyledonae
  • Summary of Invasiveness
  • Diffuse knapweed survives in a wide range of soil and environmental conditions, and produces large numbers of viable seeds that are widely dispersed by wind, water, animals and human activity. For much of the growth cycle, plants are hard to graze be...
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Top of page

Preferred Scientific Name

  • Centaurea diffusa Lamarck (1785)

Preferred Common Name

  • diffuse knapweed

Other Scientific Names

  • Acosta diffusa Sojak (1972)
  • Acrolophus diffusus (Lam.) A. Love and D. Love (1961)

International Common Names

  • English: tumble knapweed; white knapweed
  • French: centaurée diffuse

Local Common Names

  • Germany: Sparrige Flockenblume
  • Italy: fiordaliso diffuso

EPPO code

  • CENDI (Centaurea diffusa)

Summary of Invasiveness

Top of page Diffuse knapweed survives in a wide range of soil and environmental conditions, and produces large numbers of viable seeds that are widely dispersed by wind, water, animals and human activity. For much of the growth cycle, plants are hard to graze because they are protected by spines or form low-lying rosettes (Roche and Roche, 1999). It is a major rangeland weed and is considered a noxious or restricted weed in 13 US states and four Canadian provinces (Rice, 2003).

Taxonomic Tree

Top of page
  • Domain: Eukaryota
  •     Kingdom: Plantae
  •         Phylum: Spermatophyta
  •             Subphylum: Angiospermae
  •                 Class: Dicotyledonae
  •                     Order: Asterales
  •                         Family: Asteraceae
  •                             Genus: Centaurea
  •                                 Species: Centaurea diffusa

Notes on Taxonomy and Nomenclature

Top of page The currently accepted scientific name for diffuse knapweed is Centaurea diffusa, which was published by Lamarck in 1785 (USDA-ARS, 2003).


Top of page Diffuse knapweed reproduces entirely from abundant seeds that germinate quickly, forming a long, fibrous taproot and a rosette. Rosette leaves have short stalks, are alternate, deeply lobed, bipinnate, oblanceolate to oblong, up to 20 cm long and 5 cm wide (USDA, 1970; Beck, 1994).

At maturity, diffuse knapweed usually produces one upright stem with many branches. It is generally shorter than spotted knapweed (Centaurea stoebe subsp. micranthos, growing 30-90 cm tall. Branching gives a ball-shaped profile that is useful when the taproot breaks and it becomes a tumbleweed. Stems have fine short hairs, giving the plant a grey appearance. Stem leaves are stalkless, lower leaves are bipinnate, and the upper leaves are much smaller and entire (Watson and Renney, 1974; Roche and Roche, 1999).

Flowerheads are solitary or borne in clusters of two or three at the ends of branches. They are 3-6 mm in diameter and 8-11 mm long, excluding spines and flowers. Floral bracts are yellowish-green with a light-brown margin and have a terminal spine about 8 mm long, with four to five pairs of shorter, lateral spines, making them sharp to the touch (USDA, 1970).

Flowers are usually white, sometimes pink or lavender. Fruits are achenes, 2-3 mm long, dark brown, marked with several pale-brown or ivory lines. Inner achenes may have a plume or pappus of bristle-like hairs that vary from scale-like to 1/8 the length of the seed. Seeds are tightly held in the flower head, allowing them to be spread over large distances when the plant stem breaks off and becomes a tumbleweed (USDA, 1970; Roche and Roche, 1991; Roche and Roche, 1999).

Plant Type

Top of page Biennial
Seed propagated


Top of page Diffuse knapweed is native to grasslands of western Asia and south-eastern Europe. The native range includes Armenia, Azerbaijan, Georgia, Bulgaria, Greece, Moldova, Romania, Russia, Ukraine and former Yugoslavia (USDA-ARS, 2003). It was introduced into Central Europe and North America, probably through contaminated lucerne shipments (Watson and Renney, 1974). It is a serious invader of lucerne crops in the Crimea (Harris and Cranston, 1979).

