Does Pre Emergent Kill Grass Seed?

When you’re working with weed killers, it’s important to make sure the application is coordinated with the growing needs of your grass. Most pre emergent herbicides interfere with growth of grass seeds the same way they do with weed seeds, so you’ll have to be thinking several months out when planning an application.

Keeping Weeds From Growing In The Yard

Weeds can be tricky to keep out of the yard for good. Their seeds get blown over from other yards and fields, and they get deposited randomly by passing birds. These seeds might stay in the ground for several years before they germinate and sprout, so it’s an ongoing game of keeping them from germinating and eliminating them when they do.

Should I Apply Pre Emergent or Post Emergent Herbicide?

Trimec Classic Herbicide 1 Gal Post Emergent For All Major Broadleaf Weeds

Many post emergent herbicides come in targeted formulas that leave other plants and grass alone but pre emergent formulas can suppress even desirable growth. As a rule of thumb, it’s best to put any kind of herbicide on a healthy lawn to avoid damage to the existing grass root system. While post emergent can be applied during the growing season, pre emergent has to be applied before the weeds show up in the first place.

This is because certain weeds, especially grassy ones like crabgrass, foxtail, and poa annua, mix into the lawn and can’t be targeted the way broadleaf species can. The best way to prevent this is to encourage thick, dense turf so that the competing species aren’t able to grow in, but some weeds, especially crabgrass, are notoriously early risers and will show up just before your lawn comes out of dormancy. Putting a pre emergent application to prevent establishment is the only real way to keep grassy weeds out of the yard.

How Does Pre Emergent Work?

Pre emergent herbicides prevent weeds from establishing by inhibiting root growth. When you put an application down, it needs to be watered in to activate. The activated herbicide then creates a barrier in the top couple of inches of soil, which prevents seeds from emerging from the ground’s surface. This works on weed seeds as well as on grass seeds.

The Andersons Barricade Professional-Grade Granular Pre-Emergent Weed Control - Covers up to 5,800 sq ft (18 lb)

Most products, like Scotts Halts and Barricade, work disrupt the germination process by preventing root development; they won’t be effective on plants that have already have strong roots and have emerged from the soil surface.

Other products, like Tenacity, can be used as both a pre and post emergent by blocking chlorophyll production (it positions itself as a safe pre emergent for grass seed). Based on your grass type, weed type, and location, a little comparative research will help you know which weed killer is right for your needs.

When Should I Put Down Pre Emergent?

For spring growth, it’s recommended to apply a pre emergent while the soil temperatures are still below 55 degrees, the point at which crabgrass starts to germinate. It’s never too cold to put down pre emergent, and many people like to apply it in late winter to be sure its present when temperatures start to warm up.

When soil temperatures reach about 40 degrees, microbes in the soil start becoming active again and will slowly break down the herbicide. As temperatures increase, the pre emergent breaks down more quickly, so it’s better to be in early and make a second, later application than too late and be unable to get rid of the weeds. Your soil temperatures can be measured with a meter when you apply in early spring to be sure you’re getting it down before the crabgrass starts to germinate.

Pre emergent can also be applied during the late spring after your cool season grass is mature but before your summer grasses emerge. Similarly, preventive herbicides for fall and winter weeds can be applied in late summer and early fall to stay ahead of opportunistic growth.

Can You Put Down Pre Emergent Before Seeding?

Can You Put Down Pre Emergent Before Seeding

If you’re applying pre emergent herbicide across your yard, you should wait until the next season to apply grass seed since the herbicide will create the surface level barrier that prevents any sprouting seeds from establishing. If you have bare spots in your yard and you have applied preemergent, you may be able to spot-till the soil in a small area and break the barrier, although it’s not guaranteed that mixing the soil will completely prevent the herbicide from working.

When it comes to large areas, if you need to seed, you should avoid putting down pre emergent until the next growing season. On the other hand, if you feel the need to apply a crabgrass preventer to keep all areas of your lawn free of weeds, focusing on the health of the existing lawn before sowing new seeds may be the best strategy.

How Soon Can I Overseed After Pre Emergent?

If you overseed within the same couple of months after applying a pre emergent herbicide, the grass seed likely won’t sprout. You should wait at least 2 months and up to 4 months before you overseed your lawn again. This will be enough time for the herbicide to have had its effect, be broken down, and drained away with watering and rain.