Once harvest is done, many producers start planning for next year and thoughts turn to weed control.
Spring weed control is important but some weeds are best controlled with a late fall herbicide application. Producers can make next spring’s weed control easier using a post harvest herbicide application.
Winter annuals germinate in the fall and are able to continue growing after the first frost, until the ground freezes. These weeds form a few leaves in a bunch, or rosette form, and overwinter in that state.
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The plants develop their own anti-freeze, preventing them from dying, which gives them an advantage the following spring because they can send up a seed stalk and go to seed before most other plants get started.
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.
“Winter annuals deplete soil moisture and nutrients in the fall and spring. They can be very competitive against fall and spring seeded crops,” says Harry Brook, a crop specialist with Alberta Agriculture.
“Under conventional tillage, these weeds were not a big problem. A late fall tillage operation would control them easily. However, with the switch to conservation and zero tillage, these weeds are gaining prominence as serious spring weed problems. Without tillage, other control strategies need to be used, and the most effective is a late fall application of herbicide.”
The best time for a fall herbicide application is from late September to mid or late October, depending on the weather. An application at this time is the most effective because the greatest number of weed seeds are germinating and the plants are small and more susceptible.
“Most winter annuals can also be controlled in the spring, except for narrow-leaved hawk’s beard, but control after bolting is a lot more expensive and less effective,” Brook said.
“Herbicide options are very economical in the fall. Chemicals like 2,4-D and MCPA provide good control and, at recommended rates, will be safe for most crops the following spring.”
It is important to know which problem winter annuals are in a field so the best herbicide is chosen. Glyphosate works well on many winter annuals, but it may not be the best one, depending on the weed. Other common herbicides used for winter annuals include dicamba and bromoxynil.
Winter annuals can also be controlled in the spring, but to get the best control, spraying must be done before the weeds bolt.
While a spring application will control most winter annuals, it will not control hawk’s beard.
Producers who use Group 4 chemicals such as 2,4-D, MCPA and dicamba must be careful with the following spring crop. With little breakdown of the chemical over winter, there might be some carryover effects on the succeeding crop.
Producers who want to seed a noncereal crop after using dicamba, 2,4-D or MCPA may need to conduct a bioassay, which means trying to grow plants in a soil sample before seeding. If the plants die, producers shouldn’t seed.
“With the right weather conditions, a late fall spray can repay you handsomely with reduced weed competition next spring. If the weather’s right it could be worth your time and effort,” Brook said.