Distribution Table

Top 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/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes


ArmeniaPresentNative Invasive USDA-ARS, 2003
AzerbaijanPresentNative Invasive USDA-ARS, 2003
Georgia (Republic of)PresentNative Invasive USDA-ARS, 2003
IranPresentIntroduced Invasive Rechinger, 1980
SyriaPresentIntroduced Invasive Watson and Renney, 1974
TurkeyPresentNative Invasive USDA-ARS, 2003

North America

CanadaPresentIntroduced Invasive Roche and Roche, 1999
-AlbertaPresentIntroduced Invasive Duncan, 2001
-British ColumbiaWidespreadIntroduced Invasive Duncan, 2001
-ManitobaPresentIntroduced Invasive Roche and Roche, 1999; Rice, 2003
-OntarioPresentIntroduced Invasive Roche and Roche, 1999
-SaskatchewanPresentIntroduced Invasive Roche and Roche, 1999; Rice, 2003
USAPresentIntroduced Invasive Roche and Roche, 1999
-ArizonaPresentIntroduced Invasive , 2002; Duncan, 2001
-CaliforniaPresentIntroduced Invasive , 2002; Duncan, 2001
-ColoradoWidespreadIntroduced Invasive , 2002; Duncan, 2001
-ConnecticutPresentIntroduced Invasive , 2002
-IdahoWidespreadIntroduced Invasive , 2002; Duncan, 2001
-IllinoisPresentIntroduced Invasive , 2002
-IndianaPresentIntroduced Invasive , 2002
-IowaPresentIntroduced Invasive , 2002
-KentuckyPresentIntroduced Invasive , 2002
-MassachusettsPresentIntroduced Invasive , 2002
-MichiganPresentIntroduced Invasive , 2002
-MissouriPresentIntroduced Invasive , 2002
-MontanaPresentIntroduced Invasive , 2002; Duncan, 2001
-NebraskaPresentIntroduced Invasive , 2002
-NevadaPresentIntroduced Invasive , 2002; Duncan, 2001
-New JerseyPresentIntroduced Invasive , 2002
-New MexicoPresentIntroduced Invasive , 2002; Duncan, 2001
-North DakotaPresentIntroduced Invasive Duncan, 2001
-OregonWidespreadIntroduced Invasive , 2002; Duncan, 2001
-South DakotaPresentIntroduced Invasive Duncan, 2001
-TennesseePresentIntroduced Invasive , 2002
-UtahPresentIntroduced Invasive , 2002; Duncan, 2001
-WashingtonWidespreadIntroduced1907 Invasive Roche and Roche, 1999; Duncan, 2001
-WisconsinPresentIntroduced Invasive , 2002
-WyomingPresentIntroduced Invasive , 2002

South America

ArgentinaPresentIntroduced Invasive Holm et al., 1979


AustriaPresentIntroduced Invasive Tutin et al., 1976
BulgariaPresentNative Invasive USDA-ARS, 2003
Czech RepublicPresentIntroduced Invasive Tutin et al., 1976
Czechoslovakia (former)PresentIntroduced Invasive Tutin et al., 1976
FrancePresentIntroduced Invasive Tutin et al., 1976
GermanyPresentIntroduced Invasive Tutin et al., 1976
GreecePresentNative Invasive USDA-ARS, 2003
HungaryPresentIntroduced Invasive Tutin et al., 1976
ItalyPresentIntroduced Invasive Tutin et al., 1976
MoldovaPresentNative Invasive USDA-ARS, 2003
PolandPresentIntroduced Invasive Tutin et al., 1976
RomaniaPresentNative Invasive USDA-ARS, 2003
Russian FederationPresentNative Invasive USDA-ARS, 2003
-Central RussiaPresentNative Invasive Tutin et al., 1976
-Southern RussiaPresentNative Invasive Tutin et al., 1976; Sheley et al., 1998
-Western SiberiaPresentNative Invasive Tutin et al., 1976
SerbiaPresentNative Invasive USDA-ARS, 2003
SwedenPresentBohlin, 2010
SwitzerlandPresentIntroduced Invasive Tutin et al., 1976
UkrainePresentNative Invasive USDA-ARS, 2003
Yugoslavia (former)PresentNative Invasive USDA-ARS, 2003

History of Introduction and Spread

Top of page Diffuse knapweed was introduced into the USA in Washington in 1907. Canadian introduction was in British Columbia in 1936 (Watson and Renney, 1974). In the USA the primary range is the western states of Washington, Idaho, Montana, New Mexico and Arizona. Diffuse knapweed has also spread east into several mid-western states and is found in Massachusetts, Connecticut and New Jersey on the east coast (USDA-NRCS, 2002). In Canada, it can be found from the Yukon in the north, throughout most of western Canada, and east to Ontario (Roche and Roche, 1999).

