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Appendix 4: Profiles of Colorado State-Listed Noxious Weeds 163 Canada thistle (Cirsium arvense) ]ate summer and fall when shoot to root transloca[ion is the greatest. However, translocation of the herbicide is dependent on moist soil conditions. If fall is a dry period in your area, a spring application azound the flower bud stage (early June), when root carbohydrate reserves are at their lowest, is recommended. Clopyralid + 2,4-D (commonly sold as Curtail®) applied at a rate of 2-3 quarts/acre will effectively control Canada thistle. Curtail should either be applied in the late spring (when Canada thistle plants are entering the bud growth stage) or in the fall (October) when Canada thistle roots aze actively growing. The performance of Curtail can be improved when proceeded by two or three mowings under conditions when the root systems are restricted (Beck 1996, Beck and Sebastian 2000). Begin mowing when Canada thistle is 12-] 5 inches tall and repeat at about one month intervals (Beck 1996). Apply Curtail in October or about one month after the last mowing. Clopyralid alone can be applied at a rate of 2/3 to 1 pint/acre in the spring or fall. Spring applications should be timed to the rosette to bud growth stages. 2,4-D or picloram are effective when applied at a rate of 1 lb. ai/acre in the spring when Canada thistle is in the pre-bud to early bud growth stages (about ] 0-15 inches tall). For increased control, retreat with dicamba (1 ]b. ai/acre) in the Fall to prevent regrowth of plants. Cu-turaUPreventive: Reduce the spread of Canada thistle seeds by always purchasing "weed free" seeds. Quickly eliminate new seedlings before they have a chance to form swell-developed root system. Inteerated Management Summary The tendency of this species to grow in wet areas may restrict the use of certain herbicides. Control efforts should target Canada thistle plants in high-quality areas first (typically azeas that contain mostly native species and few undesirable species), and then work on controlling lower quality areas (areas that are already infested with undesirable species and have fewer desirable species present). Management strategies should be adjusted to reflect weather conditions (Nuzzo 1998). For example, drought stress reduces the effectiveness of most herbicides, but increases the effectiveness of mechanical controls (e.g., mowing or burning). It takes at,least two years of control to determine whether a particular method is effective. Several studies have recorded a temporary decline in Canada thistle in the first year of control followed by a return to the pre-treatment conditions the second growing season (Nuzzo 1998). For one example of Canada thistle control, see page 60. References Beck, K.G. 1996. Canada thistle. Colorado State University Cooperative Extension Natural Resources Series, No. 3.108. http://www.colostate.edu/Depts/CoopExt/PUBS/NATRES/03108.htm1 [24 Jan 00]. Beck, K.G. and J.R. Sebastian. 2000. Combining mowing and fall-applied herbicides to control Canada thistle (Cirsium arvense). Weed Technology. In press. FEIS • Fire Effects Information System . ] 996. Prescribed Fire and Fire Effects Research Work Unit, Rocky Mountain Research Station (producer), US Forest Service. http://www.fs.fed.us/database/feis/ [Version 12 Mar 98]. Haderlie, L.C., R.S. McAllister, R.H. Hoefer, P.W. Leino. 1991. Canada thistle control. In L.F James, J.O. Evans, M.H. Ralphs and R.D. Child, eds. Noxious Range Weeds. Westview Press, Boulder, CO. Moore, R.J. 1975. The biology of Canadian weeds. 13. Cirsium arvense (L.) Scop. Canadian Journal of Plant Science. 55:1033-1048. Nuzzo, V. 1998. Element stewardship abstract for Cirsium arvense. The Nature Conservancy, Wildland Weeds Management & Research Program. http://mcweeds.ucdavis.edu/esadocs/cirsarve.html []6 Oct 98]. Rutledge, C. R. and T. McLendon. No Year. An Assessment of Exotic Plant Species of Rocky Mountain National Park. Department of Rangeland Ecosystem Science, Colorado State University. 97pp. Northern Prairie Wildlife Research Center Home Page. http://www.npwrc.usgs.gov/resource/othrdata/Explandexplant.htm [Version 15 Dec 98]. Whitson, T.D.(ed.), L.C. Burrill, S.A. Dewey, D.W. Cudney, B.E. Nelson, R.D. Lee, R. Pazker. 1996. Canada thistle. Weeds of the West. Western Society of Weed Science, in cooperation with the Western United States Land Grant Universities Cooperative Extension Services, Newazk, CA.