Now’s the time to attack winter annual weeds

Many winter annuals are often too well established or already flowering once spring arrives

Conditions are favourable this year for some excellent post-harvest weed control, says a provincial crop specialist.

Harry Brook

Harry Brook
photo: Supplied

“Winter annuals are weeds that germinate in the fall or late fall, go through the winter in a rosette form, and go to seed quickly once spring comes,” said Harry Brook.

Common winter annuals include stinkweed, shepherd’s purse, scentless chamomile, narrow-leaved hawk’s beard, bluebur, stork’s-bill, ball mustard, peppergrass, downy brome, dog mustard, wormseed mustard, chickweed, flixweed, knawel, night-flowering catchfly, and common groundsel.

“They form a few leaves in the fall, and overwinter in that state,” said Brook. “These plants develop their own anti-freeze, preventing them from dying. It gives the plants an advantage the following spring as it will send up a seed stalk and go to seed before most other plants get started.”

Winter annuals also deplete soil moisture and nutrients in the fall and spring. A spring herbicide application is too often late as the plants are well established and may already be going to flower or seed.

Tillage is an effective control measure, but conservation and zero tillage, a late-fall application of herbicide is the best route. This should be done from late September to mid- or late October, depending on the fall and the problem weeds.

“A successful fall weed control program requires the right conditions,” said Brook. “Weed control, even after a frost, can still be very effective as long as the weeds have some green, actively growing plant material.”

Winter annuals are able to continue growing, even after the first frost, until the ground freezes, he added.

Most winter annuals can be controlled in the spring, except for narrow-leaved hawk’s beard, but control after they bolt is a lot more expensive and less effective.”

Herbicide options are very economical in the fall. Chemicals such as 2,4-D and MCPA provide good control and, at recommended rates, will be safe for most crops the subsequent spring, said Brook.

“It is important to know the problem winter annuals you have so you can pick the best herbicide for it. Glyphosate works well in mixtures, on many winter annuals but it may not be the best one depending on the weed.”

Other common herbicides used for winter annuals are dicamba, tribenuron-methyl and bromoxynil.

Problem perennial weeds such as Canada thistle, quackgrass, dandelion, and sow thistle are best controlled by a fall application of herbicide. Once again, the plants need some green leaf material and be actively growing. Dandelion seedlings are easy to control in the fall but, after overwintering, they become almost bulletproof.

Because herbicides such as 2,4-D, MCPA, and Dicamba have some residual effect, be careful with the following spring’s crop, said Brook.

“There will be little breakdown of the chemical over winter and there might be some carry-over effects on the succeeding crop. When using dicamba, tribenuron-methyl, 2,4-D or MCPA you might want to do a bioassay prior to seeding any non-cereal crop. A bioassay is simply taking some of the soil and trying to grow plants in it prior to actual seeding. If the plants die, don’t seed.”

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