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Frosty weeds and closing spray windows

Variables like weed species and recent and expected frosts will weigh into management decisions

Frosty weeds and closing spray windows

Add the weather forecast into your calculations when planning fall weed control.

It’s prime weed control season for perennial weeds like Canada thistle and winter annuals and acting now can help you, come spring.

“A lot of the winter annuals are starting to become problems because if we don’t do any kind of control in the fall, they get going early in the spring,” Lionel Kaskiw, a provincial farm production extension specialist said during a Sept. 23 CropTalk webinar.

By the next spring they’re frequently too big to kill, or if they can be controlled at all, are too expensive to control, he said, noting that makes September and October the best time to control winter annuals.

It’s the perfect time to control perennials too, as those plants are taking their cue from the colder weather and are storing sugars for the coming season, giving herbicides the perfect mechanism to hitch a ride down to the roots.

At the same time, Kaskiw cautioned, producers will need to carefully match their spray windows to the active growth of the weeds.

Why it matters: Both winter annuals and perennials are more easily controlled in fall, but producers may want to check the thermometer and the forecast to get the best spray window.

Kaskiw cautioned against spraying too early in the morning, when plants are still recovering from the overnight lows, or too late in the day, when the active ingredients in the herbicide have less time to move through the plant.

Timing will also play a role in post-harvest perennial control, he noted. Canada thistle, one of the major perennials producers look to control this time of year, is best sprayed with three or four new leaves.

“Even if you see the plants that look kind of funny, if you were straight cutting or swathing and the plant will look like it has a six-inch stem but then it’s throwing out new leaves, those leaves would still take up chemical,” he said.

Quackgrass control, likewise, is most effective when growth reaches six to eight inches; dandelion is best targeted when rosettes are six inches in diameter and foxtail barley should be showing new growth.

More producers have noted issues with foxtail barley in the last several years, Kaskiw noted.

Getting frosty

On the other end of the temperature spectrum, nature might be providing its own weed control if overnight temperatures plunge.

Kaskiw urged farmers to take stock of frost damage when making spraying decisions. Spray will do little to control a plant unless around 60 per cent of leaves are still green and growing, Kaskiw noted.

If there is active growth and 50-60 per cent of the plant is still green after a light frost, producers can likely still spray the next day, assuming temperatures rise past 10 C. Ideally, there would also be no frost in the forecast for several days following.

“If you can, try not to be spraying on a day where they’re calling again for a frost that night, because it really limits the amount of time the product can be moved through the plant,” Kaskiw said. “If you can, giving the plants an extra 24 hours after frost will be good, because they just get the opportunity to start regrowing again.”

Once temperatures dip below -4 C for hours at a time, however, producers are advised to wait several days to assess weed survival.

“Some producers have told me that sometimes when we get this frost and then get the plant growing again, they get even better kill, so just because we get a killing frost doesn’t mean that we’re done spraying for the fall,” Kaskiw said. “Keep going back out to those fields and monitoring those fields and see what kind of growth you’re getting back.”

Of the winter annuals, something like narrow-leaved hawk’s beard or stinkweed will have the most tolerance for the cold, according to the province, while something like cleavers or night-flowering catchfly are more likely to fall prey to frost.


Product rates may also play into how successful fall weed control is.

Weeds hardened from frost may take a heavier application, Kaskiw noted, while products like ammonium sulphate may increase herbicide uptake under cold conditions.

“Get out there; see what’s happening,” he said. “We’re getting some different weather events going through. We’ve had some frost. We’ve had some rainfall. We’re getting a lot of growth happening out there, so go out there, see what you’ve got for weeds, whether they be annuals, perennials or winter annuals.”

Annual weeds, much of which may be volunteer crop from seeds out the back of the combine, will often not require treatment, given the short growth window before a killing frost, he noted.

In some cases, such as volunteer canola, it is better actually for those seeds to germinate now, with winterkill around the corner, than have those seeds overwinter and become an issue next year, he added.

About the author


Alexis Stockford

Alexis Stockford is a journalist and photographer with the Manitoba Co-operator. She previously reported with the Morden Times and was news editor of  campus newspaper, The Omega, at Thompson Rivers University in Kamloops, BC. She grew up on a mixed farm near Miami, Man.



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