Planting grass seed on an existing lawn is done for several reasons including patching and overseeding. Patching is done to boost the evenness and lushness of an existing lawn by sowing seed of the same variety that is presently growing. Overseeding is done to change or complement the existing lawn grass by sowing a secondary variety of grass seed. The new grass seed is encouraged to overtake the old lawn without having to remove the old grass. Alternatively, the second variety is sown to provide year-round grass when the first variey goes dormant. In other words if you are growing a warm season grass that dies in the fall and winter you can overseed with a cool season grass seed to maintain a lawn year round. As with seeding a new lawn, planting bed preparation is the key to long term success.
Use a dethaching rake or aerating punch tool to lift portions of excess thatch from the existing lawn and expose some portions of the soil for the grass seed, nutrients and water to settle. Cover the area to be seeded twice with the tool of choice, with the second pass carried out at a 90-degree angle to the first pass to ensure an evenly prepared planting bed. Rake up and collect the loose thatch or plugs and discard orcompost.
Cast several pounds of well aged manure and good quality compost over the grass area to serve as the planting bed for the new grass seed. Use enough of each to spread an even half-inch over the entire area. Withhold a few pounds to sprinkle over the seed once it is sown.
Moisten the soil with a garden hose fitted out with a sprayer attachment. Lay down enough water to soak the soil. Allow the water to percolate into the soil fully before proceeding which takes just five to 10 minutes.
Sow the grass seed by hand over the prepared bed with a to and fro casting motion of your wrist. Grab a handful of seed and cast it over the soil as you walk backward so you don not step on the seed just sown. Use roughly half of the recommended amount of seed for your acreage when reseeding or overseeding. Excess seed on the lawn could cause excess thatch to develop, which results in an unhealthy lawn.
Scatter the remaining few pounds of compost and aged manure over the grass seed to settle it and partially cover it. Most grass seed requires some sunlight for speedy germination so a very thin and slightly uneven layer is ideal.
Water in the seed again deeply with the sprayer attachment set to mist so you don't disturb or displace the soil and seeds. Keep the soil evenly moist at all times until new green shoots appear and never allow the soil to dry out. Resume your regular fertilizing schedule after the new grass blades are at least two inches in height. Refrain from mowing until the new shoots are 2.5 inches tall and well rooted to prevent damage to the tender roots.