We apologize but the Ortho Weed B Gon Plus Crabgrass Control Ready-To-Use 2 is not safe to use on St. Augustine grass. You may want to look at Blindside Herbicide WDG which can be used to control crabgrass (along with many other weeds) in St. Augustine grass.
Ortho Weed B Gon Plus Crabgrass Control Ready-To-Use 2 is a Ready-to-Use product and intended for the treatment of individual weeds, patches of weeds or sparse weeds scattered through home lawns. It is not meant to be broadcast over the entire lawn.