If you have used a pre-emergent herbicide or “crabgrass preventer” product within the last 12 weeks, it will kill any grass seed your spread on your lawn. Review the product information for any pre-emergent herbicides you have used to see how long they remain active. Most products kill any germinating seeds for 8–12 weeks.
Pre-emergents kill all seeds that attempt to sprout in your yard. This includes weeds, grasses, trees, flowers, and garden plants. Do not spread pre-emergent if you plan to seed your lawn within the next 12 weeks or if you have seeded your lawn within the last 12–16 weeks.
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How Soon Can You Plant Grass Seed After Applying Pre-Emergent?
Wait at least 12 weeks after applying pre-emergent before seeding your yard. Any grass seed spread sooner will be killed by the pre-emergent in the soil as soon as it germinates. All your hard work will go to waste, as well as the money spent on grass seed.
Pre-emergent herbicides are non-selective, meaning they kill all types of plants. They work by entering the soil and forming a barrier near the surface. They attack seeds just as they sprout underground, but these herbicides don’t kill mature plants and grasses.
If you spread grass seed during the pre-emergent’s active window, it will kill the seeds the same way it would kill the seeds of broadleaf weeds or invasive grasses.
Can You Put Pre-Emergent on New Grass Seed?
Wait 12–16 weeks after seeding before you apply pre-emergent to your lawn. If you apply a pre-emergent too soon, it can kill young grass seedlings.
Any new grass seeded in your yard must be mature and established before you can apply pre-emergent. The rule of thumb is to wait until your new grass is mature enough that it has been mowed 3–4 times. Only then will it be able to resist damage from a pre-emergent.
Pre-Emergent and Grass Seeding Schedule
Grass seeding and pre-emergent application don’t mix. The good news is, the solution is simple. Apply pre-emergent in spring to stop spring and summer annual weeds. Then, overseed your lawn in fall to establish grass seedlings before winter.
- Apply pre-emergent in spring, when soil temperatures rise to 55℉ (13℃).
- Seed your lawn in fall, 45–60 days before the first average fall frost.
By following this simple lawn care schedule, you can stop crabgrass and invasive broadleaf weeds from cropping up in spring, then thicken your lawn in fall. Even better, many varieties of grass do best when seeded in fall.
Alternatively, you can reverse this schedule. If you have pesky winter weeds, apply a pre-emergent in fall. Then, seed your lawn in spring.
Can You Apply Crabgrass Preventer and Grass Seed at the Same Time?
Do not apply crabgrass preventer at the same time as grass seed. Crabgrass preventers are simply pre-emergents under another name. These products will kill grass seeds.
- Wait 12 weeks after applying crabgrass preventer to seed.
- If you’ve already seeded your yard, wait 12–16 weeks before applying a crabgrass preventer.
If you wish to seed in spring but want to keep crabgrass at bay, use this weed killer spray that targets broadleaf weeds and crabgrass without harming your lawn.
Is it Safe to Seed Your Lawn After Applying Pre-Emergent?
Pre-emergent herbicides kill any and all seeds in your lawn for up to 12 weeks after application. If you’ve applied a pre-emergent to your yard, wait at least 3 months before seeding. The best schedule for promoting a full, weed-free lawn is to apply pre-emergent in spring, then seed your lawn in fall.