Start Controlling Next Year's Weeds with Cover Crops

Your opportunity to start on next year's weed management program is here - look around your farm for land not producing crops right now. Early vegetables like potatoes, cucumbers, sweet corn and snap beans have been harvested recently, and many wheat fields will soon be harvested. Other fields were too wet to seed this year. And some strawberry fields have been taken out of production after several years of cropping.

This land would benefit from growing a cover crop before the next season. Most growers are aware of the benefits from cover crops: adding organic matter, protecting the soil from further water and wind erosion, and tying up leftover fertility for later release for crops. But have you thought about cover crops as the starting point in your weed management program for next year?

Cover crops tend to squeeze out germinating weeds by quickly covering the soil, out-competing small weeds and preventing further weed germination. Grassy cover crops can be mowed after they reach 12" to 18" in height, further reducing weeds by cutting them off. Mowing also encourages tillering of the cover crop, which thickens up the cover and prevents more weed germination.

In some cases, cover crops can be sprayed with herbicides to control weeds. Grassy cover crops can be sprayed with hormone herbicides like 2,4-D to control broadleaf weeds, while broadleaf crops like vetch or buckwheat can be sprayed with a grass herbicide if needed. Take extra care with hormone herbicides to prevent vapour drift. Avoid hot, humid weather, and be sure that winds are blowing away from susceptible plants like tomatoes and soybeans, as well as greenhouses and vineyards, when you apply this spray.

Cover crops can open a window to control perennial weeds. Short season summer cover crops, like oats or marigolds, could be killed with glyphosate in early fall. This targets an excellent time to control quackgrass or thistle regrowth because weeds will be actively growing, ready to absorb the herbicide. At that point, a winter cover crop like rye or wheat could still be sown to protect your soil over the winter.

Cover crops can be your first step in reducing winter annual weeds. No-till growers are learning about the challenge of winter annuals, while these weeds have plagued fruit growers for years. Winter annuals are weeds like Canada fleabane, peppergrass, and buttercups that germinate in early fall, and form a rosette. Next spring they will be difficult to control with herbicides, as they flower in early summer, and shed seeds for next year. Cover crops help suppress germination of winter annuals, as well as offering a timing for fall herbicides to control them.

Canada fleabane

Canada fleabane rosettes can be controlled by fall herbicides on grassy cover crops.

Some cover crops can actually control weeds, as well as pests like nematodes, by the chemicals that they produce. We are testing crops like marigolds, pearl millet, sorghum as well as Brassicas like Indian or white mustard. Researchers are working on the challenges on growing these crops, but wouldn't it be great to reduce weeds like hairy nightshade by growing a cover crop the summer before?

But for now, there are still lots of opportunities with traditional cover crops to get a head start on next year's weeds, in addition to all the other benefits of cover crops. Plan now to give next year's crops a head start.

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