By Richard Jauron
Iowa State University Extension
Healthy, well maintained lawns are attractive landscape additions. Lawns in poor condition, however, are somewhat unsightly. The poor condition of a lawn may be due to poor management, heat, drought, diseases, insects or other factors. In severe cases, the existing lawn may have to be destroyed and a new one established on the site. Lawns that contain over 50 percent desirable grasses can often be improved by overseeding.
Overseeding is the sowing of grass seed into an existing lawn. In Iowa, the best time to overseed a lawn is late summer (late August to mid-September).
Good site preparation is necessary for successful overseeding. If possible, identify and correct the problems causing the lawn to decline. Overseeding may only be a temporary solution if these problems are not corrected.
To reduce the competition from the established turfgrass, mow the lawn at a height of 1-1/2 to 2 inches. Successful overseeding also requires good seed-to-soil contact. Simply throwing or broadcasting seed over the lawn typically results in poor seed germination as much of the seed is resting on the thatch layer or soil surface. Rakes, core aerators, vertical mowers, and slit seeders can be used to ensure good seed-to-soil contact.
Overseeding Small Areas
Small areas can be prepared by gently raking the thin spots. When raking, it’s necessary to break the soil surface without pulling out the existing turfgrass. After raking, sow the seed by hand. Then, work the seed into the soil by gently raking the areas a second time.
Overseeding Large Areas
Large areas can be prepared by using a core aerator. 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 the 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 1/2 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 also can 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 many garden centers and rental agencies. If you would rather not do the work yourself, many professional lawn care companies can overseed your lawn.
Post Seeding Care
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 1-1/2 to 2 inches. Mow the lawn frequently to reduce the competition from the established turfgrass. When the new seedlings reach a height of 1-1/2 to 2 inches, gradually increase the mowing height over the next several weeks. The final mowing height should be 2-1/2 to 3 inches. Approximately six weeks after germination, fertilize the lawn by applying 1 pound of actual nitrogen per 1,000 square feet.
When properly overseeded, a thin, scruffy-looking lawn can be turned into a thick, lush lawn in just a few weeks.