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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Centaurea diffusa Lam. (Acosta diffusa)
Tumble knapweed, diffuse knapweed (Asteraceae)

Current level of impact
Known locations in RMNP: First observed along Highway 36 east in Boulder in 1990. Diffuse knapweed was observed within one mile of the park in 1991 and was found in the park at High Drive across from park headquarters in 1993.
Assessment: Few scattered populations, if all populations were added together, would cover an area less than 5 hectares. Found mostly in areas disturbed in the last 5 years. Capable of invading and replacing native communities.

Origin: Introduced from Eurasia.
Geographic distribution: Widely distributed throughout the U.S.
Ecological distribution: Readily colonizes soils of a wide range of chemical and physical properties. Prefers open habitat. Well adapted to semiarid areas, and does well in semi-arid west in areas of 9-16" annual precipitation.
Soils: Generally found on dry, light, porous soils.

Biennial, occasionally an annual or short-live perennial, reproduces by seeds. Re-growth from root crowns may occur for a number of years, with only occasionally regrowth one year after seed production. Does not reproduce vegetatively. Flowers July to September and plants die after setting seed. Plants are self compatible.
Seed production: Each plant is capable of producing 400-900 seeds per plant. A prolific seed producer, capable of producing up to 40,000 seeds/m2.
Seed dispersal: Mature plants break off at ground level and tumble with the wind dispersing seeds. Seeds are also dispersed by vehicles, and plants frequently colonize the sides of roads.
Germination: Seeds capable of germination over a wide range of conditions. Optimal temperature ranges from 7-34 C and moisture may be a limiting factor. Seeds below depths of 3 cm do not germinate.

Highly competitive plant, threatens to exclude more desirable species. Quickly invades disturbed sites and is also capable of invading relatively undisturbed native plant communities. Far more aggressive than annual or perennial grasses. A rapidly spreading plant which forms large, dense infestations, but solitary plants and small patches are common in recently invaded areas. Plants capable of forming densities greater than 500 plants/m2 from a single plant.
Response to shading: Prefers open habitats, and is not commonly found in shaded areas. Does not tolerate flooding or shading.

The rosette growth of the first year resists mowing and grazing. Diffuse knapweed has a taproot that deeply penetrates the soil. Also somewhat resistant to chemical control. Control efforts are of little value if native vegetation is scarce or absent. Diffuse and spotted knapweeds can be controlled with similar methods. However, reinvasion will occur unless cultural techniques which establish competitive grass cover are used.
Cultural: If desirable grasses are present, herbicide application that will not injure grasses may release them to compete with knapweeds. Irrigation, where possible, can also be used to stimulate grass competition. Usually, in disturbed sites, grass seeding is necessary to prevent weed reinfestation.
Mechanical: Pulling or cutting diffuse knapweed is a commonly recommended control measure. Studies show that plants cut below the crown regrew 38% of the time, while those which had the rosette removed along with 2-4" of taproot only survive 4% of the time (Roche 1995). Deep plowing (18 cm) can reduce knapweed. Mowing once at flowering stage or once at bud stage and again at flowering stage can reduce the number of plants that produce seeds.
Chemical: Application of 2,4-D at a rate of 1 kg/acre completely eliminated knapweed and associated fortes (Watson and Renney 1974). Research conducted at C.S.U. indicates that dicamba (Banvel) at 0.5 to 1.0 lb. ai/A (0.5 to 1.0 lb.) and picloram (Tordon) 0.25 to 0.5 lb./ai control diffuse knapweed. Also tank mixes can be used to save money and reduce harm to grasses. Tank mixes of Banvel plus 2,4-D (0.5 + 1.0 lb. ai/A), Banvel plus Tordon (0.5 + 1.0 lb. ai/A), Tordon plus 2,4-D (0.188 + 1.0 lb. ai/A) and clopyralid plus 2,4-D (Curtail; 1.5 + 8.0 oz ai/A) all control diffuse knapweed (Beck 1994).
Biological: The banded seedhead fly has been used to control knapweed. These flies reduce seed production and may devitalize the plants up to 95%. Other biological control agents have also been used with some success, including two gall flies (Urophora aphinis and Urophora quadrifasciata) which are available in Colorado. However, research at Montana State indicates that a complex of insects (perhaps 12) are needed to reduce diffuse and spotted knapweed populations. Livestock (sheep, goats, cattle) will consume diffuse and spotted knapweeds.
Other: Burning or cultivation can provide temporary control of knapweed.


Beck, K.G. 1994. Diffuse and spotted knapweed: Biology and management. Colorado 
    State University Cooperative Extension no. 3.110.

Hubbard, P. and R. Cranston. 1979. Increased forage production by reseeding and 
    chemical control of knapweed. Journal of Range Management 28:406-407.

James, L.F. J.O. Evans, and R.D. Childs (eds.). 1991. Noxious Range Weeds. Westview 
    Press, Inc. Boulder, Colorado. 466 pp.

Lacey, J., P. Husby, and G. Handl. 1990. Observations on spotted and diffuse 
    knapweed invasion into ungrazed bunchgrass communities in western Montana. 
    Rangelands 12:30-32.

Maddox, D.M. 1979. The knapweeds: their economic and biological control in the 
    western states, U.S.A. Rangelands 1:139-141.

Maxwell, J.F., R. Drinkwater, D. Clark, and J.W. Gall. 1992. Effect of grazing, 
    spraying, and seeding on knapweed in British Columbia. Journal of Range 
    Management 45:180-182.

Roche, B.F. 1995. Diffuse knapweed: Biology and Ecology. Paper given at CWMA 
    Conference in December, 1995.

Schirman, R. 1981. Seed production and spring seedling establishment of diffuse 
    and spotted knapweed. Journal of Range Management 34:45-47.

Spears, B.M., S.T. Rose and W.S. Belles. 1980. Effect of canopy cover, seedling 
    depth, and soil moisture on emergence of Centaurea maculosa and C. diffusa. 
    Weed Research 20:87-90.

Stumf, J.A. 1994. Centaurea diffusa Lam. pp. 89-95. In An Assessment of Exotic 
    Plants at Scotts Bluff National Monument and Effigy Mounds National Monument. 
    University of Nebraska, Lincoln Nebraska.

Watson, A.K. and A.J. Renney. 1974. The biology of Canadian Weeds. 6. Centaurea 
    diffusa and C. maculosa. Canadian Journal of Plant Science 54:687-701.

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