Water is Needed to “Activate” Soil-Applied Herbicides

Mark VanGessel, Extension Weed Specialist; mjv@udel.edu

Herbicides applied to the soil surface require rainfall or irrigation to move them into the soil where the plants will absorb them; or to be mechanically incorporated (field cultivator). Some areas have not received much rainfall since herbicides were applied. Weed control will start to decline if water is not received within 5 to 7 days after applications. Some products, like those that contain atrazine, mesotrione (Callisto), or isoxaflutole (Balance) may be taken up by the roots and provide some control of seedlings after they have emerged. However, Dual, Harness, and Zidua are absorbed by emerging shoots, so once weeds have emerged these products will not provide control. If you have irrigation and your corn herbicides have been applied but you have not received water, you should consider irrigating to activate those herbicides. Another caveat is that early-season competition from grass can reduce yield.