If you have an otherwise healthy lawn that’s looking a little thin, overseeding in cool climates now brings quick results that last in the following seasons. In warm climates, overseeding is an option to create green coverage all winter while the underlying grass is dormant. 

Late summer and early fall are prime for planting grass seed to establish before winter. Overseeding is successful with basic site preparation to ensure seed-to-soil contact for germination.

In both site prep and tending this time of year, the tasks are less extensive than with a total lawn renovation. Plan to water regularly after sowing to promote healthy roots, with final rounds of mowing leading into cool temperatures.

Follow this guide to find out when and how to overseed your lawn.

What Overseeding Accomplishes

Quick growth happens when new seed blends well.

If you have multiple bare patches or thinning blades and an otherwise healthy lawn, overseeding is a streamlined process to make it more lush.

Overseeding is adding a layer of seed to thicken existing grass, usually over the entire expanse. In addition to a lusher look and feel, overseeding helps prevent weeds by crowding them out with dense blades.

This type of seeding uses the same or similar grass species for infill, which is actually called interseeding. Interseeding matches existing turf or utilizes a better blend for site conditions. The term overseeding technically refers to incorporating a new species for a quick greenup.

In warm climates in the western and southern U.S., overseeding usually relates to sowing a cool-season grass atop a dormant warm-season grass. Since dormant grasses are brown in winter, the cool-season option (usually ryegrass) creates a green carpet during cold weather.

Overseeding has come to represent both types of seed layering, and we’ll stick with this term for both. The actions and preparation are the same.

Grass Seed Basics

Mixing species adapts to moisture and foot traffic.

Turfgrass falls broadly into warm-season and cool-season types based on their best growing conditions. Cool-season grasses grow in zones 6 and colder, with Kentucky bluegrass, perennial ryegrass, and tall and fine fescues as common species. Warm-season grasses generally grow in zones 7 and warmer and include Bermuda, St. Augustine, centipede, and zoysia. Ryegrass is sometimes used to overseed warm-season types in regions with mild winters.

The first consideration in how to overseed a lawn is to determine the grass type. Plant to match what you have for consistency, or mix species to better account for specific conditions. Sun exposure, moisture levels, and foot traffic/use are the main accommodations.

Source high-quality seeds in labeled packages to avoid introducing weed seeds. Check the label for seed purity, germination rates, and testing dates for viability. Grass seed is often available as blends, or combinations of varieties within the same species. Mixes are those that contain multiple grass genera and types to fit conditions like sun, shade, temperature, and moisture.

Measure the area for seeding to buy the right amount in bulk, usually in pounds per 1,000 square feet. There’s no need to scatter extra seed (beyond the recommended application), as too much leads to overcrowding and potential diseases.

When to Sow

Timing sowing carefully protects tender blades from frost damage.

To overseed existing grass, late summer and early fall offer a window of opportunity. Mild air and soil temperatures and regular seasonal moisture support healthy root development for overwintering. Sow grass seed at least 45 days before fall’s first anticipated frost date. Wait til spring if your frost date is nearer, as tender blades risk cold damage and dieback without ample time to establish.

In zones 6 and cooler, sow cool-season selections in mid-August through mid-October. These need cool air and soil temperatures with regular moisture to germinate, with an optimal soil temperature range of 50 to 60°F (10-16°C). 

In zones 7 and warmer, wrap up any warm-season infill through September. These selections rely on warm conditions to germinate, with ideal soil temperatures between 65 and 70°F (18-21°C). Overseed with winter rye as temperatures cool to the 50-60°F (10-16°C) range.

Site Preparation

Preparing the site carefully encourages strong, even growth.

Ready the site for overseeding by first removing any debris and impediments like rocks, sticks, fallen debris, or anything that may block grass seed from soil contact. It’s always a good idea to do a soil test, whether from an at-home kit or your local university extension service.

Allow a few weeks for interpreted results that outline beneficial soil amendments and nutrient levels for turfgrass. For overseeding, this information can help determine fertilizing needs, whether a starter fertilizer is useful at planting or a quick granular nitrogen about six weeks later.

To overseed the entire lawn (rather than just bare patches), mow lower than usual. Mow the blades to one and a half to two inches high. Rake the surface to remove clippings and any thatch (brown, dry blades) for good seed-to-soil contact in thin areas.

To boost soil contact, consider rental equipment like core aerators, vertical mowers, or silt seeders (for large expanses). With different functions, each serves to cut into the soil surface for aeration and to create small areas of bare earth for seeding.

For compacted soils (heavily trafficked or dense compositions, like clay), these serve to improve the texture for root development, air circulation, and drainage. They also reduce competition from existing turfgrass.

How to Overseed

Uniform seed distribution prevents bare or patchy spots.

With the site prepped, scatter the seed with a drop spreader for even distribution. Spread half of the seeds moving in one direction and the other at a right angle (criss-crossing) for uniform coverage.

Lightly rake in the seeds (again, aiming for that soil contact). Roll the site with a 50 to 100-pound roller to create better contact and to secure seeds in place. Walking on the site to tamp seeds works, too, for small areas.

Watering

Deep roots form when water penetrates slowly downward.

Seeds and seedlings need daily water to germinate and grow. Sometimes, they need it twice daily during hot, windy conditions or dry spells in early fall.

Water lightly to moisten the soil surface, avoiding excess runoff and oversaturation. Look for seeds to germinate in two to four weeks. Continue daily watering until seedlings reach two inches tall, and reduce the frequency. While spacing out watering sessions, increase the run time to promote deeper soil moisture for longer, more extensive roots. 

Mowing

Regular mowing prevents competition and supports even growth.

As grass continues to grow in the fall, mow at a low height (about two inches) to reduce competition from existing blades. As seedlings develop, raise the deck. Fall mowing at three inches promotes healthy grass for overwintering.

As growth slows with cooling temperatures, taper the mowing regimen to every other week or two. No mowing is needed when grasses are dormant. With overseeded rye grass in warm climates, plan to mow as it grows over the winter (less frequently than summertime sessions).