Herbicide Types

• Use an herbicide that is specifically labeled for use on St. Augustine grass. The University of Florida IFAS Extension website recommends using a pre-emergent herbicide before the weeds sprout, but a post-emergent formula may be the only way to control established weed populations.
  
  

Pre-Emergent Herbicides

• Pre-emergent herbicides prevent the growth of weeds, but are not effective on weeds that are already present. The University of Florida Extension recommends atrazine, benefin and pendimethalin for control of crabgrass and similar weeds. Pre-emergent herbicides should be applied in late winter/early spring, before the weeds sprout.
  
  

Post-Emergent Herbicides

• Post-emergent herbicides kill weeds that have already sprouted and are the most effective defense against summer broadleaf and grass-type weeds. The University of Arkansas Division of Agriculture website cautions against formulas containing 2, 4-D and MSMA, which can damage the turf, but notes that metsulfuron and halosulfuron can be used safely. The University of Florida IFAS Extension recommends the use of atrazine as both a pre- and post-emergent herbicide. Do not apply post-emergent formulations during drought or heat-wave conditions.