Written collaboratively by Eric Jones, Philip Rozeboom, Jill Alms, and David Vos.
Preemergence herbicide needs to be moved within the soil profile where the weed seeds are germinating, as these herbicides kill the germinating seedling, not cease germination. Most preemergence herbicides require 0.5 to 1 inch of rainfall to move the herbicide through the soil profile within several days after application; this is commonly referred to as activating rainfall. If a preemergence herbicide does not receive an activating rainfall, many weeds will emerge, as the herbicide remains on the soil surface. The South Dakota Mesonet predicts dry conditions for the foreseeable future. Therefore, recently applied preemergence herbicides may not optimally control weeds. If an activating rainfall is not realized, the timing for the postemergence herbicide application may need to occur earlier than anticipated. Not all is lost when a timely, activating rainfall does not occur within several days after application; the original preemergence herbicide application can be “activated” with later rainfall to manage later germinating weed seeds.
If preemergence herbicides have not been applied yet, there are options to hedge against dry conditions. If you are in under-tilled conditions, the herbicide can be incorporated via tillage to mimic an activating rainfall. Similarly, if an irrigation pivot/reel is present, irrigation can be used to mimic activating rainfall Use the full rate of herbicide, as there will be more herbicide available to come in contact with germinating weed seedlings. Mixing herbicides with different chemical characteristics can help combat dry conditions as well. Some herbicides are more water soluble (require less activating rain) than others. Many premixed herbicides often have both a water soluble and less water-soluble (require more activating rainfall; will not leach as readily through the soil profile after more than 1 inch of rainfall) herbicide to hedge against environmental conditions.
While weather conditions are never certain, applying a preemergence herbicide is still an asset for a weed management plan.
