Preemergent herbicide

[1] Preemergent herbicides are applied to lawns in the spring and autumn, to prevent the establishment of weed seeds.

If applied when preemergent 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 the herbicide will be ineffective.

If they are applied too late, the key enzyme inhibited is no longer active.

Depending on location, one rule of thumb is to apply when the local forsythia blooms are wilting.