Overseeding refers to the practice of adding seeds to an existing lawn in order to fill in bare spots and create a fully lush lawn. Most lawns develop thin or bare spots over time, so overseeding is a very common practice among homeowners who want to keep their lawn looking its best. September is the prime month for overseeding Kentucky Bluegrass (Poa pratensis).
Mow the lawn down to 1 inch. By doing so you'll make it so your seedlings won't have to compete with fully grown grass for sunlight.
Rake the lawn with a thatch rake, removing as much dead undergrowth from your lawn as possible, especially in the thinning or bald spots. Allow the rake to scratch the surface of the soil in the thin and bare spots.
Run a core aerator over the lawn in at least two different directions. A core aerator will pull plugs of soil from your lawn, giving the new seed a good place to begin growing.
Use a seed spreader to spread Kentucky Bluegrass seed over the entire lawn, being especially mindful to get plenty of seed in the thin and bare spots. Use 3 pounds of Kentucky Bluegrass seed for each 1,000 square feet of lawn.
Cover the entire lawn with 1/4 inch of organic manure. Not only will the manure provide plenty of nutrients for the new lawn, it will also help to hide the seeds from pesky birds.
Water the lawn well. Put at least 1 full inch of water on the lawn initially, then keep the lawn moist but not soggy for the next 21 days. This may require watering the lawn more than once a day in extremely hot or windy weather. Expect your new seed to sprout within 7 to 10 days. Do not walk on the new lawn for 21 days.
Mow the lawn 28 days after planting, with your lawnmower set to 3 or 3 1/2 inches.