Drought following the application of preemergence herbicides can result in failure, since all preemergence herbicides require some moisture for activation. A worst-case scenario for preemergence herbicides occurs when sufficient moisture prior to application allows weeds to germinate, but lack of rainfall after application prevents the herbicide from controlling the weeds. The reverse situation - excessive rainfall, is much less a problem since the solubility of most preemergence herbicides is extremely low. Most remain in a very thin layer on top of the soil, moving only a fraction of an inch from the surface. Preemergence herbicides - temperature .
Temperature has a critical effect on a few herbicides.
Casoron and Kerb are examples of herbicides which perform better in the cool season and won't last long into the summer unless incorporated into the soil. Similarly, volatilization and loss of Treflan increases with warm temperatures. Very high temperatures conceivably may reduce the longevity of all preemergence herbicides because higher temperatures speed up the biological and physical processes that break down herbicides in the soil. Longevity of herbicides in the soil is also affected by physical or chemical characteristics of the herbicide and its application rate. For example, Factor and Barricade have a long persistence because of the very low solubility of their active ingredient.