Fall Weed Control

It’s been quite the year across the Prairies with growers facing challenging conditions right through until, and during, harvest. Despite that, there are still farm tasks to be completed this fall if the weather cooperates. This time of year is generally regarded as the best time to gain a foothold of control over winter annual weeds like narrow-leaved hawk’s beard, shepherd’s purse, cleavers and stinkweed, as well as problem perennials. These weeds can get a head start in the spring and the normal pre-seed burn-off will not have much impact.

Canada thistle and quack grass are also best tackled in the fall, but pre-harvest is usually preferred as they will be actively growing. However, this year that was not always an option, depending on local conditions. “For perennial weeds like Canada thistle and foxtail barley to be controlled post-harvest with glyphosate, they must be actively growing,” says Brent Flaten, integrated pest management specialist with Saskatchewan Agriculture. “Wait for three to five leaves to be present so there is enough foliage to capture sufficient glyphosate to kill the plant.” Some perennial weeds have massive root systems that, if they do not receive enough active ingredient, will grow into a continuing problem in the following season.

“After harvesting crops like lentils, peas, and chickpeas – some of which were big acre crops this year – there likely won’t be enough weed foliage left to spray,” says Flaten. “Those crops require the grower to shave pretty close to the ground when harvesting. It may be a different story after canola or wheat, where the stubble is higher and more weed foliage is present.”

The same herbicide operation may not get both the winter annuals and the perennials – winter annuals can easily germinate long after harvest, even after a fall frost. By scouting diligently and having field records to fall back on, a decision can be made to determine the bigger or more immediate problem.

Some growers have had to desiccate the crop in order to force dry down and facilitate harvest. “If perennials are your problem weed issue, using a desiccant is not going to move the needle very far when it comes to control. Desiccants generally do not translocate to the roots to finish the job. Glyphosate, which isn’t a crop desiccant, will translocate into root systems of perennial weeds under favourable conditions but should only be applied after the crop is physiologically mature. “These are just a few of the many factors that come into play when deciding on pre-harvest and/or post-harvest weed control.”

If growers are unsure as to what is the best course of action, contact your local agriculture representative or agronomist to discuss your particular situation and come up with a weed control strategy.