Some homeowners want a green lawn all through the winter, and thus overseed their lawn with annual ryegrass. While this practice is OK for a bermudagrass lawn, it is not recommended for St Augustine grass. Here is an excellent article on overseeding grass from Texas A&M: http://www.plantanswers.com/overseeding.htm
If you are planning to “plant” annual ryegrass for a beautiful green lawn all winter long, remember to cut your existing grass shorter than normal for two reasons. First, this allows the seed to better reach the soil. Second, when the new grass germinates, it will be considerably taller than the brown stubble of your old existing grass. Keep the soil moist to speed germination.
New turf-type annual ryegrass varieties developed at the Texas AgriLife Research Center in Overton include Axcella, Axcella 2 and Panterra.
As your ryegrass grows, it should be cut no more than 2 inches tall, and be sure to mow often enough so that you’re removing no more than one-third of the new growth at a time.
Be sure to fertilize with nitrogen during the growing period for best performance. You will be the envy of your neighborhood.
Hugh Autrey, former Smith County Master Gardener
Texas A&M AgriLife Extension Service