Fall overseeding is particularly effective with lawns containing cool season grasses such as Kentucky bluegrass, perennial ryegrass or tall fescue. Plant when daily air temperatures are 60 to 75 degrees Fahrenheit. Planting too early or too late in the fall season inhibits or outright prevents the grass seed from germinating.
Time fall grass seeding so that germination occurs before freezing temperatures arrive in the area. To prepare the lawn, mow the grass as short as possible, even scalping it to allow optimum seed to soil contact and exposure to sunlight. Remove any debris and dead grass and loosen the top 1/4-inch of top soil in bare spots with a rake.
Using the recommended overseeding rates on the seed bag, sow small areas evenly by hand; first in a horizontal and then in a vertical direction. For larger areas, use a hand or lawn spreader or a mechanical seeder. A good rule of thumb is approximately 16 seeds per square inch. Too many seeds too close together makes grass blades weak and thin because seedlings have to compete for room and nutrients.
Rake the lawn gently after overseeding and apply a light layer of topsoil if desired. The key to germination is to keep the grass seed moist without saturating the lawn. To do this, water lightly and frequently, at least once a day, until the new grass is two inches high. Once it reaches mowing height, the overseeded grass needs water only at regular intervals to keep the roots moist.