Preemergence Herbicides Preemergence herbicides are applied to the soil surface and must be activated by rainfall. They must be applied to a clean soil before the weeds germinate or be applied with a postemergence herbicide that kills existing weeds. When the preemergence herbicides are activated by rainfall, they are taken up from the soil and kill weed seeds as they germinate. Preemergence herbicide rates are based on soil type. Clay soils or soils high in organic matter require more herbicide to control weeds than sandy soils.
Labels usually specify the appropriate rate for each soil type.
Do not exceed the rate specified on the label or trees may be injured. Some herbicides cannot be used on sandy soils, so be sure to consult the label before applying these chemicals. No one preemergence herbicide will control all weed species, as each herbicide differs in the species it will control. Complete reliance on only one material year after year will result in a buildup of the weeds that are resistant to that material. It is important to rotate herbicides from year to year to avoid this weed buildup, and to prevent herbicide accumulation in the soil.