New Grass This Fall: Tillering, the First Mowing, and Keeping the Leaves Off

If you overseeded this fall, the next six weeks decide how your lawn looks next spring. Your new grass probably looks thin right now, it's almost time for the first mowing, and the oaks and maples are starting to drop everything they've got. Here's what's going on with the new grass, how to mow it, and why the leaves have to come off.

Why new grass looks thin at first: tillering

Every seed sends up a single blade at first. The thickness comes later, from tillering: new shoots that grow from the base of the plant, right at the crown near the soil. Each tiller grows its own leaves and, before long, its own roots. One seedling that started as a single blade can turn into a small clump of shoots by spring.

That's how tall fescue fills in. Bermuda and zoysia spread sideways with runners, so they can creep into a bare spot on their own. Tall fescue mostly can't. It's a bunch-type grass that gets thicker by tillering in place, not by spreading. That's why bare spots in fescue stay bare unless they get seeded, and why fall overseeding matters so much here: the seed puts new plants in the gaps, and tillering thickens them up.

Seedlings start tillering once they have a few leaves, usually about 4 to 6 weeks after they sprout. Fescue tillers most in cool weather with good soil moisture, which in Raleigh means right now. So a lawn seeded in mid-September often looks thin in October and noticeably thicker by Thanksgiving.

The first mowing

Mowing actually helps. Regular mowing encourages grass to branch out and tiller, so don't put off the first cut too long.

• Wait until the new grass is about 4 inches tall. For most lawns that's 3 to 4 weeks after seeding.
• Mow to about 3 to 3½ inches, and never take off more than a third of the blade at once. Letting it get shaggy and then cutting it way down stresses the young plants, and stressed plants make fewer tillers.
• Use a sharp blade and mow when the grass is dry. A dull blade or wet grass can rip seedlings out by the roots.
• Keep mowing through the fall at the same height, as long as it's growing.

Why leaves are hard on new grass

The timing is unavoidable: fescue germinates best in September and early October, right when the leaves start falling. But a seedling needs sunlight, air and steady moisture to make food and tiller, and a layer of leaves takes away the first two and turns the third into a problem.

• They block the light. Seedlings under a mat of leaves can't make food, so they stretch, turn yellow, stop tillering and thin out.
• They trap moisture against the soil. Constantly wet, shaded seedlings are exactly what damping-off and other seedling diseases need.
• They smother. After a rain, wet leaves flatten into a layer the grass can't push through. A week or two of that and the seed under it is gone.

The result is usually a lawn that looks great in the sunny areas and thin or bare under the trees, which is the opposite of what you paid for.

How to clear leaves without hurting the seedlings

Don't let leaves sit on seeded areas for more than a few days, especially after rain. For most yards that means once or twice a week until the trees are bare, usually late November around here.

• A leaf blower on low is the easiest option. Blow from a few feet away so you're moving leaves, not seed or loose soil.
• If you rake, use a flexible plastic leaf rake and a light touch. Skip metal rakes and hard scraping.
• Do it when the lawn is dry if you can. Wet leaves are heavy and wet soil tears easily.
• Pine needles count too. A light dusting is fine, but a mat of pine straw shades seedlings just like leaves do.

What about mulching leaves with the mower? On an established lawn it's great, since it returns organic matter to the soil. On new grass, wait until it's been mowed two or three times. Before that, clear the leaves instead.

Other ways to help it fill in

• Water if fall turns dry. Fescue tillers best with steady soil moisture. If we go a couple of weeks without rain, a deep watering once a week helps. This week's watering numbers for Raleigh show how much rain we've had and how much your grass is using.
• Let the fall feedings do their job. Fall is when fescue builds tillers and roots, which is why my programs put the most feeding in September through November.
• Be patient with shade. Grass under heavy trees tillers less and stays thinner even with good care. If an area keeps thinning out year after year, ask me about it. Sometimes the answer is more seed every fall, and sometimes it's a bed or mulch instead.

What to expect

• First few weeks after seeding: single blades, a fine green fuzz. It can look thin.
• Late October into November: the seedlings start tillering and the lawn closes in.
• Spring: the payoff. Plants that tillered and rooted all fall come back thick and green.

For watering tips during the first few weeks, see How to Care for New Grass Seed. Questions about your yard? Send me a photo through the free photo checkup and I'll take a look.

About the author: David is the owner-operator of Thriving Lawns and an NC-licensed pesticide applicator caring for lawns across North Raleigh and southeast Durham. Get a free estimate →