While St. Augustine grass, with its full blades and lush cover, is one of the preferred warm-season grasses in many areas, it often must contend with crabgrass. Given the fact that both are warm-season grasses that flourish in the same conditions, controlling established crabgrass without harming St. Augustine is a difficult task. Though it may take some patience and perseverance, you can get rid of crabgrass and keep most of the St. Augustine grass relatively unscathed.
Kill all existing crabgrass with a post-emergent herbicide, pulling by hand or blocking light to the affected area. This will not kill the seeds left behind, but will stop new seeds from being produced.
Apply a pre-emergent herbicide just before the overnight soil temperature reaches 50 degrees by monitoring with a soil thermometer. The only time crabgrass seeds can germinate is once the soil temperature stays at 50 degrees or warmer for at least three days in a row.
Use St. Augustine grass plugs spaced 1 to 2 feet apart to fill in any dead spots in the grass. This helps establish the desired grass species before the crabgrass has a chance to grow back in the affected area and plugs are not hurt by pre-emergent herbicides.
Shade the area partially with an awning or some other method to prevent the crabgrass from growing back as well. St. Augustine grass can thrive in partial shade but crabgrass cannot.