There are two different reasons for overseeding in the fall.
Cool season grass (Kentucky bluegrass, perennial ryegrass, or tall fescue) is overseeded to maintain the thickness of a lawn, especially if it has thinned due to summer heat.
Warm season grass (bermudagrass, zoysia grass or buffalograss) is overseeded with perennial ryegrass, a cool season variety, when the warm season grass becomes dormant right before winter. This creates a green lawn all year round.
The method for overseeding both kinds of lawns with cool season grass seeds is the same. Below are some tips to help you use that method successfully.
1. Cool season seeds germinate when air temperatures are about 60 to 75°F, and when soil temperatures are about 50 to 55°F. Planting 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, you should aerate the soil before you plant the new seeds. Pulling out plugs of soil is stressful to an established lawn and therefore would be devastating to new seedlings. An aerated soil also has the benefit of new spaces for seeds to grow in.
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.
Perennial ryegrass is the only option for overseeding warm season lawns because it germinates quickly and has unfavorable conditions to compete with come spring, when the warm season grass begins growing again. Pick a grass seed with a high pure live seed rate, a high germination rate, and no weeds or filler material.
4. Before you overseed, mow the lawn very short, almost to the point of scalping it. When the seeds start growing, you will want them to have enough access to sunlight, and taller grass can block much of it.
5. Sow the recommended amount of seed split in a horizontal and then vertical direction across the lawn. There should be recommendations of what rate to overseed at on the bag. You can sow by hand or with a seed spreader.
6. Good soil and seed contact is vital to the germination process. For this reason you should rake the lawn lightly after you’ve overseeded. You could also add a 1/8-inch layer of topsoil and firm it with a roller, although it’s not necessary.
7. After the seeds have been planted, keep the soil lightly moist by watering briefly and frequently several times a day. If the soil is too dry, the seeds could die before germinating. If the soil is too wet, they could get washed away. Once the new grass is visible you can switch to watering deeply and infrequently.