Canada thistle is a perennial noxious weed that is widespread in many prairie and grassland areas in the midwestern United States. Field trials show that Milestone® at 5 fluid ounces per acre (fl oz/ac) or Transline® at 10.6 fl oz/ac applied prior to bud stage in June or in fall following a light frost will provide good to excellent Canada thistle control (Table 5.2, Figure 5.3). A temperature of 28⁰F in fall will cause many forbs to become dormant, but Canada thistle remains susceptible to herbicide treatments for a longer period of time, even at a lower temperature.
Although fall application timing minimizes injury to some desirable forbs, it is important for land managers to understand the species of forbs present on the site since some are sensitive to fall application. In general, the benefits of Canada thistle control, removal of other undesirable broadleaf species, and the increase in native grass cover should lead to an overall improvement in the long-term stability and composition of the restored prairie plant community (Almquist and Lym 2010).
Practical Tips for Managing Canada Thistle
by Dennis Pederson, Habitat Forever specialist
Mowing or fire alone will not control Canada thistle. For optimum Canada thistle control, use fire and/or mowing in conjunction with herbicides. Spring fire and spring mowing reduce vegetative cover, which allows for 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 has emerged and plants are at rosette to late bolting growth stage following fire or mowing.
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 fluid ounces per acre (fl oz/ac) or Transline® at 10 to 12 fl oz/ac translocate into the root system giving the best control. Of the two products, Milestone is more effective on Canada thistle. However, Transline is an option for use in areas where there is desirable woody vegetation because it is more selective. Milestone and Transline can be applied in either spring or fall; however, Milestone is more effective than Transline when applied 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 completely emerged and at rosette to bolt growth stage prior to herbicide application.
Fall applied herbicides. Thistle can tolerate temperatures as low as 25⁰F so applications can be made from September until early- to mid-October as long as green growth remains on thistle.
Use broadcast herbicide applications. Thistle patches that are completely defined and well documented can be treated with spot spraying. To ensure the greatest chance for thorough and complete application, the whole field should be treated if good inventory data is not completed.
Control infestations on edges of prairie and natural areas. 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.