Managing Canada Thistle In Lawns
Photo by Noelle Orloff, MSU.
Canada thistle (Cirsium arvense) is a very persistent, perennial forb that is listed as a noxious weed in Montana and in many other states. It is challenging to control, in part because it has a large underground root system that allows it to store resources and re-sprout after control efforts. Management efforts should aim to deplete the root system over time. Controlling Canada thistle in lawns is possible, but it will take repeated efforts over consecutive years. Using an integrated approach by applying more than one management technique will improve chances of success. Cultural, mechanical, and chemical control options are all available for lawns.
One important aspect of managing Canada thistle in lawns is making sure the lawn itself is healthy and competitive. Fertilize and water the lawn properly and consider using techniques like overseeding with a desired turfgrass species if needed. Using mechanical or chemical control efforts will be much more successful if Canada thistle must compete with healthy grass. See the MSU Extension publication Maintaining Successful Lawns in Montana (850KB PDF) for more information.
If there are only a few plants, hand-pulling or digging Canada thistle can be useful. During the growing season, pulling or digging will need to be repeated about every two or three weeks or whenever re-sprouting stems are visible. Dispose of plants in the trash as Canada thistle can reproduce by root fragments.
Photo by Noelle Orloff.
Herbicides labeled for use in lawn and ornamental settings are available for controlling Canada thistle. Repeated applications over multiple years will be necessary. For best results, apply herbicides when plants are actively growing, usually in spring to early summer or fall. Herbicides that contain the active ingredients (a.i.) 2,4-D, dicamba, MCPA, and fluroxypyr or triclopyr can be effective when applied at the right time. Examples of trade names for lawn and ornamental herbicides include Foundation™, Escalade 2®; and SpeedZone®. Make sure to follow all label instructions and take precautions to avoid injuring desired plants in the landscape. For example, do not apply these herbicides too close to trees or ornamental flowers. To limit the risk of injury to desired plants due to exposure via drift, do not use the herbicides mentioned above on windy days or if daytime temperatures rise above 80 degrees. Some active ingredients like dicamba can volatilize (change from liquid to vapor) when applied during high temperatures. Spraying when cooler will not only increase efficacy of the herbicide on Canada thistle, but it will limit risk of injury to non-target plants due to drift of volatilized active ingredient.
This month's Weed Post is also available as a Printable PDF (780KB).
Disclaimer: Herbicide recommendations are provided only as a guide. It is always the pesticide applicator’s responsibility, by law, to read and follow all current label directions for the specific pesticide being used. Due to constantly changing labels and product registration, some of the recommendations given in this writing may no longer be legal by the time you read them. If any information in these recommendations disagrees with the label, the recommendation must be disregarded. No endorsement is intended for products mentioned. The authors and Montana State University assume no liability resulting from the use of these recommendations.