Overseeding is the process of planting new grass seed into an already-existing lawn. Overseeding is often required when lawns start to thin due to poor cultural management, disease or environmental stresses like drought or heat. If the damaged lawn contains 50 percent or more of desirable grass species, it can most likely be improved through overseeding, suggests the Iowa State University Extension. Overseeding achieves best results when done at the right temperature.
The ideal temperature for overseeding is when the soil and air are warm enough for the grass seed to germinate but not too hot as to cause stress, recommends the Illinois State University Extension. Grasses germinate best between when the average midday temperatures are in the low 70s or when the soil temperature at a 4-inch depth is in the mid 70s.
The time for overseeding warm- and cool-season grass differs. The ideal time to overseed cool-season grass is during later summer or early fall, in September or October, at least 20 to 30 days prior to the first hard frost -- with the next best choice in March or April. Warm-season grasses are best overseeded in late spring. The recommended time for seeding is between mid-April to mid-May.
Preparation of the site at the right time is an important factor in the success of overseeding. There needs to be good seed-to-soil contact with warm temperature soil for healthy germination. Even if the site is well prepared, seeding too late in the fall, when soil starts to get cold, results in poor germination. Start by mowing the existing lawn at a low setting, where the blade is a little over the soil line. This will help reduce competition with seedlings. Rake away clippings and debris. In case of thatch buildup, use a dethatcher or verticutter to dethatch the lawn. Aerate the lawn after dethatching to allow better penetration of air and water into the soil.
Overseed the lawn using a handheld, drop or broadcast spreader. Use two to three times more than the recommended amount of seed on bare areas. Rake in seed in the soil to a depth of 1/16 to 1/8 of an inch. Keep the seeded area moist by watering lightly and frequently until seeds have germinated. Once grass is established, increase the duration and reduce the frequency of watering. Do not apply any starter fertilizer for three weeks after overseeding.