St. Augustine grass, a warm-season turf common in the southern United States, is prized for its dense, carpet-like appearance. Its naturally thick growth habit aids in weed suppression, but the turf is notoriously sensitive to many common herbicides used on other lawn types. Applying the wrong chemical can severely damage the broad blades of St. Augustine grass, leading to discoloration, thinning, or even complete loss of the lawn. The solution lies in a careful, multi-pronged strategy that prioritizes cultural practices and utilizes only specific, labeled herbicides to safely manage weeds.
Cultural Practices for Weed Suppression
The most effective defense against weeds is maintaining a consistently healthy and dense St. Augustine lawn, which naturally limits the space and light available for weed seeds to germinate. This turf thrives when mowed at a higher setting, specifically between 3 and 4 inches, allowing the grass blades to shade the soil surface. Shading the soil prevents sunlight from reaching weed seeds, reducing the germination rate for many common annual weeds.
Watering habits also directly influence weed populations. Deep, infrequent irrigation encourages a robust, deep root system. Conversely, frequent, shallow watering promotes shallow root growth and creates a moist surface environment where weeds, such as dollarweed, rapidly take hold. Proper fertilization during the active growth period, typically spring and summer, ensures the turf maintains its density, crowding out invaders by monopolizing soil nutrients and light.
Safe Post-Emergent Weed Control
When weeds have already sprouted, a post-emergent herbicide is necessary. Selecting the correct active ingredient is paramount to avoid injuring St. Augustine grass. The safest and most frequently recommended active ingredient for broadleaf weed control is Atrazine, which is effective against common invaders like clover, dollarweed, and chickweed. Atrazine works by interfering with photosynthesis and often provides a secondary short-term pre-emergent barrier in the soil.
For a wider range of control, especially for stubborn weeds, homeowners should look for products containing specialized combinations of active ingredients labeled explicitly for St. Augustine. These often include compounds like thiencarbazone-methyl, iodosulfuron-methyl-sodium, and dicamba, which together target numerous weed species. Specialized products are also available for sedges and nutsedge, which Atrazine does not effectively control; these typically contain halosulfuron-methyl.
Application timing is important when using any post-emergent product, as the turf is sensitive to chemical burn under high heat or stress. Herbicides should be applied when the grass is actively growing and healthy, avoiding drought stress. Avoid applying post-emergent chemicals when air temperatures are consistently above 85 to 90 degrees Fahrenheit, as this can lead to severe turf damage.
Pre-Emergent Strategies for Future Protection
A long-term strategy for a weed-free lawn involves applying pre-emergent herbicides, which create a chemical barrier in the soil to stop weed seeds from sprouting. This preventative approach is particularly useful for grassy weeds like crabgrass, as few safe post-emergent options exist for St. Augustine once they are established. Application timing is determined by soil temperature, not air temperature, and requires two main applications each year to manage both summer and winter weeds.
The first application should occur in late winter or early spring when the soil temperature at a two-inch depth consistently reaches 55 degrees Fahrenheit, signaling the germination of summer annuals like crabgrass. Safe and effective active ingredients include Prodiamine and Pendimethalin, which form a protective layer lasting several months. A second application is necessary in the late summer or early fall when soil temperatures drop to about 70 degrees Fahrenheit to prevent the germination of winter annual weeds, such as annual bluegrass (Poa annua).
For the pre-emergent barrier to be effective, the product must be “watered in” immediately after application, typically with about half an inch of water, to activate the barrier in the top layer of the soil. Pre-emergent herbicides prevent seeds from establishing, so they should not be applied if you are trying to plant new St. Augustine seed, sod, or plugs, as the chemical will interfere with the establishment of the grass’s roots.