Recent comment by a reader of Indiana Prairie Farmer prompted a call to Glen Nice, Purdue University weed control specialist. The reader claimed the magazine erred when it said in the June issue that Canada thistles couldn't be controlled by spraying in June. And the reader even quoted a line from the Purdue weed guide claiming a good time to control thistles is 'late spring.'
This one boils down to semantics, agrees Nice. 'Late spring' was meant to infer April and May, not mid-June. "If you spray a new colony of thistles (first year for that patch) that don't have a fully-developed root system yet, and the weather hasn't turned hot yet, you might get lucky and control them," he says. But if you're up against an established patch of Canada thistle and the weather has already turned up the heat, don't expect control from spray applications in that time frame.
That's why July and August aren't hardly ever good times to try to control Canada thistles. Perhaps the best time of the year is in early fall, once thistles begin putting out new shoots. "They're moving materials down into the roots, getting ready for winter, and so it's a good time," he says. They would also translocated herbicide into the roots, giving you a lot better chance for control.
Perhaps the other semantic here is what 'control' means. If you just want to burn them down so you don't have to look at them or perhaps so they won't go to seed, you can likely accomplish that with high rates, even in June, Nice says. "But that's not what we consider control," he adds. "Chances are very high that the patch will be back there next year. Spraying or mowing thistles that time of year may weaken the patch some, but you're not going to wipe it out."
That's because thistles have a tremendous underground root system. Merrill Ross, now retired, was a Purdue weed control specialist who dedicated more than two decades to studying Canada thistle, johnsongrass and how to control them. He often displayed Canada thistle at field days, grown in a box with a plexiglass side to show their extensive roots.
One shot with herbicide at a time of the year when the plant isn't sending much material down to the roots simply won't damage the root system to any great degree, Nice agrees.