St. Augustine grass is a vigorous, stolon producing, warm season grass capable of crowding out most invasive weeds over time, if it is cared for properly. Crab grass is an annual grassy weed that can be treated with a pre-emergence herbicide, if cultural practices do not keep it under control, according to Texas A&M University. Keep your St. Augustine lawn as healthy as possible by perfecting watering, fertilizing and mowing regimens and then consider if chemical intervention is needed.
Water your St. Augustine liberally to keep it green, lush and growing at as rapid a rate as possible, to crowd out the crabgrass. Apply a minimum 1 inch of water each week, in one or two deep-soaking sessions to keep the roots well hydrated. Less water may be needed when it rains and considerably more may be needed in dry climes, drought conditions or during the summer.
Fertilize your St. Augustine lawn regularly, once a month or once every other month from spring through late fall. Apply according to the product label directions for your expanse of lawn, but do not exceed 1 lb. actual nitrogen for every 1,000 square feet of grass.
Mow your St. Augustine grass at the proper height for its species and light conditions. Keep St. Augustine grass growing in direct sun between 1 and 3 inches in blade height. Keep grass that is growing in moderate to full shade conditions at 3 inches in blade height. Taller St. Augustine grass tends to aid in shading out weed species such as crab grass by denying sunlight to the seeds and plants, disrupting or slowing their growth patterns.
Spray your St. Augustine lawn with a pre-emergence herbicide to control annual grasses, like crabgrass, in each spring that comes after a fall when the crabgrass had been prominent. Apply according to the product label directions each year, in the spring, as needed.