Overseeding vs. Interseeding
Sometimes, these two terms are used interchangeably, and they are quite similar. Both are a practice of sowing seed to improve the density and appearance of the turf. Interseeding is the practice of seeding the same species or species mix into the lawn to increase the density of the turf and help it recover from die-out or thinning. Overseeding is the practice of introducing a new species of grass to an existing stand of turfgrass to temporarily improve its appearance. Sometimes, this is sowing an annual species into the lawn for quick green-up. It can also be the practice of sowing the seed of a cool-season grass into a stand of warm-season grass to green it up in the early spring or late fall when the warm-season grass is dormant. This practice is much more common in warmer parts of the country than in Iowa.
Because the true practice of overseeding (sowing a different species into a lawn) is not frequently done in Iowa, we often use this more familiar term instead of the more appropriate term, interseeding, when we sow the same species of grass into the lawn to improve density (a common practice in Iowa). Practically speaking, the only difference between the two terms is the type of seed sown, so calling it "overseeding" when it's really "interseeding" is not a big problem. Both overseeding and interseeding are done in the same way.
Overseed in Late Summer/Early Fall
Mid-August through mid-September is the best time to renovate a thin lawn. Sowing grass seed in late summer has several advantages over spring seeding. Cool-season grass seeds germinate quickly in the warm soils of late summer. Once the grass germinates, the warm days and cool nights of fall promote rapid turf growth. Also, there will be less competition from weeds as fewer weed seeds germinate in late summer and fall.
Use the Right Seed
When purchasing grass seed, choose a high quality seed mix that is best adapted to the site. In sunny areas, Kentucky bluegrass is the best choice. Select a seed mix that contains at least 2 or 3 bluegrass cultivars. Use a mixture containing Kentucky bluegrass, perennial ryegrass, and fine-leaf fescues in partially shaded areas. The fine-leaf fescues (creeping red fescue, hard fescue, chewings fescue, etc.) are the best grasses for shady locations.
Make Sure You Get Good Seed-to-Soil Contact
To reduce the competition from the established turfgrass, mow the lawn at a height of 1½ to 2 inches. Successful overseeding requires good seed-to-soil contact. Core aerators, vertical mowers, and slit seeders can be used to insure good seed-to-soil contact.
Core aerators are machines with hollow metal tubes or tines. They remove plugs of soil when run over the lawn. To prepare the site, go over the lawn 3 or 4 times with a core aerator. When finished, there should be 20 to 40 holes per square foot. Apply the seed with a drop seeder. Afterward, drag the area with a piece of chain link fence or drag mat to break up the soil cores and mix the seed into the soil.
It's also possible to prepare the site with a vertical mower. When run over the lawn, the knife-like blades of the vertical mower slice through the thatch and penetrate into the upper 1/4 to ½ inch of soil. One or two passes should be sufficient. Afterwards, remove any dislodged debris from the lawn. Sow grass seed over the lawn with a drop seeder. Work the seed into the soil by again going over the site with the vertical mower.
Large areas can also be overseeded with a slit seeder. A slit seeder makes small grooves in the soil and deposits the seed directly into the slits.
Core aerators, vertical mowers, and slit seeders can be rented at some garden centers and rental stores. If you would rather not do the work yourself, many professional lawn care companies can overseed your lawn.
Care After Overseeding
After seeding, keep the seedbed moist with frequent, light applications of water. It's usually necessary to water at least once or twice a day. Continue to mow the lawn at a height of 1½ to 2 inches. Mow the lawn frequently to reduce the competition from the established turfgrass. When you begin to mow the new seedlings, gradually increase the mowing height over the following weeks. The final mowing height should be 3 inches. Approximately 6 weeks after germination, fertilize the lawn by applying 1 pound of actual nitrogen per 1,000 square feet.