Recent dry weather raises concern about effectiveness of preemergent herbicides. Preemergent herbicides applied on the soil surface need to be moved into the soil where the target weed seeds are germinating (normally 1-2 inches deep) in order to be effective. The process normally requires 0.5 to 0.75 inch of water within a few days of herbicide application. For example, product labels state that Curbit 3EC® needs 0.5 inch of water within 5 days of application, and Dual Magnum® needs at least 0.5 inch water within 10 days of application. In nonirrigated areas where rain is not anticipated, a shallow cultivation is needed to move the herbicide into the zone of weed seed germination.
Most areas of Indiana were dry the last week of May and first week of June. If preemergent herbicides were not incorporated in a timely manner by water or machine, unsatisfactory weed control might be observed. In that case, cultivation and/or a postemergence herbicide application is needed to control emerged weeds. It is important to keep in mind that the weeds need to be controlled before they cause yield loss, depending on crop and weed density this may be 3 or 4 weeks after planting.