Plan to Attack Winter Weeds

August 27, 2008 11:07 AM
If you're moving into a no-till system, clobbering winter annual weeds this fall will eliminate yield robbers in your next crop.
"The first year or two of no-till, winter annuals aren't so numerous because you've been taking them out with tillage,” says Jim Kinsella, a long-time no-till farmer from Lexington, Ill. "But if you don't control them in no-till, the population could explode within three to five years.”
Weed scientists Mark Loux of Ohio State University (OSU), Bob Hartzler of Iowa State University and Aaron Hager of the University of Illinois have all noticed increased populations of winter annual weeds in recent years. Less fall tillage, they agree, is one of the reasons. Other factors may be warm fall weather, which gives the weeds more time to grow after harvest, and mild winter weather, which helps them survive and continue to grow in the spring.
Winter annuals can also sometimes emerge after early fall tillage, making them a threat even in conventional fields, Loux notes.
"In Illinois, winter annual weeds seem to be moving north,” Kinsella says. "In the 1960s, I noticed that fields would green up after harvest as far north as Pana. Later, the line moved to Decatur, then Bloomington, and now all the way to Interstate 80.” I-80 now seems to mark the northern boundary for fall flushes of winter annuals in
Illinois, Hager agrees.
There are lots of reasons to wage war on winter annuals. "Winter annuals survive the winter with little or no further growth, start growing again in late winter or early spring and flower and go to seed in late spring or early summer,” Loux says. "So, the negative effect of most of them occurs when you are establishing the next crop. Marestail, however, does not flower until late summer, so it competes directly with the growing crop.”
A lush growth of winter annuals keeps soil cool and shady in the spring and slows drying, Kinsella says. In a dry spring, the weeds suck out moisture you may not want to lose. They also tie up nitrogen and other nutrients, slowing early crop growth.
Some winter annuals, such as prickly lettuce, henbit and dandelions, can produce an allelopathic effect on crops, Kinsella adds. The weedy cover attracts rodents and provides egg-laying sites for cutworms and common stalk borers. Chickweed and henbit can harbor diseases and nematodes. Any type of weed growth, however, makes no-till look less appealing to landlords.
"A heavy infestation of chickweed can actually impede seed placement with a no-till planter,” Hager adds.
All in the timing. "Fall is a good time to kill winter annuals,” Hager continues. "The weeds are smaller than they are in the spring, so you get better coverage with herbicides. And in the spring, it may be more difficult to spray the weeds at the ideal time.”
"OSU studies have shown effective control with herbicides applied from mid-October to early December,” Loux says. "But to control dandelion and Canada thistle, herbicides should be applied by early November. In most treatments, we include 2,4-D because it's economical and helps control dandelion, marestail and mustards.”

Carpets of fall-germinating weeds are becoming a more common sight in central and southern parts of the Corn Belt, and control measures are more important than ever.       Photo: University of Illinois

You can email Darrell Smith at

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