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.
Distribution
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.
Reproduction
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.
Competition
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.
Control
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.
References
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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