When overseeding your lawn, it is better to use compost as a topdressing instead of topsoil. This is because compost is rich with nutrients and organic materials which will feed the grass seed and work their way into the existing soil as the compost breaks down.
Adding topsoil will put a smothering layer of dirt atop your existing grass. The topsoil you bring in usually won’t have enough nutrients to feed your grass seed, and it won’t break down over time. Compost is the key when overseeding.
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Why is Compost Better than Topsoil for Overseeding?
Grass seed needs 2 things to sprout and thrive: water and nutrients. Dark, rich compost retains moisture from watering, creating a habitat that encourages grass seed to sprout. Also, it is extremely rich in nutrients needed for grass growth, giving seedlings an initial boost.
- Compost retains water, keeping grass seeds moist.
- Compost contains nutrients grass seeds need for initial growth.
- Topsoil dries out more easily than compost and contains fewer nutrients.
- Compost breaks down to feed existing soil. Topsoil sits atop the yard and can smother existing grass.
- Topsoil is used to level yards prior to sodding or overseeding. Compost is best when overseeding an existing yard.
In comparison to compost, topsoil typically does not retain moisture well. A thin layer of topsoil topdressing will dry out quickly, which can kill grass seedlings. Additionally, topsoil is not rich in nutrients since it contains little organic matter.
How Use Compost When Overseeding: 5 Steps
Now that the choice is obvious, it’s important to use your compost top dressing correctly to achieve the following goals:
- Create a moist, safe environment for grass seeds.
- Feed your existing soil with nitrogen-rich compost.
A good compost topdressing results in higher seed germination rates, better grass survivability over time, and reduced dependency on fertilizers to jump start your lawn. Follow the process below for great results.
Mow your grass shorter than usual, to a height of 1.5–2 inches (4–5 cm). This will make all the following steps easier. It’s a lot easier to aerate a yard, apply a layer of compost, and ensure good seed-to-soil contact when long grass isn’t hindering the job.
- Mow tall turf grasses (Bluegrass, Fescue, Rye, St. Augustine, etc.) to a height of 1.5–2 inches (4–5 cm).
- Mow short turf grasses (Bermuda, Zoysia) to 1 inch (2.5 cm).
- Soil should be visible between grass blades after mowing.
Mowing at this height ensures a good working space as you overseed. It’s a simple and highly important step.
Use a core aerator to aerate your entire lawn prior to overseeding. This will not only break up thatch, but it will also create small holes for your compost to enter, where it can break down and feed grass roots.
- Rent a core aerator from your local hardware store.
- Aerate your yard thoroughly, in an east-west pattern, then a north-south pattern.
- Leave the cores on the ground where they fall. Do not discard them. They will break down over time.
This step is essential because of the nature of compost. Most of the nitrogen in compost comes in the form of uric acid, which evaporates when exposed to direct sunlight. By aerating to create many small holes in your yard, you allow the compost to fill these holes. Here, the compost will hide from the sun and release helpful nutrients into the soil to feed your grass seed.
Top Dress with Compost
Spread compost on your aerated yard at a rate of 1 cubic yard per 1,000 square feet of lawn. This will provide a layer of compost between 0.25 and 0.33 inches thick. This amount of compost is perfect for feeding and protecting your new grass seed without smothering existing grass.
- Use 1 cubic yard of compost per 1,000 square feet of yard.
- Spread the compost evenly, making sure it does not bury or smother existing grass.
- The compost will naturally fill the holes left by aeration—this is like injecting fertilizer right into your lawn.
Keep in mind, compost breaks down quickly. Although the compost may have a strong smell now and make your lawn seem dirty, in a few weeks all you’ll see are beautiful new grass seedlings springing up in your existing grass.
Spread Grass Seed
Although some advise spreading seed before compost, you’ll get better results by seeding after your spread compost. This process will prevent grass seed from collecting in the holes left by aeration, where it may be buried too deeply to sprout.
- Spread grass seed according to bag rates.
- Use this broadcast spreader, going first in an east-west pattern, followed by a north-south pattern.
Your grass seed may seem unprotected at the moment, but this is only temporary. We will protect it from birds, as well as being washed or blown away, in the next step.
Cover Grass Seed
Use a garden rake to gently cover your grass seed. Grass seed will typically only sprout if it is within 0.25 inches of the surface. Grass seed buried any deeper than this will not sprout. The seed will lie dormant or rot in the soil.
- Rake compost over the grass seed, burying it no deeper than 0.25 inches. It is okay if some grass seed is visible on the surface—this is better than burying seeds too deep.
- Use this lawn roller to roll over the lawn. This will encourage good seed-to-soil contact and increase the number of grass seeds that sprout.
By rolling your lawn until it is firm, you press the seeds and compost together, where the compost can provide a cocoon of nutrients and moisture. Now that seeding is done, you can move on to watering and adding starter fertilizer, so your green lawn takes off.
Is Topsoil or Compost Better for Grass Seed?
Compost before overseeding is a tried and true method for getting the most out of your overseeding efforts. Topsoil doesn’t bring as many benefits for overseeding. When choosing between the two, use compost as your top dressing.
- When overseeding, use compost, not topsoil.
- Compost provides more nutrients and moisture than topsoil.
- Compost breaks down to increase soil performance.
- Topsoil will not provide the optimal habitat for grass seeds and it won’t break down over time, leading to an unsightly, dirty yard.
To encourage a thriving yard, mow low, aerate, and then spread a high-quality compost product in a layer 0.25 inches thick. Then, spread your grass seed and rake the compost to cover the seed. This organic layer of compost will serve as a nursery, encouraging your new seeds to sprout and take root.