Seeding and overseeding a lawn
Grass seed is cheap but the labor isn't. You can do it yourself. Almost all of it, essentially, comes down to four things: the date, the application rate, whether the seed reached soil, and whether it stayed damp for germination.
New seeding or overseeding
A new seeding starts from bare soil: a lawn being grown initially, or one having been torn out and will be replaced. Nothing will be competing with the seedlings, so the rate is the full one and the whole area has to be prepared especially for this.
Overseeding puts seed into a lawn that already exists, to thicken the canopy or to work another grass variety into the lawn over a few seasons. The existing turf shades the new seedbed and takes the water first, so the rate is roughly half. Doubling it does not double the canopy: seedlings crowded too tightly compete with each other as well as with the lawn.
The steps below are the same for both. Where the numbers differ, both are given.
When
Timing matters more than anything else on this page, and it is actually decided by soil temperature rather than the calendar. Cool-season grasses (bluegrass, ryegrass, the fescues) germinate best with soil around 50 to 65°F. Warm-season grasses (bermuda, zoysia) like it above ~65 to 70°F.
For a cool-season lawn, that means late summer into early autumn is the best window of the year: the soil is still warm from summer, air is cooling off, autumn rains, and months of root growth before the next summer. Weed pressure is also at its lowest, because crabgrass is finishing up their lifecycle rather than starting.
Spring works fine and is the second choice. However, the seedlings are only able to grow a fraction of the root system before their first summer, and they grow while most annual weeds in the soil are also germinating.
How much seed
Rates are recommedned in pounds per 1,000 square feet and vary by species, because seeds vary enormously in size. There are roughly two million bluegrass seeds in a pound and about 230,000 ryegrass seeds, which is why the bluegrass rate looks so low in comparison.
Pounds per 1,000 square feet for a new seeding, with the overseed rate beside each. A bag labelled for a species is not always bagged at these rates: read the tag.
Getting seed onto soil
This is where most overseeding fails. Seed broadcast over an established lawn lands on leaf blades and gets stuck in the canopy, dries out, and might never reach soil at all.
- Mow low first, around 1.5 to 2 inches and collect the clippings. It opens the canopy and gets light to the seedbed.
- Open the surface. Core aeration is the usual answer on an established lawn: the holes are seedbeds, and the cores break down over the following weeks and cover the seed. Vertical slicing or a light dethatching does a similar job.
- Spread half in each direction. Half the seed walking north to south, half east to west, at half the spreader setting each pass. One pass at full rate leaves stripes that show all season.
- Then drag or rake lightly. A quarter of an inch of soil over the seed is plenty; an inch is likely too much. Rolling is optional on a new seedbed and unnecessary on an overseed.
- Stay off until after the first mow.
Starter fertilizer at seeding is worth it where a soil test shows phosphorus is low, since seedling roots cannot forage far for it. Several states restrict phosphorus on established lawns and exempt new seedings specifically.
Watering
A germinating seed has no roots. It can only use water in the top half inch of soil, and that half inch dries out in a few hours of sun and wind. A seed that swells and then dries will die.
So the pattern for the first couple of weeks is the opposite of normal lawn watering: light and often. Enough to keep the surface dark, two or three times a day in warm weather, rather than one long soak. Once seedlings are up and mowing has started, water the other way: fewer, deeper waterings drive roots down, and staying on the light schedule keeps them at the surface where the first hot week will find them.
What sets it back
A crabgrass pre-emergent stops grass seed from establishing by exactly the same mechanism it stops crabgrass, and most of them stay active for two to three months. A spring application means no seeding during that timeframe. There are a couple of actives sold specifically as safe at seeding time; check the label.
- Burying the seed. A quarter inch is the limit. Seed under an inch of topsoil has spent its whole energy reserve before it reaches light.
- Buying by the bag rather than the tag. The tag carries the species, the germination percentage, the year it was tested, and the weed seed content. Cheap mixes carry annual ryegrass, which dies in a season, and can carry weed seeds.
- Mowing too early or too wet. Wait until the seedlings are about 3 inches, mow high with a sharp blade, and take no more than a third off.
- Expecting a bluegrass overseed to look like anything for a month. Nothing is wrong; it germinates in 14 to 30 days.
What to take away
- Soil temperature sets the seeding date. Late summer beats spring for a cool-season lawn by a wide margin.
- Overseed at about half the new-seeding rate. More seed is not a thicker lawn.
- Seed that does not reach soil does nothing. Mow low, open the surface, spread in two directions, lightly rake.
- Light and frequent water until germination, then deep and infrequent, and switch promptly.
- A pre-emergent applied in spring may rule out seeding for two to three months after it.
Keep reading
How to order and read a soil test report
Step-by-step: From sample collection to interpreting results. What each line on a lab report means and why they are important.
Lime: why, how much, and how
Why two soils reading pH 5.5 require different lime rates, what is ENM and ECCE, and why pH might drift over time.
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