Risk of Introduction

Top of page The risk of introduction of C. diffusa is high in contaminated lucerne seeds (Zouhar, 2001). It is a restricted noxious weed in 13 US states and four Canadian provinces (Rice, 2003).


Top of page Diffuse knapweed is tolerant of a wide range of conditions, but prefers arid or semi-arid habitats. Infestations stop abruptly with an increase in soil moisture near temporary and permanent streams or with flooding and irrigation (Berube and Myers, 1982). It invades disturbed or overgrazed rangeland and dry, open forest sites where it forms dense, often monotypic stands (Lacey et al., 1990). Other areas of invasion include road and railway right of ways, waterways, gravel pits and industrial areas (Roche and Roche, 1988).

Habitat List

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Terrestrial – ManagedCultivated / 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-naturalNatural grasslands Present, no further details Harmful (pest or invasive)
Riverbanks Present, no further details Harmful (pest or invasive)

Hosts/Species Affected

Top of page Diffuse knapweed is a serious weed of lucerne in its native range (Harris and Cranston, 1979) and will sometimes invade orchards and vineyards. However, it is mainly a rangeland pest (Roche and Roche, 1999).

Host Plants and Other Plants Affected

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Plant nameFamilyContext
Medicago sativa (lucerne)FabaceaeMain

Biology and Ecology

Top of page Diffuse knapweed is a herbaceous annual, biennial, or short-lived perennial that exploits disturbed arid or semi-arid areas. It spreads entirely by seeds, forming monotypic stands that dominate through successful competition for soil moisture and nutrients and through allelopathy (Callaway and Aschehoug, 2001, 2000).


Diffuse knapweed is normally diploid (2n=18) but is occasionally tetraploid (2n=36) (Watson and Renney, 1974). It forms rare hybrids with spotted knapweed (Centaurea maculosa) and the hybrid is Centaurea x psammogena (Ochsmann, 2003). Populations have large genetic diversity due to obligate outcrossing and multiple introductions (Harrod and Taylor, 1995; Roche and Roche, 1999).

Physiology and Phenology

Diffuse knapweed reproduces by seed and is generally biennial. Seeds germinate in spring, very late summer or autumn, and over a wide range of environmental conditions. Germination requires more than 55% soil moisture, and seeds germinate best on the soil surface. Emergence rate decreases as seedling depth increases, with little or no emergence below 2.5 cm. More than 80% of seeds germinate at temperatures between 13 and 28°C and optimum moisture (Watson and Renney, 1974).

Physiologically polymorphic seeds give the plant maximum advantage. There are nondormant seeds that germinate in the dark, light-sensitive seeds that germinate after exposure to red light, and dormant seeds that do not respond to exposure to red light. Open canopy conditions provide favourable light conditions for germination, but seeds will germinate in closed canopies (Spears et al., 1980; Nolan and Upadhyaya, 1988).

Most seeds sprout in the first year, producing rosettes that bolt in the second or third year. Root development occurs during the rosette stage, and exposure to cold temperatures primes the plant for bolting. Plants completing juvenile growth by autumn overwinter as rosettes, and usually bolt in early May. Plants that do not complete the juvenile stage by the end of autumn remain as rosettes through the second year and may bolt during the third year. Under conditions of severe crowding, a plant may not flower for 5 or more years (Thompson and Stout, 1991; Zouhar, 2001).

The phenology is highly variable and dependent on the site. Moisture, temperature and plant density are all factors in development. Flower buds are formed in early June. Flowering occurs from July through September or later as permitted by adequate moisture and mild temperatures. Mature seeds are usually formed by mid-August, followed by the death of the plant. Dead plants break off at ground level in the spring and tumble with the wind, spreading seed as they roll (Watson and Renney, 1974; Thompson and Stout, 1991).

