Practical Tips for Effectively Managing Canada Thistle in Natural Areas

by Dennis Pederson, Habitat Forever specialist 

See "Pheasants Forever–Conserving Wildlife Through Habitat Protection and Restoration" 

  • Mowing or fire alone will not control Canada thistle; Use fire and/or mowing in conjunction with herbicides. Spring fire and spring mowing reduce vegetative cover allowing better herbicide coverage on thistle. Fire and mowing also add an additional stress to Canada thistle. It is important to delay herbicide application until all Canada thistle have emerged following fire. Wait at least two weeks after mowing to apply herbicides. 
  • Control the root system. About 95 percent of thistle biomass is underground, so the root system has to be killed to effectively control Canada thistle.
  • Use the most effective herbicides at labeled rates. Milestone® at 5 to 7 fl oz/A or Transline® at 10 to 12 fluid ounces per acre (fl oz/A) translocate into the root system giving the best control. Milestone is more effective on Canada thistle and less costly than Transline, but Transline can be more selective and can be used to control thistle in areas where there is desirable woody vegetation. Milestone and Transline can be applied in either spring or fall; however, Milestone is more effective in the fall. 
  • Spring/early summer herbicide applications should be made when thistle is fully emerged, and when the largest plants are at early bud growth stage. Mowing or fire prior to application will allow for more consistent Canada thistle growth, but be sure plants are at rosette to bolt growth stage prior to herbicide application.
  • Fall applied herbicides. Thistle can tolerate about 25 F so applications can be made from September until early- to mid-October as long as green growth remains on thistle. 
  • Use broadcast herbicide applications. Unless thistle patches are completely defined and well documented, spot spraying is usually self-defeating and a false economy. To ensure the greatest chance for thorough and complete application, the whole field should be treated.
  • Control infestations on field edges. If field edges, ditch banks, wetland edges and fence lines are not treated along with the main grassland body, thistle colonies will maintain a foothold in those locations and reestablish quickly.

 ®™Trademark of The Dow Chemical Company (“Dow”) or an affiliated company of Dow. Milestone is not registered for sale or use in all states. Contact your state pesticide regulatory agency to determine if a product is registered for sale or use in your state. Label precautions apply to forage treated with Milestone and to manure from animals that have consumed treated forage within the last three days. Consult the label for full details. State restrictions on the sale and use of Transline and apply. Consult the label before purchase or use for full details.
Always read and follow label instructions. 

Active ingredients for herbicide products mentioned in this article: Milestone (aminopyralid), Transline (clopyralid).