St. Augustine grass is a popular warm-season turf, admired for its dense, carpet-like appearance and its ability to tolerate heat and moderate shade. It thrives in the warm, humid climates of the southern United States. Maintaining this lush appearance requires careful weed management because St. Augustine is notably sensitive to many common herbicides. A general-purpose weed killer can easily damage or destroy the lawn. This guidance offers specific, safe solutions to control weeds while preserving the health and density of your St. Augustine turf.
Understanding St. Augustine Sensitivity
St. Augustine grass (Stenotaphrum secundatum) is susceptible to certain chemical classes of herbicides. Unlike tougher grasses, its physiological structure reacts poorly to active ingredients found in many standard broadleaf weed killers. This intolerance is referred to as phytotoxicity, resulting in visible damage such as leaf yellowing, stunted growth, or turf dieback.
The grass spreads aggressively via above-ground runners (stolons), creating a thick, dense canopy. While this growth habit helps crowd out weeds, it does not prevent chemical damage. Herbicides containing common ingredients like 2,4-D, dicamba, or quinclorac, which are safe for Bermuda or Zoysia grass, must be avoided or used only in specialized, turf-safe formulations. This sensitivity necessitates a highly selective approach to target weeds without harming the turf.
Safe Chemical Control Options
Post-Emergent Control (Existing Weeds)
Controlling weeds that have already sprouted requires a selective post-emergent herbicide that can eliminate the invader without injuring the St. Augustine turf. The primary and most trusted active ingredient for this purpose is Atrazine, effective against a wide range of broadleaf weeds. Atrazine is a systemic herbicide that inhibits photosynthesis, offering both post-emergent and limited pre-emergent control.
Atrazine is useful for tackling problematic broadleaf weeds such as dollarweed, clover, and chickweed. While highly effective, its action is often slower than other herbicides, meaning it may take several weeks to see the full effect. It is crucial to use products where Atrazine is the sole or primary active ingredient and is specifically labeled for use on St. Augustine grass.
For more difficult weeds, such as sedges (nutsedge), a different approach is necessary, often involving the active ingredient halosulfuron-methyl. This chemical specifically targets sedges and their underground tubers (nutlets), providing long-term control.
Another powerful option for a broader spectrum of difficult-to-control broadleaf and some grassy weeds is the herbicide Celsius WG. It contains a combination of three active ingredients: thiencarbazone-methyl, iodosulfuron-methyl-sodium, and dicamba.
Combination products that include 2,4-D or dicamba must be specifically formulated and labeled for St. Augustine grass. These specialized formulations use lower concentrations or safer derivatives to minimize the risk of phytotoxicity. Always check the product label to ensure it explicitly states safety for your specific turf variety.
Pre-Emergent Control (Prevention)
The most effective strategy for long-term weed management is preventing annual weeds from germinating using pre-emergent herbicides. These products form a chemical barrier at the soil surface, inhibiting the initial root growth of emerging weed seedlings. This is the best defense against annual grassy weeds like crabgrass, which are difficult to control once established in St. Augustine turf.
The most common and safe active ingredients for pre-emergent control are prodiamine and pendimethalin. Both create a long-lasting barrier that prevents the germination of a wide spectrum of annual weeds. Dithiopyr is another reliable option, offering the added benefit of providing early post-emergent control for very young crabgrass seedlings.
Timing the application is paramount, as it must be applied before the weed seeds begin to sprout. For spring annual weeds, apply the product when the soil temperature consistently reaches 55°F at a four-inch depth. A second application in late summer or early fall is often recommended to prevent the germination of winter annual weeds like annual bluegrass.
Cultural Practices for Weed Minimization
A healthy, dense stand of St. Augustine grass is the most effective defense against weed invasion, significantly reducing the need for chemical intervention. Adjusting lawn care routines to favor turf growth naturally minimizes weed pressure. Cultural practices create an environment where the grass thrives and weeds struggle to compete for sunlight, water, and nutrients.
Mowing at the correct height is a powerful tool for weed control. St. Augustine grass should be maintained between 3 and 4 inches tall. This height allows the grass blades to cast shade over the soil surface, blocking the sunlight required for many weed seeds to germinate. Keeping the grass taller also promotes deeper root growth, making the turf more resilient to environmental stress.
Proper watering techniques are equally important. St. Augustine benefits from deep, infrequent watering, where the soil is soaked to a depth that encourages the roots to grow downward. Applying about one inch of water once or twice a week is typically better than light, daily watering. Shallow watering promotes weak roots and keeps the surface soil moist, which is ideal for weeds such as dollarweed to flourish.
Maintaining a balanced fertilization schedule ensures the turf remains dense and vigorous throughout the growing season. Providing necessary nutrients, particularly nitrogen, allows it to grow thick enough to crowd out opportunistic weeds. For isolated or sparse populations, manual removal is a chemical-free solution, provided the entire root system is carefully pulled to prevent re-sprouting.
Applying Herbicides Effectively and Safely
The success of chemical weed control relies heavily on the method and timing of application to maximize efficacy and ensure turf safety. The single most important factor when applying post-emergent herbicides is temperature control. Applying herbicides when temperatures are too high can cause severe damage or turf burn.
It is advised not to apply most post-emergent herbicides, especially those containing Atrazine, when the air temperature exceeds 85°F to 90°F. The best time for application is typically early morning or late evening when the turf is not under heat stress. Avoid applying any herbicide during periods of drought or when the lawn is otherwise stressed, as the grass is more vulnerable to chemical injury.
The decision between a targeted spot treatment and a broadcast application depends on the severity of the weed infestation. Spot treatments are the preferred method for isolated weed patches, as they concentrate the chemical only where it is needed and reducing the risk of widespread turf injury. A full broadcast application should only be performed when the weed pressure is widespread and requires comprehensive coverage.
Accurate calibration and dilution are necessary to prevent over-application, which is a common cause of turf damage. Always read the product label for the correct mixing ratio and application rate per area. Using a calibrated sprayer ensures the herbicide is applied at the precise concentration required for effectiveness. For pre-emergent granular products, watering the lawn immediately after application is necessary to activate the chemical barrier in the soil.