Watch for Early Flush of Winter Annual Weeds

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Marestail from top view

When April-like temperatures hit in February, you are not alone in seeing some green weeds in your fields. Coupled with a lack of snow cover, these above-average temperatures in many regions could jump-start early winter annual weed growth.

That makes March critical to reexamine your 2024 herbicide plans and watch for potential insects hosted by these weeds that could harm emerging corn.

“Farmers often have a good handle on their winter annual weeds, especially in my area [central Iowa] where fall tillage is common,” says Meaghan Anderson, Iowa State University field agronomist.

Occasionally, farmers have a problem field with a vast weed seedbank that can look like a cover crop of winter annuals come spring. “Pay attention to emerging and existing common chickweed, henbit, field pennycress, marestail (possibly glyphosate- or ALS-resistant), purple deadnettle and other winter annual weeds,” Anderson advises.

Winter annuals germinate in early fall, overwinter, and then flower and produce seeds in mid- to late  spring. Fall control is most effective on younger plants if attention is paid to possible herbicide resistance (marestail, for example). Spring control of winter annuals becomes more challenging once plants mature and begin to bolt (stem elongation) beyond the easier-to-control rosette stage.

Start scouting

Anderson recommends boots on the ground now to halt yield loss in fields targeted for corn planting. “Late-winter and early spring weather will determine whether winter annual weeds get a jump on us,” she explains. “If we don’t see any arctic air blasts on these snowless fields, these winter annuals will have a head start going into spring.”

An earlier start could have detrimental downstream effects. Weedy fields attract migrating spring insects like seedcorn maggots, black cutworms and green armyworms. Even soybean cyst nematode (SCN) populations can ramp up when purple deadnettle, henbit and field pennycress winter annuals serve as hosts.

Several universities, like Iowa State University’s Integrated Pest Management (IPM) team, offer a moth-trapping network to help with this effort and keep farmers informed when potential pest injury could occur. The North Central IPM Center distributes a monthly newsletter that contains valuable information.

Spring control options

Given a milder winter and the likelihood of early winter annual growth, control with herbicides or tillage will be challenging due to bolt-stage weeds. Combine that with early emerging summer annual weeds like marestail and giant ragweed — both with widespread herbicide resistance — and weed control decisions become critical.

“It’s a tough and delicate balance to control both winter and summer annual weeds due to their different emergence timing and size in the spring, so paying attention to herbicide labels is critical,” Anderson says.

“By planting time, winter annuals are often at the end of their growing season, but mature populations can contribute seed to worsen problems in future years.”

For problem winter annual weed fields, spring tillage may offer some help but won’t kill larger weeds with big roots. Moving your burndown herbicide application to early March in the Corn Belt comes with risk due to erratic spring weather and a shortened residual control window.

“For effective herbicide control, the rule of thumb is low temperatures of 40 F and above, preferably for several days before spraying,” she says. “Make sure temperatures won’t drop down to freezing after application or control success may decline.”

Active scouting is critical to identify problem weeds and growth stage. Even small weeds ahead of corn planting can steal valuable moisture and nutrients the crop needs.

Get more tips for managing winter annual weeds in this podcast segment with Brad Burkhart, market development specialist, Corteva Agriscience.

Cover Crop Benefits

Anderson quickly points out how farmers adopting cover crops see significant benefits beyond soil health, erosion control and nutrient recycling.

“Cover crops are absolutely helping with weed control, particularly with marestail,” she says. “There is a lot of good research, and farmers are seeing visual control as weeds are crowded out, depending on fall seeding success. Spring cover crop competition reduces winter annuals and even spring germinating species.”

The best bet ahead of spring corn planting is to scout now to understand your specific weed populations by field. Discuss herbicide options on a case-by-case basis to reduce weed competition and stop increasing weed seed populations for future years, Anderson adds.

Content provided by DTN/The Progressive Farmer