It is essential to plant grass seeds on an existing lawn every few years. After five or six years, grass growth slows down and weeds start to compete for room. A good way to judge whether your lawn needs more grass seed is to separate pieces of grass and look between them. If you can see soil, it's time to overseed. Overseeding every three years will keep your grass looking full and lush, and it will keep disease at bay.
Buy grass seed that is the same variety as the existing lawn. This will keep the grass looking consistent. If you are not sure the type of grass, use a database such as the one offered by the Scott's Miracle-Gro Company or the University of California (see Resources). You can also bring a sample of the grass to a local plant center. Overseed the lawn in late September to give the grass the winter months to germinate.
Mow the grass as short as possible. Put your lawnmower on the lowest setting. It's okay if your hitting soil as you mow. Bag the clippings or collect them if you don't have a bag on the mower.
Rake the existing lawn and remove all debris such as stones and sticks. Leave the stubble of the grass and soil in place. The new grass seeds needs to lay on top of soil to germinate. It will not grow successfully on top of debris or grass clippings.
Apply the grass seed per the manufacturer's instructions. If there are guidelines for overseeding and planting a new lawn, follow those to get the thickest growth.
Spread the seed with a hand spreader if you're overseeding a small portion of an existing lawn. If you're reseeding the entire lawn, use a mechanical spreader.
Set a sprinkler to an oscillating setting to lightly water the area. Do so until it's moist but not soaking wet. Water it twice daily for at least two weeks so it doesn't dry out.
Mow the grass when the blades are 3 inches tall. Cut them down to 2 1/2 inches.