Overseeding? Here’s How to Keep Weeds Out

Whether you're planting new grass seed or overseeding, the process of planting grass usually also stirs up weeds. Tilling soil, raking soil, watering all benefit grass seed, but they also stimulate weed seed that’s already in your soil. Fortunately, there's a way to nurture your grass seed without also encouraging weed growth. By using a little known preemergent that’s safe for grass, patching with sterilized starter material, and using good mowing practices, you can keep the weeds out.  

Related: Here's How I Spring Seed My Lawn as a Yardcare Expert

1. Mesotrione based preemergent

You might have read that you can’t put down a preemergent or weed and feed while also spreading new seed. Typically you have to wait for four months after spreading seed to apply any weed control. That’s because most weed preemergents have prodiamine, which kills grass seed along with weed seed. However, there is a preemergent that does work well with grass seed–mesotrione. Mesotrione, which is sold under such brands as Tenacity, is selective. It will prevent crabgrass and clover from growing by allowing grass seed to germinate. It will even kill off weeds that do manage to sprout. While mesotrione is safe, you must apply it at the time of overseeding or before the new seed germinates. 

Note: While mesotrione works well with cool season grasses (fescues, KentuckyBluegrass, perennial ryegrass), it can damage some warm season grasses. 

2. Use Sterilized dressings

Most of us use topsoil as a dressing to reseed those bare spots in our lawn. That’s a mistake. Top soil is loaded with weed seed. Skip the topsoil and instead use a sterilized top dressing. Go with sterilized peat moss or coconut coir, the same starter material you would use for indoor seeding. That way you’re only planting grass seeds and not weed seed along with it. 

Pro-Tip: Coconut coir is the eco-friendly choice here as peat moss harvesting has more of an impact on the environment.

Related: Salt Ruining Your Lawn? Do This Before You Reseed

3. Give your seedlings an advantage

Seedlings don’t just have to compete with weeds when they’re growing, they also face stiff competition from your existing turf. While you don’t want to harm your existing grass, you can handicap it to level the playing field for your new seedlings. Before you overseed, cut the grass down to a height of 2 inches. This stunts the existing grass, giving your new seed time to germinate and grow. Eventually the new seed and existing grass will grow together, creating denser lawn. 

4. Mow early and often

Weeds like open space in which to grow. If you have bare patches or gaps in your lawn, expect weeds to move in. Get your grass to fill those sparse spots instead through frequent mowing. The more you mow, the more grass grows. Mowing encourages your grass to grow sideways, a technique called “tillering,” which fills in gaps in your lawn faster. The fewer gaps you have in your lawn, the harder it is for weeds to move in. Aim to mow about once every 5 to 7 days or when the grass grows an inch beyond its ideal height, which is typically 3 inches. 

Pro Tip: Make sure your mower blade is sharp. A dull blade can pull young grass plants right out of the ground.