No matter what herbicide is used, Canada thistle cannot be controlled with one herbicide application. Seedlings may emerge in treated areas for up to 20 years. Once you have controlled the perennial plant you must be on the lookout for seedlings. The two most effective times for herbicide application to Canada thistle are at the bud stage and during regrowth in the fall. Table I. Results from Canada thistle control studies conducted in western Nebraska.
Persistence is the key to a successful Canada thistle control program.
You must use multiple herbicide treatments to achieve control. Choose control measures that can be used for at least two to three successive years. Thoroughly plan your control program in advance and adhere to it until control is complete. Apply atrazine preemergence to corn growth, follow up with Banvel or Stinger postemergence before corn is 24 inches tall. Treat Canada thistle again in the fall of the year with Banvel, Roundup or Stinger.