Pre-Emergent Herbicide After Heavy Rain: Still Effective?

If you applied pre-emergent herbicide just before a deluge, you're likely fine—unless significant soil erosion occurred. The active ingredient dissolves and binds to soil particles; if the soil stayed put, the chemical remains in place and will work as intended. However, if runoff carried away topsoil, the herbicide may have moved with it, reducing effectiveness. On flat or gently sloping ground with no visible erosion, no reapplication is needed. On steep slopes where soil washed away, consider a second application once conditions stabilize.