It is often necessary to overseed an existing lawn to strengthen weak areas, fill out bare spots or thicken the entire lawn growth. Warm season grasses should be overseeded in the early spring months and cool season grasses in September. Refrain from using weed killer and weed prevention chemicals at least one month prior to seeding, as these can prevent grass seed germination. Follow an aggressive watering plan to keep the seeds moist once they are sowed in order to induce germination.
Test your soil so that you can make the necessary amendments prior to seeding. Contact your local county university extension office for a soil test kit and follow the instructions for submitting a soil sample of the lawn area being planted.
Prepare the lawn area by mowing the existing grass to a length of 1 1/2 inches. Use a grass bagger on the lawn mower or thoroughly rake and remove the clippings from the lawn.
Remove thatch from the lawn by renting a de-thatching machine and running it across the lawn area being seeded. Aerating the lawn is also beneficial and can be done instead of de-thatching if the equipment is available to rent.
Make amendments to the soil based on the soil test results. The results will tell you the soil pH and if there are any nutrients that are low in the soil.
Sow grass seeds over the lawn area at double the normal rate. You need a higher seed application when sowing over an existing lawn.
Apply a nitrogen fertilizer in a slow-release formula over the lawn after seeding. Water the entire area after fertilization to stimulate absorption.
Water the seeded area on a regular basis to keep the seeds moist for successful germination. Watering lightly three to four times a day is best until sprouts are visible.
Apply a nitrogen fertilizer in a quick-release formula approximately five weeks after the seeds have germinated. Repeat this application six weeks later.
Mow the lawn to a length of 2 inches once it has reached a height of 3 inches.