Iowa State University

Winter Annuals:  Spray Now or At Planting?
by Bob Hartzler

April 2, 2004 - Winter annuals are an increasing problem in southern and central Iowa no-till fields.  While winter annuals rarely compete with corn or soybeans, they can hinder planting operations by reducing soil warming and drying in wet springs.  Thick masses of winter annuals also may cause problems with accurate seed placement and uniform stand establishment.

Winter annuals generally are fairly easy to control while in the vegetative stage of growth.  However, once they enter the reproductive stage of growth they become relatively tolerant of traditional burndown herbicides.  Winter annuals sprayed after flowering has initiated usually respond to the herbicide very slowly and therefore continue to interfere with planting and crop development for a period after application.  The current weather forecast suggests we should have a week of favorable weather for treating fields infested with winter annuals.  The risk of winter annuals interfering with planting operations can be minimized by spraying fields early prior to the weeds entering reproductive stages.  In most situations only fields with moderate to heavy infestations of winter annuals will benefit from an early burndown treatments.  There usually is little benefit to early burndown treatments in fields with less than one winter annual per square yard.  The relative effectiveness of several herbicide treatments on winter annuals and dandelion is described in Table 1.

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