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An Assessment of Exotic Plant Species of Rocky Mountain National Park

Centaurea maculosa Lam. (Centaurea biebersteinii D.C., Acosta maculosa)
Spotted knapweed


Current level of impact
Known locations in RMNP: First documented in RMNP in Moraine Park campground in 1992 between campsites 22 and 23. Likely introduced accidentally by park visitors.
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.

Distribution
Origin: Introduced from Europe as a contaminant of alfalfa and clover seed.
Geographic distribution: Northeastern states, north central states and Pacific northwest. Throughout all eastern half of U.S., from southern California to east-central Montana.
Ecological distribution: Adapted to well-drained, light textured soils that receive summer rainfall, including habitats dominated by Ponderosa Pine and Douglas fir, as well as foothill prairie habitats.
Soils: Especially in sandy soils. Found in light, porous, fertile, well-drained, often calcareous soils in warm areas. Inceptisol soils are susceptible to invasion in western Canada.

Reproduction
Biennial forb, reproduces by seeds. Lateral root sprouting may result in rosettes that remain attached to parent, but expansion is dependent on seeds. Flowers August through September. Plants are self fertile, and also cross-pollinated by insects.
Seed production: Each plant produces 4-5 capitula in the first year (range 1-25), and 8-15 capitulas in succeeding years. The mean number of achenes/capitula range from 9-37 (36-555 seeds/plant). Plants may produce up to 140,000 seeds/m2.
Seed longevity: Seeds remain viable after eight years of burial.
Seed dispersal: Short pappus and weight of the seed keep dispersal distances relatively short. Seeds generally fall within a 3-12 dm radius of the parent plant. Seeds dispersed by livestock, rodents, vehicles, in hay or commercial seed.
Germination: Seeds may germinate over a wide range of soil depths, soil moisture contents, and temperatures. Seed dormancy may be induced by exposure to light.

Competition
Knapweed readily establishes on disturbed soils, and its early growth makes it a good competitor for soil moisture and nutrients. There is evidence that knapweeds release chemical compounds that inhibit the growth of other plants. Knapweed is usually found around disturbed areas. However, once a plant colony is established, it may invade areas that are relatively undisturbed or in good condition.
Response to shade: Prefers open areas, not commonly found in shaded areas.

Control
(See diffuse knapweed). Seeds are relatively long lived, and may reduce the chances of successful eradication using mechanical control methods. Most literature has focused on reestablishing range, pasture or cropland, and not on restoration of native plant communities.
Mechanical: Mowing is possible method of control, but not eradication. Mowing can greatly reduce seed production. However, a well-established seed bank such as those likely to be present in areas of large infestations would likely compensate for any seeds removed by mowing. Mowing, just after flowering, but prior to seed maturation is an option for areas with little other vegetation that are not steep or rocky. Hand removal and hand clipping prior to seed maturation are other management options, although soil disturbance may promote other weed colonization.
Chemical: (See diffuse knapweed) 2,4-D is a temporary solution because it does not inhibit germination from seeds in the soil. Picloram persists in soils for 4 years, but enough herbicide is lost from a 0.4-0.6 kg/ha treatment to allow germination and re-infestation(Harris and Cranston 1979). Other studies have shown that spotted knapweed can be controlled with 2,4-D applied at a rate of 1.5 kg/ha with dicamba at 1 kg/ha (Stumpf 1994).
Biological control: (See diffuse knapweed) Four insect species have been introduced into North America to help control knapweeds including two gall flies (Urophora aphinis and Urophora quadrifasciata) which are available in Colorado. A moth which attacks the capitula, and a beetle which attacks the roots also show promise as control agents. Sheep have been used in Montana to reduce spotted knapweed and release grasses from competition (Beck 1994).

Notes
Spotted knapweed has a bitter taste, but is sometimes grazed by deer.

References

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

Chicoine, T.K. 1984. Spotted knapweed (Centaurea maculosa L.): Control, seed 
    longevity, and migration in Montana. MS Thesis Montana State University.

Harris, P. and R. Cranston. 1979. An economic evaluation of control methods for 
    diffuse and spotted knapweeds in Western Canada. Canadian Journal of Plant 
    Science 59:375-382.

Hubbard, W.A. 1975. Increased forage production by reseeding and the chemical control 
    of knapweed. Journal of Range Management 28:406-407.

Maddox, D.M. 1982. Biological control of diffuse knapweed (Centaurea diffusa) and 
    spotted knapweed (Centaurea maculosa) in Washington, Montana, Idaho, Oregon, 
    and California. Weed Science 30(1):76-82.

Muir, A.D. and W. Majak. 1983. Allelopathic potential of diffuse knapweeds extracts. 
    Canadian Journal of Plant Science. 63(4):989-996.


Mauer, T. M.J. Russo, and M. Evans. 1995. Element Stewardship Abstract for Centaurea 
    maculosa: Spotted knapweed. The Nature Conservancy, Arlington, Virginia.

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 disuse 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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