Ryegrass for Winter Color
Some species of warm-season turf grass commonly grown in U.S. Department of Agriculture hardiness zones 6 to 10, such as zoysiagrass (Zoysia spp., hardy in USDA zones 6 to 9) and Bermuda grass (Cynodon spp., hardy in USDA zones 7 to 10), have the disadvantage of going dormant and turning brown when the weather turns cool in the winter. A solution to that problem is to overseed the turf with ryegrass (Lolium spp.), a cool-season grass that maintains its green color through the winter and then dies out in the spring, leaving the warm-season grass to take over again as it regains its color. When they're used in this way, the combination of the two grasses are often referred to as winter ryegrass.
Annual and Perennial Ryegrasses
Perennial ryegrass (Lolium perenne) is the preferred choice for winter overseeding in warm climates because it produces a better turf than annual ryegrass (Lolium multiflorum). Annual ryegrass will die in the spring, so it is less likely to compete with the established warm-season turf grass as the weather warms. Perennial ryegrass is hardy in USDA zones 7 and below, so it will likely persist later in the spring than annual ryegrass. It's not tolerant of heat and drought, though, so it's unlikely to survive for long in warm, dry climates.
Ryegrass in the Summer
Perennial ryegrass may be able to survive in sheltered, shady areas of the lawn, and when it persists through the summer season, it can cause problems as it competes with the warm-season turf grass. Ryegrass is also susceptible to diseases in warm weather, so it's not a good all-season turf grass on its own in warm climates. Both annual and perennial ryegrasses may compete with the established warm-season grasses during especially cool springs, so it may be necessary to discourage the growth of ryegrass in the spring. Stopping fertilization in March and mowing very closely will help to kill the ryegrass and allow the warm-reason grass to take over.
Ryegrass vs. Rye
Ryegrass is sometimes confused with rye (Secale cereale), an annual cereal grass usually grown for its grain or as a forage crop for livestock or wildlife. Further confusing the distinction is that some varieties of rye can survive through cold winters and are grown as a winter cover crop in farm fields and pastures; these varieties are often called winter rye, but they are unrelated to ryegrass and are never used as a turf grass.