Preemergent herbicide

Preemergent herbicides prevent the germination of seeds by inhibiting a key enzyme. In some areas of the world, they are used to prevent crabgrass from appearing in lawns. Preemergent herbicides are applied to lawns in the spring and fall to prevent the germination of weed seeds. They will not affect any established plant. In the spring, they should be applied when air temperatures reach 65–70°F for four consecutive days. In the fall, they should be applied when nighttime lows reach 55–60°F for four consecutive nights.