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Preemergent herbicides prevent the germination of seeds by inhibiting a key enzyme. In some areas of the world, preemergent herbicides are used to prevent crabgrass from appearing in summer lawns. Preemergence 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 degrees for four consecutive days. In the fall, they should be applied when nighttime lows reach 55-60 degrees for four consecutive nights. 
"Weed and feed" products which contain both preemergence herbicide and fertilizer in a single product should not be used on southern lawns or warm-season grasses. If applied when preemergence herbicide is needed, the fertilizer may burn or stress the lawn. If applied after the lawn "green-up", weed seeds will have already germinated and will be ineffective.
Preemergent herbicides must be applied at a critical time. If they are applied to the soil too early they get washed too deep into the soil by rainwater. If they are applied too late the key enzyme inhibited is no longer active. Depending on where you live, one rule of thumb is to apply when the local forsythia blooms are wilting.
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