If your lawn is looking a little thin and worse for wear, overseeding is an easy way to fill in the bare spots and restore your lawn to its original thickness. You can also use the overseeding technique to add cool-season grasses to a lawn composed of warm-season grass, adding year-round green to southern and mid-southern lawns. Overseeding is most appropriate for healthy lawns that are in need of a boost.
Mow your lawn as closely as possible and rake up the clippings. Set your mower to its lowest setting, 1½ to 2 inches if possible. If your grass is more than 6 inches long, mow to a height of 3 inches, followed by a second pass at the lowest setting.
Rent a power vertical mower, or power rake and dethatch your lawn. The knife-like blades of these machines will slice through the matted thatch layer to a depth of 1/4 to 1/2 inch and expose a layer of topsoil. The Iowa State Extension recommends one or two passes with the dethatcher to open up the soil. After dethatching, rake up any debris left behind by the process.
Aerate your lawn with a core aerator as an alternative to dethatching. This is another piece of rentable equipment. It has hollow tines that pull cores of soil out of your lawn to allow water and air to penetrate to your lawn's roots. The Iowa State Extension recommends making two or three passes with the aerator to obtain about 20 to 40 holes per square foot. This process will leave cores of soil behind. These can be raked up or broken up by dragging a piece of chain link fence behind a rider mower or tractor.
Spread the seed mix at the rate recommended for the species you've chosen. Use a drop, broadcast or handheld spreader. Rake lightly to help the seed make firm contact with the soil.
Water regularly. Water once or twice a day for the first two or three weeks. Once the seedlings are established, cut back on watering, applying only as much as needed to prevent wilting.