How to Overseed a Lawn

Overseeding means spreading new grass seed into an existing lawn without tearing it up. It’s the cheapest way to thicken a thin lawn, bring in better grass varieties and crowd out weeds over time.

Most overseeding fails for one of three reasons: the timing is wrong, the seed never touches soil, or it dries out in the first two weeks. Get those three right and the rest is easy.

Everything below is based on extension guidance from Iowa State, Nebraska, Kentucky, UC IPM and Georgia. I look after my own lawn with EGO battery tools, and a seeding job is mostly about patience, not gear.

When to overseed

Cool-season lawns

Late summer to early fall is the best window by a wide margin. Iowa State gives mid-August through mid-September. Nebraska says August 15 to September 15. The soil is still warm, so seed germinates fast, and fewer weed seeds sprout at that time of year. The new grass then has two months of cool weather to root before winter, and another cool spring before summer heat.

Spring is the backup. Nebraska suggests April 1 to May 15 and notes that seeding after mid-May gets harder as heat, dry weather and weeds build. Spring seedlings have only a few weeks before summer stress hits.

There’s also a pre-emergent problem in spring. Most crabgrass pre-emergents, including prodiamine and dithiopyr, kill germinating grass seed too. Nebraska notes that only a few products, such as mesotrione and siduron, are safe to use at seeding. If you’ve already put down a standard pre-emergent, check the label for how long to wait before seeding.

Warm-season lawns

Warm-season grass is seeded in late spring and early summer, when NC State says soil should hold 65 to 70°F. UGA rates late spring and summer as the best time to seed bermuda.

Many warm-season lawns, though, aren’t overseeded to thicken them. Zoysia and St. Augustine are usually patched with sod or plugs. When southern homeowners talk about overseeding, they usually mean winter ryegrass for color. UGA lists 5 to 10 lb of ryegrass per 1,000 sq ft over established bermuda, in fall. That ryegrass dies out as the bermuda wakes up, but it competes with the bermuda during spring green-up, so it’s a trade-off.

Full timing for both grass groups is in the lawn care calendar.

Choosing the seed

Match the seed to what you already have, or to what you want the lawn to become.

Iowa State’s recommendations for the north:

• Full sun: Kentucky bluegrass, ideally a blend of at least 2 or 3 varieties.
• Partial shade: a mix of Kentucky bluegrass, perennial ryegrass and fine fescues.
• Shade: fine fescues.

Tall fescue is the common choice through the transition zone because it handles heat and drought well.

Buy good seed. Nebraska recommends certified seed (blue tag) and a blend of 3 or 4 varieties, and says to avoid coarse pasture grasses and annual grasses. Check the label for weed seed and “other crop” content. Cheap seed with a high weed percentage is no bargain. Our best grass seed guide compares specific products.

How much seed

Overseeding uses less seed than starting a lawn from bare soil, because grass is already there. Nebraska puts it at up to 50 percent less than the full rate.

Sources: Nebraska Extension (bluegrass, tall fescue) and UGA (ryegrass). Always check the bag, too, since seed sizes vary between mixes. To figure your lawn’s size, measure length times width in feet and divide by 1,000.

What you’ll need

• Grass seed at the right rate for your square footage
• A mower you can drop to 1.5 to 2 inches
• One of: a core aerator, a vertical mower (power rake) or a slit seeder; all are commonly rented by the day
• A drop or broadcast spreader. See best lawn spreaders.
• A leaf rake, or a drag mat or a piece of chain-link fence
• A hose and sprinkler you can run once or twice a day

How to overseed, step by step

1. Fix the cause first

Seed won’t solve what killed the grass. Thin turf from deep shade, compacted paths, poor drainage or grubs will thin again. If you have patches rather than general thinning, how to fix bare spots in your lawn covers diagnosis.

A soil test every few years tells you whether pH or nutrients are holding things back.

