When and How to Overseed Your Lawn (Cool-Season Grasses)

The best time: late summer

For cool-season grasses — Kentucky bluegrass, tall fescue, perennial ryegrass and fine fescues — Purdue Extension puts the optimum seeding window at August 15 to September 15 in its region. Soil is still warm, so seed germinates quickly; air is cooling, so seedlings are not stressed; and fall weeds compete less than spring weeds. Seedlings then have the whole autumn and the next spring to root before summer heat.

Spring seeding can work, but young grass then meets its first summer with shallow roots, and crabgrass is germinating at the same time. Warm-season grasses such as Bermuda and zoysia follow a different calendar — they are seeded in late spring and early summer.

How much seed

Purdue’s new-lawn rates vary widely because seed size varies widely:

For a 5,000 sq ft tall fescue lawn, that is about 37.5 lb of seed for a new lawn or about 19 lb for overseeding.

Step by step

1. Mow short — about 1½–2 in — and bag the clippings so seed can reach the soil.
2. Open the soil. Core-aerate, or rake hard to scratch the surface. Seed lying on thatch rarely survives.
3. Spread half the seed in one direction and half at right angles for even coverage.
4. Rake lightly or roll to press seed into the soil.
5. Water little and often until germination — keep the top inch damp — then switch to deeper, less frequent watering.
6. Mow the new grass once it reaches your normal mowing height plus about a third.

Should you fertilize?

A starter fertilizer helps new seedlings. UConn’s soil lab notes that phosphorus may be added when seeding, sodding or overseeding, while established lawns should get phosphorus only when a recent soil test calls for it. Many states restrict phosphorus on lawns, so check local rules.

Common overseeding mistakes

• Spreading seed on a thick thatch layer without raking or aerating.
• Using a pre-emergent weed killer in the same season — it stops grass seed too.
• Letting the seedbed dry out for a single hot afternoon during germination.
• Seeding far more heavily than the label rate; crowded seedlings compete and get diseases.

Choosing good seed

The seed label tells you more than the picture on the bag. Look for named varieties suited to your region, a high germination percentage, a low weed-seed percentage and little or no “other crop” seed. Cheap mixes often contain annual ryegrass or other short-lived grasses that make the lawn look green quickly and then thin out.

• Full sun, high traffic: tall fescue or bluegrass with some perennial ryegrass.
• Shade: blends with fine fescues, which tolerate shade and low fertility.
• Fast cover for erosion: annual ryegrass as a temporary nurse crop, planning to reseed within a year.

How long until you see grass?

Perennial ryegrass is usually the quickest to germinate, often within about a week or two in good conditions. Tall fescue is a little slower, and Kentucky bluegrass can take two to four weeks. Keep the surface moist for the slowest grass in your mix, not just the first sprouts you see.

Ways to get seed into soil

• Core aeration then broadcast: seed falls into the holes and loosened soil. Good for compacted lawns.
• Slit seeding: a rented slit seeder cuts grooves and drops seed directly into them. Very effective for thin lawns.
• Rake and broadcast: fine for small areas — rake hard enough to see soil.
• Topdressing: a thin layer (about ¼ inch) of compost over the seed helps keep it moist.

Watering new seed

Until germination, keep the top inch of soil damp — often a light watering once or twice a day, more in hot or windy weather. Once seedlings are established, shift to deeper, less frequent watering of about an inch per week including rain, which encourages deep roots.

Dormant seeding

If you miss late summer, some homeowners seed in late fall, after soil is too cold for germination. The seed sits through winter and germinates in spring. It avoids summer heat but competes with spring weeds, and you cannot use most pre-emergent crabgrass products that spring.

Frequently asked questions

Sources

• Purdue Extension AY-25: Establishing lawns
• UConn Soil Nutrient Analysis Lab: Suggested fertilizer practices for lawns

This guide is for planning and general information. Check product labels, local codes and a qualified professional for your specific project. How we research and check our content.