Reproductive Biology

Diffuse knapweed has male and female flowers on the same plant (monoecious) and is pollinated by insects, mostly honey bees and bumble bees. Some experiments have shown that the plant is self-compatible, but consensus is that fertilization requires cross-pollination (Watson and Renney, 1974; Harrod and Taylor, 1995).

Monotypic stands of up to 500 plants/m² can produce a massive seed rain of 11,000 to 40,000 seeds/m². However, many seeds remain on the plant until the taproot breaks and a tumbleweed is formed. Plants and seeds are dispersed by wind, water and by vehicles over long distances (Watson and Renney, 1974; Roche and Roche, 1999).

Environmental Requirements

Diffuse knapweed is commonly found on well-drained soils such as sandy or gravelly loams or loamy fine sands containing coarse fragments. It is less competitive on shallow, very coarse textured soils such as sand or loamy coarse sand, although it may thrive on these sites when disturbance removes other vegetation (Zouhar, 2001).

It grows in soil with a pH range of 5.9 to 8.4. In Canada, most infestations are found where mean annual temperatures are between 7.2 and 9.4°C. It grows well where the annual rainfall is between 240 and 420 mm (Watson and Renney, 1974). However, it has been found at locations with mean annual temperatures ranging from 3.6 to 13.8°C (Harris and Cranston 1979; Strang et al., 1979; Roche and Roche, 1999; Climate, 2003). Diffuse knapweed is not competitive in moist microsites such as gullies, depressions or poorly drained soils. It does not grow well in saline soils (Zouhar, 2001).

In British Columbia, Canada, it is commonly found on south-facing slopes below 900 m. In Washington, USA, it is found below 1500 m in 150-900 mm of annual rainfall (Roche and Roche, 1999).


A number of grasses and herbaceous plants are found with diffuse knapweed. These include bluebunch wheatgrass (Agropyron spicatum [Pseudoroegneria spicata]), western yarrow (Achillea lanulosa [A. millefolium]), Idaho fescue (Festuca idahoensis), sheep sorrel (Rumex acetosella) and downy brome (Bromus tectorum) (Watson and Renney, 1974).

It may also be dominant in bitterbrush (Purshia tridentata), habitat types with or without an overstorey of ponderosa pine (Pinus ponderosa) (Roche and Roche, 1999). In the southern interior of British Columbia, Canada, it may be found with Douglas fir (Pseudotsuga menziesii), lodgepole pine (Pinus contorta), Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa) and understorey components such as pinegrass (Calamagrostis rubescens) and fireweed (Epilobium angustifolium) (Hubbard, 1975; Berube and Myers, 1982; Myers and Berube, 1983).

Latitude/Altitude Ranges

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Latitude North (°N)Latitude South (°S)Altitude Lower (m)Altitude Upper (m)
0 0 0 1500

Air Temperature

Top of page
Parameter Lower limit Upper limit
Mean annual temperature (ºC) 4 14
Mean maximum temperature of hottest month (ºC) 22 37
Mean minimum temperature of coldest month (ºC) -15 0


Top of page
ParameterLower limitUpper limitDescription
Dry season duration412number of consecutive months with <40 mm rainfall
Mean annual rainfall219844mm; lower/upper limits

Rainfall Regime

Top of page Winter

Soil Tolerances

Top of page

Soil drainage

  • free

Soil reaction

  • acid
  • alkaline
  • neutral

Soil texture

  • light

Special soil tolerances

  • shallow

Natural enemies

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Natural enemyTypeLife stagesSpecificityReferencesBiological control inBiological control on
Agapeta zoegana Herbivore Roots
Bangasternus fausti Herbivore Seeds
Bangasternus orientalis Herbivore
Chaetorellia acrolophi Herbivore Seeds
Cyphocleonus achates Herbivore Roots
Larinus minutus Herbivore Seeds
Larinus obtusus Herbivore Seeds
Melanoplus sanguinipes Herbivore
Metzneria paucipunctella Herbivore Seeds
Oedaleonotus enigma Herbivore
Pelochrista medullana Herbivore Roots
Pterolonche inspersa Herbivore Roots
Puccinia jaceae Pathogen
Puccinia jaceae var. diffusa Pathogen
Sphenoptera jugoslavica Herbivore Roots Canada
Subanguina picridis Parasite
Terellia virens Herbivore Seeds
Urophora affinis Herbivore Seeds Canada
Urophora quadrifasciata Herbivore Seeds Canada

