Warm season grasses go dormant during the winter months and can be overseeded with a cool season grass variety to maintain green color and adequate quality. Annual ryegrass, perennial ryegrass or rough bluegrass are the standard grasses used for overseeding. The best time to overseed the home lawn is mid to late October, but more accurately after the first frost. Annual ryegrass is the fastest germinating of the three varieties and probably the cheapest. It looks very similar to perennial ryegrass with a dark green color and shiny leaves. Annual and perennial ryegrass both grow quickly and require frequent mowing (around 2 to 2.5 inch height) especially during late fall and early spring. There water use use rates are moderate and fertility requirements are low - maybe one to two pounds of nitrogen over the winter months. You should overseed at a rate of about 10-12 pounds of seed per thousand square feet. And keep the lawn irrigated for several weeks to ensure germination.
Rough bluegrass has a much smaller seed and can be used successfully in the higher density varieties like fine bladed zoysiagrasses and hybrid bermudagrasses. They require a little less nitrogen and more infrequent mowing (2 to 2.5 inch height). The color is more of a medium to apple green and the seed take a little longer to germinate. You should overseed at a rate of about 6-8 pounds of seed per thousand square feet. And keep the lawn irrigated for several weeks to ensure germination.
Don't be surprised to find these cool season grasses still competing with your warm season lawns in the late spring and early summer. Breeding programs around the country have made new varieties of these grasses that are very resisitant to even Texas weather. Many people stay away from overseeding altogether because of the spring and summer competition with the warm season grass. The warm season grass takes longer to establish and is less healthy in the early summer months, which could spell disaster later on.