Crabgrass is not actually a grass at all but is an invasive weed that can overpower your lawn and destroy your lush front yard. Especially common in the south and eastern United States, crabgrass can reach heights up to three feet. It can be extremely hard to get rid of, and you may find yourself in an endless battle to save your lawn in the warmer months. But crabgrass does not have to be a problem--you can kill crabgrass in St Augustine grass with the right materials.
Monitor the temperature of your soil in the mornings in early spring by sticking a meat thermometer one inch into the ground.
Mark each morning's temperature on a calendar and wait until the temperature of the soil is above 50 degrees for three days in a row.
Spray your lawn with a pre-emergence herbicide.
Water your lawn with one-half of an inch of water to activate the herbicide.
Monitor your grass and note the first emergence of crabgrass.
Mix your crabgrass killer for St. Augustine grass as per the instructions, and spray the area of crabgrass growth.
Refrain from watering your lawn for at least 24 hours to prevent spreading of the herbicide.
Begin watering your grass normally after at least 24 hours and be patient, as the surrounding St. Augustine grass may yellow or brown a bit but will grown back in on its own.