Lawn grasses are categorized according to their preferred season of growth. The kinds that grow best at temperatures in the low to mid 70s are called cool season grasses. These are the grasses of the northern and cool regions of the country. They include bent grass, Kentucky bluegrass, fescues, and ryes. The others, the warm season grasses, are the lawn grasses of the Sun Belt. They grow best at higher temperatures, and usually become brown and dormant during cool seasons. Warm season grasses include bahia, Bermuda, blue grama, buffalo, carpet, centipede, seashore paspalum, St. Augustine, and zoysia.

Seeding rates vary primarily because the size of seeds varies. You'll need fewer pounds of small seeds, and more pounds of larger grass seeds. For instance, a pound of Kentucky bluegrass contains more than 2 million seeds; a pound of tall fescue is only 230,000 seeds.

Because lawn seeds vary in size, a lawn seed mixture that is 50 percent Kentucky bluegrass and 50 percent fine fescue does not mean your lawn will be half of each. Two pounds of such a mixture would equal about 2 million seedlings of bluegrass and 600,000 seedlings of the fescue, or a lawn of two-thirds bluegrass and one-third fescue.

When overseeding dormant lawns, increase seeding rate by 25 to 50 percent.

While we recommend many specific varieties of different grasses, most lawns are either straights, mixtures, or blends of seed. Check with your extension service for varieties that are proven to perform best in your area. Always read the label on the seed mix you buy. Look for the names we recommend in the grass descriptions. If you don't see any named varieties, shop further.

Straights. Warm-season grasses such as Bermuda and zoysia are usually sold as straights, or just one variety of grass. That's because these kinds of grasses don't mix well with other kinds of grasses.

Blends are composed of varieties of one type of grass. Blends increase the genetic diversity of the lawn somewhat, and combine the strengths of the different grasses. The result is a healthier, more resilient lawn.

Lawn seed mixtures: These are combinations of two or more different species of grasses, such as Kentucky bluegrass, fine fescue, and perennial ryegrass. Mixtures have the practical advantage of introducing a much greater genetic diversity into your lawn. A pest or disease is much less likely to gain a foothold, and then less likely to cause serious damage, in a genetically diverse lawn.

Mixtures are usually created in order to tailor the grasses to the situation. For example, you'll see mixtures labeled "Show Lawn," "General Purpose," "Shady Lawn," or "Tough"or "Play" lawn." Typically, fancier lawns mixtures are predominently bluegrass, mixtures for shade are mostly fescue, and tough mixtures are tall fescue or Bermuda, depending upon where you live.

Occasionally warm- and cool-season grasses are combined in order to maximize the strengths of each. An examples is Bermuda and perennial rye: the Bermuda does well in the summer; the rye in the winter. While such combinations can work very well and provide an attractive green lawn year round, you will have to manage such a lawn carefully so that both grasses remain in a sort of balance.