Using a pre-emergent weed killer on your grass helps keep pesky weeds from marring the lush, even look of your lawn. Whether you use granule or spray types, don't plant grass seed for several weeks after applying the herbicide. This holds true if you're using a standalone herbicide or a weed-and-feed product.
Waiting to Plant Seed
When you spread an herbicide over your lawn and then decide to patch bare spots, overseed or change your grass type to grow a new species from seed, you should wait at least a month with most pre-emergent weed killers. If you used a crabgrass preventer, that time frame might extend to four months. Check the herbicide manufacturer's label to confirm how long you must wait, as it can differ between herbicide products. The pre-emergent herbicides stop weed seeds from germinating, but they can stop grass seeds as well. After you seed, wait until you've mowed the new grass at least four times, which could be six to eight weeks after seeding, before adding herbicides. This keeps the chemicals from stunting the seedling's growth.
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