Canada thistle control
Curtail is clopyralid plus 2,4-D and is effective on Canada thistle but control tends to be less than from Transline. Recent research at Colorado State University shows that the performance of Curtail to control Canada thistle can be improved when preceded by two or three mowings. When Canada thistle infestations occur in situations where root growth would be restricted, such as habitats with high water tables, begin mowing when it is 12 to 15 inches tall. Repeat mowings at about one month intervals. Apply Curtail at 2 to 3 quarts/acre in October or about one month after the third mowing. Follow this regimen for two consecutive years.
Mowing hay meadows can be an effective tool if combined with herbicide treatments. Mowing alone is not effective unless conducted at one-month intervals over several growing seasons. Always combine mowing with cultural and chemical control. Mowing at hay cutting stimulates new Canada thistle shoots to develop from its root system.
In irrigated grass hay meadows, fall herbicide treatments that follow mowing can be an effective management system because more Canada thistle foliage is present after cutting to intercept herbicide. Additionally, root nutrient stores decrease after mowing because the plant draws on them to develop new shoots.
If a Canada thistle infestation exists in a field that will be rotated to alfalfa, control the weed before seeding alfalfa. Alfalfa is an effective competitor only after it is established. It will not adequately establish in a well-developed Canada thistle infestation. A Canada thistle management system can start with crop or grass competition combined with herbicides, with the field rotated to alfalfa when the management plan ends.
Additional information about Canada thistle management and control can be found at the Kansas Department of Agriculture web site: http://www.ksda.gov/plant_protection/content/49/cid/895.
Current recommendations on chemical control of Canada thistle in Kansas can be in the 2012 Chemical Weed Control for Field Crops, Pastures, Rangeland, and Noncropland SRP 1063 publication available at your local county extension office or online at: http://www.ksre.ksu.edu/library/crpsl2/SRP1063.pdf.