St. Augustine grass thrives in moist, warm areas with mild winters. Unfortunately, this is just the sort of climate that weeds enjoy as well. The best defense against the takeover of weeds in St. Augustine grass is a lush and thick lawn. St. Augustine will grow if neglected, but its thin coverage is a welcome mat for weeds. If weeds do manage to wheedle their way into your lawn, they are easy to get rid off with a combination of mechanical and chemical control.
Keep your St. Augustine grass short in spring and summer, around 2 inches in height, by mowing it once every 5 days. Most weeds cannot survive frequent mowing and their seeds have trouble germinating in full sunlight. In the fall, let your St. Augustine grow to 3 inches in height. This will help the grass store energy for the winter and the increased coverage will prevent weeds from invading while the grass is dormant.
Identify the type of weeds that are invading your St. Augustine grass. Annual, perennial, broadleaf and grass-type weeds all require different measures of control. If you are unsure of the type of weed in your yard, uproot it and take it to your local garden center or county extension office for identification.
Pull all of the weeds up by hand, taking care to pull as much of their root systems up as possible. This may be enough to rid your lawn of annual weeds. Perennial weeds are likely to grow back, but this will stop their spread and better prepare them for chemical control.
Spray any weeds that grow back with a post emergent herbicide prescribed for use on that specific type of weed. Coat the weeds evenly in herbicide, but stop just before the point of run-off.
Re-spray the weeds, if needed, at the interval prescribed by the herbicide. Do not treat sooner than that or you may kill your grass along with the weeds.