Early spring flowers remind us that winter is nearly over. But it's a painful reminder when those purple blooms indicate a field filled with henbit and purple deadnettle.

“Winter weeds are becoming a major issue. And, it's one of the most challenging weed problems we've dealt with,” says Bill Johnson, University of Missouri (MU) extension weed scientist. “Controlling winter annuals is a complex thing that depends on weed size, time of year, field history, drainage and costs versus benefits.”

While control can prove a quandary, it wasn't a surprise to Johnson that winter annuals became a problem. “It was inevitable that with the popularity of Roundup Ready technology, year after year, the weed population would shift,” he says.

No-till also tends to help winter annuals flourish, according to Mark Loux, Ohio State University (OSU) weed specialist. “Most winter annuals emerge in the fall, and the warm weather in late fall during the past several years has resulted in higher populations. In no-till fields there's nothing to disrupt their emergence,” he says. “However, winter annuals can emerge after an early fall tillage and have been a problem in tilled, as well as no-till, fields.”

Winter annuals cause numerous problems in spring. Their heavy growth prevents soil from drying and warming up. They also can interfere with tillage and crop establishment and the longer-lived weeds, such as marestail, compete with crops through the growing season and interfere with harvest.

Researchers agree that winter annuals are best controlled in fall, although early spring herbicide applications provide considerable control also. “Fall treatments provide more consistent control than spring applications,” says Loux. “Among soybean herbicides we studied, only three provided 80% control when applied in March. But 10 of the herbicides provided 90% control when applied in November.”

There are cost/benefit considerations for when to spray as well, according to Johnson. “The benefit of a fall herbicide application depends greatly on spring weather and drying characteristics of a field. If planting is early, the fall application can replace a spring burndown application. And, fall weed control may get you into the field earlier, particularly on poorly drained soils.”

Not every field needs to be sprayed. “If a field has a long-term problem of heavy winter annual weed pressure or is historically a field that stays wet, it's a candidate for treatment,” he says. “You may not need to spray the entire field, but use those same criteria for deciding what spots in a field should be sprayed.”

Johnson recommends that fall herbicide applications be made when the soil temperature drops just below 50∞. ”It's just the reverse of what we recommend in the spring,” he says. “You want to spray when it's warm enough to get a good kill. But spray after the ground is cool enough to prevent a re-emergence of another crop of weeds.”

The costs for herbicide control can vary from just $2/acre for 2,4-D to $15-18/acre for some of the more expensive treatments, Johnson says.

Find the OSU herbicide recommendation for fall and spring control of winter annuals at http://corn.osu.edu/archive/index.html. Select issue 01-35. For help identifying weeds, check out www.psu.missouri.edu/fishel or order Early Spring Weeds of No-till Crop Production (NCR614) from MU by calling 800-292-0969.