Chemical weed control can be significantly affected by dry weather. Greatest effects are observed on preemergence and postemergence herbicides. Sunlight degrades preemergence herbicides on the soil surface, and if rainfall or irrigation does not follow within seven to ten days after application, poor weed control often results. Even for highly persistent herbicides, failure to move the compound into the soil due to the lack of rainfall allows weeds to germinate just after planting. With subsequent rainfall, these persistent compounds usually provide residual weed control of later-germinating weeds.
Shallow incorporation can provide some weatherproofing of preemergence herbicides.
In other words, incorporation with implements such as a rotary hoe or shallow-rolling cultivator can substitute partially for rain to activate herbicides. The key is limiting depths of soil disturbance to the upper inch of the seedbed to avoid damage to the planted crop. To a lesser extent, dry conditions also influence the performance of soil-incorporated herbicides. Uniform herbicide incorporation in excessively dry soils is difficult to achieve. In extreme dry soils, some herbicides are tightly bound to soil particles and become unavailable for weed uptake.