Notes on Natural Enemies

Top 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 Dispersal

Top of page Natural Dispersal (non-biotic)

Dispersal of diffuse knapweed seed is mainly by wind. Some seeds are shaken loose as plants sway in the wind. Dispersal over longer distances occurs when plants break loose from the taproot and tumble in the wind, dispensing seeds individually from the small opening at the top of the seedheads. Seeds from tumbleweeds ride on the frames of vehicles and colonize roads. Seeds and plants may also be transported in waterways and colonize riverbanks. In British Columbia, Canada, logging trucks, off-road vehicles and trail bikes have greatly contributed to the spread of both spotted and diffuse knapweed (Watson and Renney, 1974; Strang et al., 1979; Roche and Roche, 1999).

Vector Transmission (biotic)

Diffuse knapweed may be spread on rangelands by grazing animals (Watson and Renney, 1974).

Agricultural Practices

Seeds are dispersed by infested hay, vehicles and equipment, animals or shoes. Contaminated lucerne seeds have led to the international spread of C. diffusa (Watson and Renney, 1974; Spears et al., 1980).

Accidental Introduction

Diffuse knapweed was accidentally introduced into the USA and Canada in shipments of contaminated lucerne seed (Roche and Roche, 1999).

Intentional Introduction

As it is not an ornamental plant, C. diffusa has never been deliberately introduced.

Pathway Vectors

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VectorNotesLong DistanceLocalReferences
Land vehiclesautomobiles, farm vehicles Yes
Soil, sand and gravel Yes

Plant Trade

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Plant parts liable to carry the pest in trade/transportPest stagesBorne internallyBorne externallyVisibility of pest or symptoms
True seeds (inc. grain) seeds
Plant parts not known to carry the pest in trade/transport
Fruits (inc. pods)
Seedlings/Micropropagated plants
Stems (above ground)/Shoots/Trunks/Branches

Impact Summary

Top of page
Animal/plant collections None
Animal/plant products None
Biodiversity (generally) Negative
Crop production None
Environment (generally) Negative
Fisheries / aquaculture None
Forestry production None
Human health None
Livestock production Negative
Native fauna None
Native flora Negative
Rare/protected species Negative
Tourism None
Trade/international relations None
Transport/travel None


Top of page Many economic losses have been attributed to diffuse knapweed infestations. Examples include the loss of forage on rangeland and pasture, depletion of soil and water resources, displacement of native species on wildlands, reduction of biodiversity, reduced land value and increased road maintenance costs. Costs for forage loss alone in Washington state, USA, have been estimated at about $1-3 million each year (Lacey and Olson, 1991; Roche and Roche, 1999; Zouhar, 2001).

Environmental Impact

Top of page Diffuse knapweed replaces traditional forage plants on range and pasturelands. While it is not poisonous, it can lead to reduced nutrition for livestock and wildlife. Where there is overgrazing or drought, flower shoots are sometimes grazed whereas immature rosettes are not. Young rosettes are nutritious and edible, but are difficult for cattle to eat because they grow so close to the ground (Watson and Renney, 1974).

Mature knapweed plants are coarse and fibrous and the spines on the bracts can be very irritating, or may even cause injury to the mouths and digestive tracts of grazing animals. However, diffuse knapweed is grazed by deer and domestic sheep and by elk and cattle, at least through the bolting stage (Sheley et al., 1998; Roche and Roche, 1999).

Impact: Biodiversity

Top of page Though it has no effect on the growth or survival of conifer seedlings and antelope bitterbrush (Purshia tridentata), it does impact perennial grasses. It has rapid growth rates, large seed output and extended growing periods. Continuous seed rain and sequential emergence leads to maximum site dominance and monotypic stands. Bluebunch wheatgrass (Pseudoroegneria spicata) may offer little resistance to knapweed invasion (Sheley and Larson, 1996). Nutrient competition combined with allelopathy leads to dominance and suppression of biodiversity (Zouhar, 2001).

