How to overseed a lawn, step by step

Fix the cause before you seed

Penn State Extension says to correct the main causes of a declining lawn before renovating: drought, heavy shade, very acid soil, weeds or insects, disease, thatch, poor mowing and grass that does not suit the site. Seed sown into the same problem thins out again.

Take a soil test first. Missouri Extension calls a pH of 6.5 to 6.8 excellent for establishing turf and 6 to 7 acceptable, and any lime need appears on the test report. If you need to kill broadleaf weeds first, it says those product labels usually ask for about three weeks between spraying and seeding, so start early.

Open up the surface

Seed that lands on thatch or matted grass does not root. Penn State's renovation steps begin by killing the old lawn with glyphosate, which is more than a thin lawn needs: Missouri says that if a lawn only needs overseeding to fill thinned areas, a nonselective herbicide is not required and seeding can proceed. The surface preparation is the same either way. Penn State describes removing excess thatch with a slicing or dethatching machine run in several directions, raking it up, then making several passes with a core aerator. Slicer blades should cut only about a quarter to half an inch into the soil.

Missouri puts it as good seed-to-soil contact. The best tool is a slit-seeder, which plants seed about a quarter inch deep and lowers the chance of seed washing away in a heavy rain. A power rake or vertical slicer, which works up the top half inch of soil, also works. Both can usually be rented. For a small thin patch, loosening the surface with a rake achieves the same goal by hand, which is our own suggestion.

Choose the seed and the rate

Use a species that matches your lawn and your light. Penn State says perennial ryegrass is the best species for renovating because it germinates quickly and its roots can penetrate slightly compacted soil; Kentucky bluegrass, fine fescues and tall fescue also work if the soil has been aerated well. Read the seed label for the species, the purity, the germination rate and the weed seed content, since low-quality seed gives a thin, weedy lawn.

The rates in the table are pounds of seed per 1,000 square feet. More is not better. Penn State warns that an excessively dense stand from a high rate can bring disease problems and mower scalping, while a rate that is too low leaves a thin lawn where weeds do better. Illinois Extension lists 1 to 3 pounds for a Kentucky bluegrass blend and 6 to 9 for tall fescue, tells you to read the label for your seed rate and warns that exceeding rates may give weak, spindly seedlings and disease. When the bag label and the table differ, follow the label.

Spread, feed and cover

Penn State and Missouri both advise seeding in two directions, and Illinois Extension says the same: half the rate in one direction, then the other half at right angles. Set the spreader to deliver half the rate, cover the lawn once, then cover it again at right angles. A drop spreader suits small areas, and Penn State says the area should have enough bare soil showing after aeration and slicing to seed with one.

Apply starter fertilizer just before or just after seeding. Missouri gives 0.5 pound of nitrogen per 1,000 square feet as the usual rate, depending on the soil test. Mulch is optional on an overseeded lawn: Penn State says there is no need if enough of the old vegetation stays on the surface, while on tilled bare soil Missouri recommends one bale of straw per 1,000 square feet.

Water for the first weeks

Missouri says the first two weeks after seeding are the most critical: keep the surface moist, not wet, never let germinating seed dry out, and avoid puddles and runoff. On warm, windy, dry days that may take several light waterings a day. Penn State says new seedlings have shallow roots and will probably need regular, sometimes daily, irrigation, less in rain or cool, overcast weather.

Seedlings should be up in 10 to 14 days, according to Missouri. Penn State calls the whole 4 to 8 weeks after seeding critical and says more new lawns fail from poor follow-up care than from anything else. Once seedlings root, cut back to less frequent, deeper watering; the exact schedule depends on your soil and weather.

Mow, feed again and what to avoid

Start mowing with a light mower when the new grass is no more than one third above your cutting height. At a 3 inch height, mow just before it reaches 4 inches. Penn State's test: walk onto the grass, and if the plants spring back upright, you can probably mow.

Add a second feeding later. Penn State suggests half a pound of nitrogen per 1,000 square feet 4 to 8 weeks after the starter, and Missouri says to apply 1 pound one month after you see seedlings. Avoid herbicides on new turf: Penn State says many broadleaf products should wait 4 to 6 weeks after establishment, and Missouri notes that in its spring seeding advice many crabgrass preventers cannot be used when seeding, and it names Tupersan as the one that can go on the same day. Read every label first.

Our summary of what the sources warn against: seed left on thatch, a surface that dries out while seed is germinating, puddles and runoff, a seed rate far above the label, mowing too early and foot traffic. Penn State says seedlings can be bruised or uprooted if trampled.