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Management Of Russian Knapweed
In Colorado, Russian knapweed is the most aggressive of the knapweeds infesting an estimated 168,00 acres. Russian knapweed has the ability to invade healthy plant communities and to establish a monoculture. Russian knapweed monocultures have been recorded to remain stable for 80 years. Russian knapweed is believed to be allelopathic, producing chemicals that inhibit the growth of other plants. Successful management of Russian knapweed will involve an integration of the following control methods.
Mechanical Control
Handpulling for perennial weed control is rarely effective. While two mowings, 8 weeks apart (first at bud-growth stage), suppressed Russian knapweed during that year, the weed recovered vigorously the subsequent season (Beck 1999). Research has shown that mowing alone will not control this weed.
Chemical Control
Russian knapweed is controlled with Tordon 22K (picloram) at 1 to 2 quarts/A. Tordon may be broadcast sprayed up to 1 quart/A but one can spot spray at rates up to 4 quarts/A. Tordon plus 2,4-D (1 to 1.5 pints plus 1 quart/A) also will control Russian knapweed. If low rates of Tordon or Tordon plus 2,4-D are used, application for two consecutive years may be necessary to achieve adequate control. Apply Tordon anytime the weed is actively growing. The Tordon label prohibits seeding perennial grasses the same year this herbicide is applied (Beck 1999). Curtail (clopyralid plus 2,4-D) at 1 oz/A may also be used. These herbicides can be purchased at an agricultural products supplier (please check the yellow pages for specific stores).
The use of herbicides to control Russian knapweed before establishing perennial grasses is an important part of a management system. Research has shown that control is greater when herbicides are applied when Russian knapweed was either at the bloom or seed stage, rather than the rosette or early growth stage (Whitson 1999).
Biological Control
At this time, an effective biological control agent for Russian knapweed in Larimer County has not been found.
Cultural Control
Russian knapweed might be managed temporarily with herbicides or, in the future, with insects, but long-term reductions must include planting competitive plant species to occupy bare ground once infested by the weed. Many improved grass species can be seeded in late fall or winter when seedbeds are properly prepared. An initial herbicide treatment is important to suppress problem perennial weeds before seeding.
Important grass characteristics to be considered for long-term control of problem weeds such as Russian knapweed include 1) adaptation to the soil and climate; 2) ease of establishment; 3) competitiveness with weeds; 4) palatability and nutritive value, particularly for late-season use; 5) dry matter productivity; and 6) stand longevity (Whitson 1999).
Resources
Beck, K.G. 1999. Russian knapweed. Colorado State Cooperative Extension. www. Colostate.edu/Depts/CoopExt
Whitson, T.D. 1999. Russian knapweed. In : Biology and Management of Noxious Rangeland Weeds., Sheley, R.L. and J.K. Petroff. Oregon State University Press, Corvallis. Pp.315-322.