Overseeding an existing lawn can be a useful solution for a number of lawn challenges. With overseeding, you can create a year-round lawn by mixing cool and warm season grasses. It can also be used to slowly switch over from one varietal of grass to another or create a blend of compatible grasses to suit a specific look or set of conditions. Overseeding is a straightforward job for a DIYer and can easily be completed within a day for the average size patch of lawn.
Dethatch or aerate the existing lawn to remove excess thatch and create entry points for the new grass seed roots to reach and knit into the soil. Make two passes with either tool with the second pass made at a 90-degree angle to the first to ensure an even removal of thatch. Rake up all of the loose thatch or plugs and discard them in the trash.
Lay down a 1- to 1 1/2-inch layer of good quality compost over the entire area to be overseeded. Rake the compost lightly to spread it evenly.
Sow seed in an even layer over the compost. Spread the seed by hand using casting motions with your wrist. Alternatively, use a seed spreader to achieve an even distribution of seed. Run the rake lightly over the seeded area to nestle the seed into the compost without completely burying it, as most grass seed requires some sun for optimal germination.
Water your freshly sown grass seed deeply with a long and gentle rain or mist so as not to displace the seed or compost. Keep the soil and seed evenly moist at all times through germination which can take up to 3 weeks or more. Water once or twice daily as needed and never allow the soil and seed to dry out.