Fall is the best time of year to overseed cool-season lawns like tall fescue, but warm-season grasses like zoysia are planted in late spring. Zoysia will soon be going dormant, so new plants will not establish. Plus, zoysia is rarely seeded, as zoysia seed costs more than plugs (how it is usually planted), which are tiny pieces of sod that are ready to root-in. Zoysia plugs tend to be sold for planting around early May or so.

Pure zoysia lawns require dethatching every few years, but tall fescue lawns do not. (Kentucky bluegrass, which isn't often used as a pure lawn type in Maryland, would also need periodic dethatching. Dethatching needs to be done while a grass is in active growth, so that would be early autumn (prior to overseeding) for fescue/bluegrass and early summer for zoysia.

A lawn that has mixed warm-season and cool-season grass should ideally be converted to one or the other, as such mixes are hard to care for since fescue and zoysia require different maintenance regimes, from mowing height to fertilization timing and other aspects of lawn care. If you were overseeding fescue, then you can look for products containing one or more of the recommended turfgrass cultivars that are listed on the linked page. Our Grass Seed page contains other information, like seeding rate, so you know how many pounds of seed to look for based on the size of the lawn are you want to overseed.

Miri