Threatened Species

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Threatened SpeciesConservation StatusWhere ThreatenedMechanismReferencesNotes
Artemisia campestris var. wormskioldii (Northern wormwood)NatureServe NatureServe; USA ESA candidate species USA ESA candidate speciesOregon; WashingtonCompetition (unspecified)US Fish and Wildlife Service, 2014
Silene spaldingii (Spalding's catchfly)USA ESA listing as threatened species USA ESA listing as threatened speciesIdaho; Montana; Oregon; WashingtonCompetition - monopolizing resources; Competition - smotheringUS Fish and Wildlife Service, 2007

Social Impact

Top of page Diffuse knapweed has no effect on human health (Zouhar, 2001).

Risk and Impact Factors

Top 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
  • Highly mobile locally
  • Has high reproductive potential
  • Has propagules that can remain viable for more than one year
Impact outcomes
  • Damaged ecosystem services
  • Ecosystem change/ habitat alteration
  • Negatively impacts agriculture
  • Negatively impacts tourism
  • Reduced amenity values
  • Reduced native biodiversity
Impact mechanisms
  • Competition - monopolizing resources
  • Competition - smothering
  • Competition
  • Produces spines, thorns or burrs
Likelihood of entry/control
  • Highly likely to be transported internationally accidentally
  • Difficult/costly to control


Top of page Diffuse knapweed is grazed by bighorn sheep, white-tailed deer, mule deer and elk in the winter and early spring. Knapweed rosettes are an important part of the diet during these times. The impact of knapweed consumption on the welfare of these animals, and the effects of heavy utilization of rosettes need further examination (Miller, 1990). There may be an interaction between biocontrol and wildlife forage, as deer eat knapweed seedheads as winter browse after the establishment of Urophora spp. seedhead flies (Harris, 1988).

Diffuse knapweed is a source of pollen and nectar for honey bees. Birds and rodents, including chipmunks, use diffuse knapweed seeds for food. Chipmunks probably cache some seed for later use (Zouhar, 2001).

Prevention and Control

Top of page Introduction

Integrated control is most effective against diffuse knapweed. Central to the strategy is the prevention of seed formation, which can be accomplished by grazing, mowing, hand-pulling and biological control. Any control method must be integrated with a programme of revegetation to prevent re-infestation. As knapweeds thrive in direct sunlight, they can be suppressed by shading from competing vegetation (Kennett et al., 1992; Woo et al., 2002).

Cultural Control

Grazing by sheep and goats can suppress knapweeds (Olson and Wallander, 2001). Continual grazing of the tops of young plants can retard plant development, seed formation, and gradually deplete root reserves. As animals usually prefer to eat nearby grasses instead 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).

Diffuse knapweed should be grazed during the bolting stage for 10 days, and again after 14 days for an additional 10 days to reduce seed production. Although grazing can reduce seed production, it can also cause diffuse knapweed to become a short-lived perennial and when grazing is removed, populations often return to their former levels (Popay and Field, 1996).

In knapweed-infested rangeland, any patches of bare ground can lead to knapweed invasions (Lacey et al., 1990). Thus, reseeding of rangelands with competitive perennial grasses is important. Crested wheatgrass (Agropyron cristatum) has been a successful competitor against knapweeds in dry areas of Canada, reducing the rate of vegetative spread, limiting weed density and reducing weed seed production (Berube and Myers, 1982). Crested wheatgrass is more competitive than Russian wildrye (Elymus junceus [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 (F. altaica) are more competitive than bluebunch wheatgrass (Pseudoroegneria spicata) (Lacey et al., 1990; Steinger and Muller-Schärer, 1992).

Manipulation of plant ecology is important for the long-term suppression of knapweed. The kinds and numbers of plants at a site change slowly over time. Initial colonizers, usually seedy, weedy and fast-growing, are replaced successively with other plants, until a final set of plants known as the climax vegetation predominates. 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).

When the soil is rich in nitrogen, aggressive plants such as knapweed, which appear early in the succession, will out-compete perennial grasses. Controlled colonization can be used to force plant succession towards the preferred climax vegetation. This strategy is to plant simultaneously intermediate and late colonizers in the natural plant succession scheme. For instance, Herron et al. (2001) found that plantings of annual rye and bottlebrush squirreltail (Elymus elymoides) can deplete the soil of nutrients, especially nitrogen. As a result, late seral plants such as bluebunch wheatgrass (Pseudoroegneria spicata) will out-compete the early colonizer knapweed. Low nitrogen availability also makes knapweed more susceptible to root-feeding biological control agents (Steinger and Muller-Schärer, 1992).

