To over-seed an existing lawn, prepare it by raking and aerating it, spread starter fertilizer to give the grass a healthy start, and apply grass seed using the recommended seed rate for the selected grass species. Keep the seeded area evenly moist until germination occurs, and refrain from mowing until the new grass blades are at least an inch in height.
Over-seeding a lawn in late summer or early fall ensures a healthy lawn the following spring. Cut the grass to a 2-inch height, bagging the grass clippings. Use a rake or riding mower with a dethatcher attachment to work across the lawn, loosening and removing the season's accumulated clippings. Make two passes over the area, with the second pass at a 90-degree angle to the first one. Collect and dispose of loose thatch, and then aerate the lawn using an aerator attachment for a riding mower or a walk-behind model. Aeration breaks up heavy clay soils, adding air pockets that increase water absorption.
Prepare the planting area by spreading 1/2 inch of well-aged manure and compost over it. Apply grass seed to the lawn with a rotary spreader, or hand cast the seed with a to-and-fro casting motion of the wrist. To determine the amount of grass seed, use the general rule of 2 to 4 pounds of seed per 1,000 square feet of lawn. Top dress the area with several pounds of remaining manure and compost mixture. Sprinkle the newly seeded lawn three times daily for the first two weeks until germination occurs. Once the seedlings sprout, water the lawn once daily, allowing the soil to become somewhat dry between watering times.