2008-11-13 / Local & State

Scout For Winter Annual Weeds Now

Winter annual weeds have become much more prevalent over the last five years in Pennsylvania. This may be related to less residual herbicide use in soybeans, more mild winters allowing better winter survival, more weed seed infested manure ending back in the field, or other reasons.

These winter annuals include common chickweed, purple deadnettle, henbit, horseweed or marestail, a number of mustards, and some winter annual grasses such as ryegrass and downy brome. As a general rule, winter annuals emerge between late August and early November although depending on the year, some spring emergence can also occur. Winter annuals can be competitive in winter grains like wheat and barley, but also at planting time in no-till corn and soybeans.

In addition, some winter annuals can serve as alternate hosts for soybean cyst nematode (SCN) and can result in increased SCN egg numbers in soil if the weeds are not eliminated early in their lifecycle. Purple deadnettle is one of the more prominent hosts for SCN. Researchers at Ohio State have been studying this interaction and recently published a newsletter article in C.O.R.N. discussing management of winter annuals weeds as it relates to SCN.

Late fall and early spring are the best times to control winter annuals with herbicides. For best activity, apply herbicides when daytime temperatures are above 50 F and night time temperatures are above 40 F for several days during application time. In fallow fields, a combination of gylphosate plus 2,4-D ester is fairly effective for control of most winter annual weeds. Delaying application until corn or soybean planting time can reduce the effectiveness of the treatment plus allow weed seed production, which perpetuates the problem. Also, glyphosphate resistant and tolerant weeds like marestail should be targeted early in their lifecycle. Excerpted from Field Crop News: Bill Curran, weed specialist, Penn State University If you have any questions contact the Fulton County Extension office 717-485-4111.

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