St. Augustine grass spreads itself by stolons and can be vigorous and aggressive against invading species when watered, fertilized and mowed correctly. According to Texas A&M University, St. Augustine grass effectively crowds out most weeds yet can be susceptible to cool season weeds like henbit, chickweed and clover during winter when it is dormant. Proper lawn care practices combined with wise use of hormone-based or pre-emergence herbicides can check the weed colonies and restore the St. Augustine to prominence.
Irrigate your St. Augustine grass regularly, deeply applying a bare minimum of 1 inch of water per week in one or two sessions. Keep the thatch and soil just beneath moist at all times and drench the soil immediately if it feels dry to the touch.
Raise the mowing height on your St. Augustine grass to between 2 and 3 1/2 inches to allow the plants to direct the bulk of their energies to re-establishing healthy and dominant roots. Taller blade length will also help to shade competitive weed seeds from the sun they require to germinate.
Fertilize your St. Augustine grass three times each year in the spring, summer and late fall applying 1 pound of actual nitrogen for every 1,000 square feet of lawn. Cast evenly over the lawn as recommended on the label and water in well until the thatch and top 6 inches of soil is drenched with nutrient laden water.
Combat cool season weeds that invade St. Augustine while dormant, like henbit, chickweed or clover with hormone-oriented herbicides designed to combat those specific cool season grasses but not harm the warm season St. Augustine. Apply in the spring according to product label dosing directions and repeat as recommended and needed.
Treat unwanted competitive grasses or weeds like bluegrass, fescue or crabgrass with a pre-emergence herbicides applied in the early spring according to the product label directions.
Spray unwanted perennial grasses like dallis and Bermuda grasses with non-selective herbicides directly on the plant mass, as these are difficult to control in St. Augustine in any less invasive manner. Apply according to the product label directions at the center of the weed colony and be careful not to misdirect any overspray onto the St. Augustine. Repeat as needed to get the weed population under control.
Spend a little time hand weeding after a deep water session when it is easier to pull the weeds out by the roots. Grasp the entire weed plant around the base between your thumb and forefinger and pull it up and out of the soil. Discard all pulled weeds and do not compost them; not all compost processes are hot enough for long enough to kill the weed seeds.