Crab grass is a weed that emerges in the spring and continues to grow until the first frost in the fall or early winter. Once crab grass appears, it tends to get out of control. For this reason, it is best to start ridding your lawn of crab grass even before it grows, while it is still seed. Fortunately, if any still grows, you can take several measures to kill it, and since you have St. Augustine grass, you will also be able to create an environment unsuitable for crab grass to thrive in.
Apply a crab grass pre-emergence herbicide in the early spring, before the crab grass begins to grow. Many types of herbicides are available, such as ones that contain benfin or altrazine. However, before you purchase a product, check the label to be sure it is safe to use on St. Augustine Grass.
Pull out crab grass with your hands. Do this when the soil is wet and grab the crab grass right at the soil and pull up. Keep pulling until you get all the roots. Throw it out, away from your lawn.
Mow your grass to a height of 2 ½ to 3 inches. This will help shade out existing crab grass and prevent more from growing, including from the roots you may have inadvertently left behind from pulling up the crab grass.
Keep your grass thick and dense so crab grass does not have room to grow. Water it often with about an inch of water, especially during dry hot spells. Also, fertilize St. Augustine grass from spring (when the ground reaches 65 degrees Fahrenheit) and thereafter every six weeks until early fall. Use a granular grass fertilizer labeled for lawns and apply it following label instructions.
Apply a crab grass post-emergence herbicide to kill existing crab grass. Some herbicides, such as those labeled with DSMA, MSMA, CAMA and AMA, are not safe for St. Augustine grass. Rather use one, such as Asulox, that is labeled safe for St. Augustine grass.