2. Kill weeds early, or wait

Broadleaf weed killers can harm young grass. If weeds are a problem, treat them well before seeding and check the herbicide label for how long to wait before you seed. Many people simply seed in late summer and handle weeds the following fall once the new grass is established.

3. Mow short

Iowa State says mow to 1.5 to 2 inches before overseeding. That’s lower than you’d normally mow, and it’s on purpose: it cuts competition and light-blocking from the existing grass so seed can reach the soil and the seedlings get sun. Bag the clippings this time.

4. Open up the soil

This is the step people skip, and it’s the one that matters most. Seed lying on top of thatch dries out and dies. It needs seed-to-soil contact.

Iowa State lists three ways:

• Core aerator. Make 3 to 4 passes until you have 20 to 40 holes per square foot. Spread the seed, then drag a mat or a piece of chain-link fence across the lawn to break up the cores and work the seed in.
• Vertical mower (power rake). One or two passes to cut through thatch into the top 1/4 to 1/2 inch of soil. Rake up loose debris, sow the seed, then go over the area once more to press seed into the grooves.
• Slit seeder. Cuts small grooves and drops seed directly into them. Best for large areas, and it gives the most even result.

Nebraska says power raking only makes sense if thatch is over 1/2 inch, and that aeration holes don’t need topdressing. For a small area, hard hand-raking with a metal leaf rake does the job.

5. Spread the seed

Use a spreader set for your seed and rate. Split the amount in half and make two passes at right angles. Iowa State recommends this crisscross for new lawns, and it avoids stripes of thick and thin grass.

For small amounts of seed, Nebraska suggests mixing it with dry sand or sawdust so it spreads evenly.

6. Fertilize (when and how much)

Iowa State suggests waiting about 6 weeks after germination, then applying 1 lb of actual nitrogen per 1,000 sq ft. Some homeowners use a starter fertilizer at seeding instead. Either way, follow the product label for rate and timing. When to fertilize your lawn has the full schedule.

7. Water lightly and often

The seedbed has to stay moist until the seed sprouts. Iowa State says keep the top inch of soil moist with frequent light watering, usually at least once a day. On hot, windy days, that can mean twice.

Don’t flood it. Puddles wash seed into piles and can rot it.

Once seedlings reach 1 to 2 inches, Iowa State says water less often but more deeply. Over the next few weeks, move toward the normal deep-and-seldom routine in how often to water your lawn.

How long until you see grass

It depends on the species. UC IPM lists these germination times:

So if you seed a bluegrass mix, the ryegrass in it will show up in a week and the bluegrass may take a month. Don’t stop watering when the first green appears. Iowa State says to expect germination in 2 to 3 weeks for a typical fall overseeding.

Mowing after overseeding

Iowa State says keep mowing at 1.5 to 2 inches while the seedlings come in, so the older grass doesn’t shade them. Once the new grass is tall enough to mow, raise the deck gradually over the following weeks up to about 3 inches.

Use a sharp blade so you cut the young grass instead of pulling it. Mow when the soil is dry enough that you don’t leave footprints or wheel ruts. How to mow a lawn covers heights and technique.

What to skip after seeding

• Pre-emergent. Most will stop your new grass, too. Check the label before using any.
• Broadleaf weed killer on seedlings. Check the label for how long after seeding it can be used.
• Heavy foot traffic. Keep kids and dogs off as much as you can for the first few weeks.
• Leaves piling up. In fall, blow or rake leaves gently off the new grass so they don’t smother it.

Common overseeding mistakes

Seeding in late spring. It often works for a month, then the seedlings die in July heat.

Throwing seed on thatch. Without aerating, slicing or raking, a lot of the seed never reaches soil.

Letting it dry out once. A germinating seed that dries out on day 6 is dead. This is the most common cause of failure.

Too much seed. More seed doesn’t mean a thicker lawn. Crowded seedlings compete with each other and get weak. Stick to the rate.

Mowing too high. Leaving the old lawn tall shades out the seedlings.