A more short-term approach is to fertilize or combine fertilization with herbicides. Nitrogen fertilization has been used with mixed results. When fertilizer is applied to areas with a substantial grass understorey, grass yields increase on those sites (Sheley and Jacobs, 1997). Where knapweed has already invaded, nitrogen fertilization with no other treatment may favour knapweed over native perennial grasses (Prather and Callihan, 1991; Velagala et al., 1997; Sheley et al., 1998; Herron et al., 2001).

Effects of fertilization may be complicated by moisture conditions at the site. 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). However, Myers and Berube (1983) found that the addition of nitrogen had no effect on biomass of either diffuse knapweed or competing grasses in dry areas of British Columbia, Canada.

Mechanical Control

Control of diffuse knapweed by hand-pulling is feasible for scattered diffuse knapweed plants. It is important to remove the entire taproot with as little soil disturbance as possible. Plants should be pulled in the spring and when the soil is moist, and again in June to remove bolting plants before they flower and set seed. Finally, plants are pulled just before seed dispersal, taking care to remove the plants from the site. Hand-pulling is tedious and not always effective (Roche and Roche, 1999).

Mowing decreases flower and seed production, and in the long-term it may affect knapweed densities. It can also reduce weed competition during a revegetation programme. Mowing diffuse knapweed in British Columbia, Canada, at the bud stage and again at flowering reduced the number of plants producing seed by 77-99%. Mowing treatments also reduced seed germination by 79%. Plants mowed early in the growing season produce few viable seeds; however, mowed plants usually resprout and flower again. In some instances, diffuse knapweed densities increased after a single mowing (Watson and Renney, 1974; SBNM, 1997).

Cultivation and irrigation will kill diffuse knapweed. However, cultivation causes soil disturbance and may not be a management option on rangelands. In a location where cultivation is acceptable, regular deep ploughing can slice knapweed roots and bury seeds, thus weakening the plant. Cultivation in combination with reseeding competitive perennial grasses may minimize re-invasion by the knapweeds (Roche and Roche, 1999).

Chemical Control

In general, herbicides are expensive and do not provide lasting control of diffuse knapweed. They can 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 and are not destroyed by composting (Fay et al., 1991; Bezdicek et al., 2001; Houck and Burkhart, 2001). If herbicides are used, they should be applied at the rosette or early bolting stage and before seeds are produced. They should always be combined with a revegetation programme (Woo et al., 1999, 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 (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 biocontrol agents have been released (Clark et al., 2001a). The weevil Larinus minutus is having a serious impact on plant growth and density at many locations (Story and Piper, 2001). The 13 species 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 had spread onto diffuse knapweed. The 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. The larvae feed inside the seedhead, 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). The 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. The adults feed on knapweed rosettes. The 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; Lang, 1997; Story et al., 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 (Jacobs et al., 2000; Clark et al., 2001b).

Sphenoptera jugoslavica, a root borer, is a flat, blue-black, copper-coloured beetle which attacks the 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 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. The 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 the 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).

Integrated Control

Combination treatments of fertilizer and herbicides have given inconsistent results. Sheley and Roche (1982) found knapweed suppression and grass growth was greater at sites receiving both treatments. Hubbard (1975) obtained similar results in dry areas, but not in areas where rainfall was greater. Sheley and Jacobs (1997) found that 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.

It is hard to control diffuse knapweed if the treated area is constantly challenged with seeds. One study with small plots combined burning, cultivation, herbicide, competitive planting and nitrogen fertilizer. At the end of the third year, grass production peaked. For 3 years after that, grasses declined due to drought. Diffuse knapweed had re-established after 8 years. The authors attributed the re-invasion to 3 years of drought and to small plot size, which allowed reseeding from adjacent plots (Roche and Roche, 1999).

Public information systems and bounty programmes can be useful as part of an IPM programme. Bounty programmes have had some success in Montana (Lacey et al., 1988).


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