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Fall is a great time to control perennial weeds

Winter annuals are also ripe for cleanup at this time of year

It’s time to control winter annual, biennial and perennial weeds.

“The perennials are going to start moving things down to the roots (including weed-killing herbicides),” Manitoba Agriculture’s weed specialist Jeanette Gaultier said in an interview Sept. 14. “So it is the perfect time now to be thinking about perennial weed control.”

And there are no shortage of weeds. During a provincial weed survey earlier this season Gaultier saw lots of perennial weeds in farmers’ fields, including dandelions, Canada thistle and perennial sow thistle.

“My recommendation is to go in with a Group 4 (2,4-D, dicamba, MCPA) or glyphosate or making a combination of the two, which is the best way to tackle some of those problems for sure,” she said.

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But before spraying farmers need to consider what crop they will plant next year and whether the herbicide they want to spray this fall will leave a residue that can harm that crop. For example, higher rates of dicamba applied in the fall can injure oilseed crops, including canola, flax, sunflowers and mustard planted next spring.

“Typically at lower rates of our Group 4s residuals shouldn’t be an issue before soybeans, but if guys are uncertain they should talk to their chem rep,” Gaultier said.

Lower rates of dicamba tank mixed with glyphosate isn’t usually a concern, but some pre-mixes, can have higher rates of dicamba, which can leave a harmful residue.

Lionel Kaskiw, Manitoba Agriculture’s farm production adviser at Souris, says he has seen dicamba residues injure sunflowers the following spring.

“There are other products you can maybe look at or go with a higher rate of glyphosate,” he said during a webinar Sept. 14. “Glyphosate isn’t overly expensive so a higher rate would probably do a fairly good job.”

Group 2 herbicides can be an option, but can have residue issues with oilseeds in the spring, Gaultier said.

Frost effect

Research shows a fall-applied herbicide works best after a light frost, Gaultier said. But waiting for that is a gamble because it could be a killing frost instead, shutting down plant growth and rendering herbicides ineffective.

“Going in a little bit early makes sense seeing that we have had some light frost and cooler temperatures (Sept. 13 and 14),” she said. “I think now is the time to start.”

After frost assess the damage to weeds.

“You are looking for probably 50 to 60 per cent of the (weed) material still being green and undamaged. You definitely want to wait a few days.”

Watch the forecast too.

“When we start getting those cooler nights and days make sure that we do have a day or a couple of days when we are getting daytime temperatures of 8° to 10° C for at least a couple of hours otherwise it is harder to get the (herbicide) efficacy that one might expect,” Gaultier said.

Winter annuals

There are several winter annuals to watch for, including narrow-leaved hawk’s-beard.

“If that is a problem, farmers want to steer clear of using something with 2,4-D because it is not as effective, or you need really high rates and that can affect your recropping,” Gaultier said. “Going in with one of the other Group 4s or a Group 2 combined with glyphosate is probably a better pick. I also like the Group 2 products. The Group 4s and Group 2s all seem to have activity on stinkweed, shepherd’s purse and flixweed. But I really like the Group 2 combinations often for folks who have issues with some of the pink family of weeds — white cockle (a perennial) and night-flowering catchfly (an annual or winter annual).

“Last year we had such a mild winter and a lot of chickweed overwintered as well. That’s another one Group 2s will have activity on.”


Cleavers is hard to control, especially in canola. There is an effective herbicide registered for control but grain companies won’t buy canola sprayed with it because China — Canada’s biggest foreign buyer — hasn’t approved it.

“It definitely seems to be moving eastwards across the province,” Gaultier said. “It has been an issue along the Saskatchewan border, especially in the northwest.”

Glyphosate will control small cleavers.

Gaultier said trials have shown BASF’s Distinct herbicide, which contains Group 4 and 19 products, has had “good success” on cleavers.

Kochia is another weed farmers may find in their fields this fall. If farmers suspect they have herbicide-resistant kochia they can submit samples to the Pest Surveillance Initiative Laboratory for testing.

Farmers also need to let weeds regrow if they were cut during the harvesting process. Look for four to six leaves.

“The spray has to be intercepted by something,” Gaultier said. “With thistles try to make sure you have at least six leaves so the product is getting in there.”

Old is new

Old soil-applied products Avadex and Fortress are regaining popularity, Gaultier said. Avadex (triallate) is in Group 8 and Fortress (triallate and trifluralin) contains Groups 8 and 3. Neither group is used much making them a good fit for fighting herbicide-resistant weeds.

About the author


Allan Dawson

Allan Dawson is a reporter with the Manitoba Co-operator based near Miami, Man. Covering agriculture since 1980, Dawson has spent most of his career with the Co-operator except for several years with Farmers’ Independent Weekly and before that a Morden-Winkler area radio station.



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