If you have Cool season grasses (Kentucky bluegrass, perennial ryegrass, or tall fescue) it is better to overseed earlier - late February to mid March or so. The idea is to get grass planted a good three weeks before it is time to start mowing the lawn. Anytime in the spring is fine, but you are likely to get a better result if you start early.If you have Warm season grass (bermuda grass, zoysia grass or buffalograss) it is better to overseeded towards the end of season - late April to the beginning of June. The warmer weather is associated with the beginning of the growing season for these types of grasses
The steps for overseeding both kinds of lawns is the same. Below are some tips to help you use that method successfully.
1. Cool season grass starts to grow when air temperatures are about 60 to 75°F, and when soil temperatures are about 50 to 55°F. Warm season grass grows better when the temperatures get up to around 80°Fand the ground temperature is above 70. Too early or too late in the season will lengthen the time it takes for the seeds to germinate, or prevent them from germinating at all
2. If your soil is compacted, areate the lawn before planting seed .It will make it easier for the new grass to establish.
3. You can choose seed that matches the cool season species in your lawn, but you could also introduce a new variety, which would increase your lawn’s tolerance to drought and disease.
4. Mow the lawn very short before overseeding, but do not scalp it. When the new grass starts to grow you will want it to get enough sunlight and the taller grass around it could block that out.
5. Sow the recommended amount of seed split in two different directions across the yard. There should be directions for how much seed to use in the instructions on the bag. You can sow by hand or with a seed spreader.
6> Good seed to soil contact is important to the germination process. Raking the lawn after seeding, especially in thinner spots, is helpful. Add an 1/8th inch layer of topsoil to bare spots.
7. The most important step is to keep the grass seed moist. Water briefly & frequently until the grass starts to grow. Once the grass is visible, switch to watering once or twice a week for longer periods of time