Iowa State University

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Scout for winter annuals now   
Bob Hartzler

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October 03, 2007Winter annual weeds have steadily increased in the past decade due to expanding no-till acres. In the majority of Iowa no-till fields winter annuals do not generate sufficient foliage by planting time to interfere with planting or early-season crop growth. Because of this, our recommendation generally is to control winter annuals with at-planting burndown treatments. However, some fields may have infestations that can interfere with planting or early-season crop growth. The risk of winter annuals interfering with crop production increases in dry springs when the weeds deplete soil moisture reserves, or wet springs when planting is delayed allowing excessive growth of the winter annuals. Keep in mind that the establishment of winter annuals last summer and fall was reduced by dry conditions in many areas of the state. Thus, the only way to know whether an early herbicide application is warranted is to get out and walk the fields to determine the presence of winter annuals. Unless significant areas of field have densities of more than 3-5 weeds per square foot there probably is no need for an early treatment. In most fields winter annuals are present in patches, complicating the decision whether to make a special trip specifically for these weeds.

The images at the right illustrate the growth of field pennycress in a southern Iowa field. By April 15 the plant was flowering and providing complete ground cover. At this stage of development the plant is difficult to kill with herbicides and the foliage slow to decompose. Field pennycress is one of the more aggressive winter annuals; other species such as buttercup or shepherd's purse are less likely to interfere with early-season crop growth. Decisions on the need to control winter annuals early should be based on: 1) density of weeds, 2) percentage of field infested, and 3) competitiveness of the species.

Most people are not as adept at ID'ing winter annual weeds as they are summer annuals. An Extension booklet is available from the University of Missouri that focuses specifically on common winter annual and other weedy species that are found in crop fields prior to planting:
Early spring weeds of no-till crop production. NCR 614.

It can be obtained in a hard copy ($3) or as a pdf file.




Prepared by Bob Hartzler, extension weed management specialist, Department of Agronomy, Iowa State University

For more information contact:
ISU Extension Agronomy
2104 Agronomy Hall
Ames, Iowa 50011-1010
Voice: (515) 294-1923
Fax: (515) 294-9985
Submit questions or comments here.  

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Common chemical and trade names are used in this publication. The use of trade names is for clarity by the reader. Inclusion of a trade name does not imply endorsement of that particular brand of herbicide and exclusion does not imply nonapproval.