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Time to take out Canada thistle

Here’s a question you might find on an agricultural version of “Are you smarter than a fifth grader?” If you could only spray Canada thistle during one season of the year, which would it be?

If you said “fall,” you’re extremely smart. Two Indiana Certified Crop Advisers provide advice on how to put together an attack plan against Canada thistle. Early fall is a good time because thistles regrow and begin to pull substances down into their roots. Your goal is to make one of those substances a herbicide.

Key Points

• Catch thistles in the early fall while they’re actively growing and not damaged.

• Pick herbicides that translocate into the roots.

• It’s unrealistic to think one fall application will knock out whole patches.


“The first step if you have Canada thistle is to make sure it’s part of your plan for fall weed control,” says Steve Gauck, with Beck’s Hybrids, Westport. “I would use a quart of 2,4-D and glyphosate to control plants that emerge in the fall.”

Gauck believes in spraying early, while thistles are relatively small. Good growing conditions make it more likely that you’ll take out at least part of the patch. The second “must” for fall application on Canada thistles is good coverage. Make sure you’re using tips and pressures that allow you to thoroughly cover plants.

Willis Smith, with Senesacs, Otterbein, believes the key is to spray a herbicide that translocates into the roots. The most commonly used systemic herbicides, he notes, are glyphosate and Stinger. The latter is expensive. However, if you’re primarily spraying patches, you might get the economics to pencil out.

If he’s going after Canada thistle in the fall, Smith also prefers to hit it when it’s actively growing and about 8 inches tall. The application will likely be more effective if the thistles don’t have damaged leaves. He also stresses it’s important not to disturb the patch for a couple of weeks after application.

Be realistic

Both CCAs have strong opinions about taking the right approach to fighting Canada thistle. “Thistles also germinate in the spring, so a total fall program won’t control them,” Gauck says. “You’ll also need to plan a program for spring control while the thistles are small.”

The least-effective time to spray thistles is once they form seed heads in late spring and early summer. They simply aren’t receptive to pulling herbicide down into the roots at that time.

Smith is even more blunt about your odds of controlling thistles. “Unless you’re serious about Canada thistle control, you will dismiss my comments as making absolutely no economic sense,” he explains. “There’s no reason for a halfway approach toward control of thistles. It’s a waste of time, effort and money.

“Just burning off top growth of thistles does nothing for long-term control. Using tillage only causes thistles to spread as you drag plants through the field.

“It’s very unrealistic to think that you will control Canada thistles in one fall application,” Smith continues. “Consider a spring application at early bud stage, along with fall applications, for successive years. Is it expensive? Yes, but how much have thistles cost you over the years?”

This article published in the September, 2010 edition of INDIANA PRAIRIE FARMER.

All rights reserved. Copyright Farm Progress Cos. 2010.