First Mow After Fall Overseeding: Height, Timing and Which Mower to Use

Fall is the best time to overseed a northern lawn, which means October and November are when a lot of people stare at a patchwork of old grass and new fuzz and wonder when they can mow. Mow too soon and you pull up seedlings or rut soft soil. Wait too long and the old grass gets shaggy and shades out the new.

This guide covers when new grass is ready, how high to cut it, which mower to use, and what to do when the season runs out. It’s for cool-season lawns such as Kentucky bluegrass, tall fescue and perennial ryegrass. For southern lawns overseeded with ryegrass for winter color, see mowing ryegrass over bermuda. MowReview quotes no prices and hasn’t tested the products mentioned. How we research explains why.

When new grass is ready for its first mow

Ohio State University’s turfgrass specialist, Pam Sherratt, says new grass “can be mowed for the first time four to six weeks after seeding.” That window is shorter for irrigated lawns and longer in dry weather. Germination itself varies by grass: Pennington, for example, lists 7 to 21 days for its perennial ryegrass.

Weeks are only a guide, though. The real test is height.

Measure the new grass, not the old. In an overseeded lawn, the existing grass will reach mowing height long before the seedlings do. If the old grass needs cutting first, mow it with a sharp blade at your normal height on a dry day, and step carefully around the thinnest areas.

How to mow new grass without damaging it

1. Sharpen the blade first. A dull blade tears young grass instead of cutting it. Ohio State recommends sharpening after every 25 hours of use, about twice a season.
2. Use a rotary mower. Ohio State says the first mow is done with a rotary. University of Georgia advice is to switch to a reel mower, if you use one, only after the second or third mowing.
3. Wait for dry grass and firm soil. Ohio State warns that where bare soil shows and the ground is wet, mower wheels cause ruts and other damage.
4. Don’t turn sharply on new grass. Make wide turns, or turn on a path or driveway.
5. Leave the clippings. Short clippings from a one-third cut fall between the blades and don’t smother seedlings.
6. Hold off on weed killer. Most herbicide labels restrict use on new seedlings, so check the label before treating an overseeded lawn.

If the season runs out

Overseed late enough, and the new grass may not reach mowing height before growth stops. That’s fine. Purdue’s turf team says to keep mowing as long as grass grows, and the corollary is that you don’t need to mow grass that isn’t growing. Leave seedlings at whatever height they’ve reached, keep the old grass from getting shaggy, and make the first proper cut in spring. When to stop mowing in fall covers the end of the season, and mulching leaves explains how to keep leaves from smothering new grass.

What about robot mowers?

A robot mower is gentle compared with a person pushing a heavy mower, but it runs often and turns on the spot, which can scuff seedlings and soft soil. Pause it while the seed germinates, and restart it once the new grass has had a few normal mows. Robot mowers on new sod covers a similar situation, and will a robot mower damage your lawn explains wheel marks and turning.

The picks

Any sharp rotary mower will do. These three, from Bob Vila’s hands-on electric mower tests, show the range from very light to full-size.

Questions

How long after overseeding can I mow?

Usually four to six weeks. Ohio State’s turf specialist Pam Sherratt says new grass can be mowed for the first time four to six weeks after seeding, sooner on irrigated lawns and later in dry conditions.

How tall should new grass be before the first mow?

About one-third taller than your target height. Ohio State says that for a 3-inch target, the first mow comes when the grass reaches about 4 inches. University of Georgia advice for overseeding is the same idea: start mowing when seedlings are 30% taller than the height you want.

Should I use a reel or rotary mower on new grass?

A rotary mower for the first cuts. Ohio State says the first mow is done with a rotary mower, and University of Georgia advice is to use a rotary for the first mowing so seedlings are cut, not torn, then switch to a reel mower if you use one after the second or third mowing.

Can I mow new grass when the ground is wet?

Avoid it. Ohio State warns that where bare soil is visible and the ground is wet, mower wheels can cause ruts and other surface damage. Wait until the soil is firm and the grass is dry.

What if winter arrives before the new grass is tall enough to mow?

Don’t mow it. Leave the seedlings to go into winter at whatever height they’ve reached, and make the first cut in spring once they’re about one-third above your target height.

Can a robot mower mow newly overseeded grass?

Wait until the new grass is established. A robot mower runs often and turns on the spot, which can scuff seedlings and soft soil. Pause it during germination and resume once the new grass has had a few normal mows.

The short version

Wait until the new grass is about one-third taller than your normal height, usually four to six weeks after seeding. Mow with a sharp rotary blade, on a dry day, on firm soil, and take off only the top third. If the season ends first, leave the seedlings and make the first cut in spring.