Source: Iowa State Extension
Overseeding is one way to improve an existing lawn that is in poor condition. Iowa State University Extension and Outreach horticulturists offer tips on how to overseed, what seed to use and how to ensure good germination. For more information about growing and maintaining a lawn, contact the ISU Hortline at 515-294-3108 or firstname.lastname@example.org.
A dense stand of grass can be established by overseeding -- sowing grass seed into an existing lawn. In Iowa, late summer (mid-August to mid-September) is the best time to overseed a lawn.
To reduce the competition from the established turfgrass, mow the lawn at a height of 1.5 to 2 inches. Successful overseeding requires good seed-to-soil contact. Core aerators, vertical mowers and slit seeders can be used to ensure 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 three or four 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 one-fourth to one-half 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.
In addition, lawns can be overseeded with a slit seeder. It makes small grooves in the soil and deposits the seed directly into the slits.
When purchasing grass seed, select 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 two or three 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.
After seeding, keep the seedbed moist with frequent, light applications of water. It’s often necessary to water once or twice a day. Continue to mow the lawn at a height of 1.5 to 2 inches. Mow the lawn frequently to reduce the competition from the established turfgrass. Gradually increase the mowing height when beginning to mow the new seedlings.