Mid-February is a good time to start controlling crabgrass in your lawnIn established turfgrass, members of the dinitroaniline herbicide family ( e.g., oryzalin [Surflan], benefin [Balan], prodiamine [Barricade], or pendimethalin [Pendulum, Pre-M]) provide control of annual grass and some broadleaf weed species when applied prior to weed seed germination.
Oxadiazon (Ronstar), bunsulide (Betasan, Pre-San) and dithiopyr (Dimension) also provide preemergence annual grass and broadleaf control.
Atrazine and isoxaben (Gallery) provide preemergence control of primarily broadleaf weeds. More more details refer to EDIS Publications EP141 (http://edis.ifas.ufl.edu/pdffiles/EP/EP14100.pdf)
Caution: Some so called "Southern Lawn Weed Control" products cannot be used on Floratam St. Augustine lawns without causing harm. Read and follow all label directions for your safety and the safety of your enviornment.