When Is the Best Time to Overseed a Lawn?

The short answer

Exact dates depend on where you live. A common rule of thumb is to seed cool-season grass about six weeks or more before your usual first hard frost, so seedlings can grow roots before winter.

Why early fall works for cool-season grass

In late summer and early fall the soil is still warm from summer, which speeds germination, while cooler air reduces heat stress on young grass. Many annual weeds that compete with spring seedlings, crabgrass in particular, are dying back. Seedlings then get both fall and the following spring to establish before summer heat. Spring seeding can work, but seedlings face summer stress sooner, and pre-emergent weed control used in spring usually stops grass seed as well.

Why warm-season grass waits for late spring

Bermuda and other warm-season grasses need warm soil to germinate and grow. Seeded too early, they sit dormant while weeds move in. Seeded too late, they do not have enough summer to establish before going dormant in fall.

It is late fall. Is it too late?

For most cool-season lawns in colder regions, it is too late for a normal fall seeding, because seedlings will not have time to establish before freezing weather. You have two options.

• Wait until spring. Seed as soon as the soil can be worked, and skip pre-emergent crabgrass products on the seeded area, or choose one labeled as safe for new seedings.
• Dormant seeding. Spread seed in late fall or winter once the soil is too cold for it to germinate. The seed sits in the soil, is worked in by freezing and thawing, and sprouts as the soil warms in spring. It works best on bare or thin areas where seed reaches the soil, and on ground that will not wash away. Some extension services suggest a slightly higher rate to allow for losses to birds and runoff.

In mild climates where the ground rarely freezes, cool-season seeding can still succeed into late fall. In the southern US, fall is the season to overseed dormant Bermuda with ryegrass for winter color.

How much seed to buy

Overseeding usually uses about half the rate for a new lawn, because existing grass already covers part of the ground.

These are typical figures within published ranges; your seed label's rate takes priority. Enter your own area and grass in the grass seed calculator.

Worked example

A thin 6,000 sq ft tall fescue lawn: 6,000 ÷ 1,000 = 6, and 6 × 3.5 lb = 21 lb of seed. Bags are sold in fixed sizes, so round up to what the store stocks.

How to overseed, step by step

1. Mow short. Cut a little lower than usual and bag the clippings so seed can reach the soil.
2. Open the surface. Rake out thatch, dethatch or core aerate. Seed-to-soil contact is the single biggest factor in success.
3. Spread the seed. Use a spreader, half in one direction and half crosswise.
4. Feed lightly. A light fertilizer application helps seedlings; follow the product label and local rules. The fertilizer calculator converts a nitrogen target into pounds of product.
5. Water little and often. Keep the top of the soil moist until the seedlings are established, then water deeper and less often.
6. Mow when ready. Wait until the new grass reaches normal mowing height, and use a sharp blade.

Signs a lawn needs overseeding

• Thin areas where you can see soil between grass plants.
• Bare patches after summer drought, insect damage or heavy wear.
• Weeds filling gaps, which usually means the grass is too thin to compete.
• An older lawn of a variety that struggles with disease, shade or drought, which you want to improve with newer seed.

Match the new seed to the existing lawn, or to a compatible mix. Overseeding a fine-bladed bluegrass lawn with coarse tall fescue, for example, leaves visible clumps.

When overseeding is not enough

If more than about half the lawn is weeds or bare soil, renovating or reseeding from scratch may be more effective than overseeding. For an instant result, compare the cost of sod with the sod calculator.

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