St. Augustine (Stenotaphrum secundatum) is a warm-season grass that grows in tropical, subtropical and Mediterranean-type climates, primarily U.S. Department of Agriculture plant hardiness zones 8 through 10, where it creates a thick green lawn when its basic cultural requirements for sunlight, nutrients, moisture and warm temperatures are met. A lush turf provides excellent weed control, but when necessary, the use of selective pre- and post-emergence herbicides and local application of nonselective herbicides can help to control unwanted weeds and grasses in your St. Augustine lawn.
Opportunistic cool-season weeds such as chickweed, clover and henbit can take hold in your St. Augustine lawn during the winter dormancy period. A selective hormone-based herbicide that kills broadleaf weeds rids your St. Augustine lawn of these weeds when applied according to label directions in the spring after the lawn greens up. Avoid the use of products containing 2,4-D, dicambra and mecoprop on St. Augustine, as damage from these herbicides is likely to occur on this grass. Product formulas designed specifically for St. Augustinegrass can contain atrazine, but follow cautions for use carefully to avoid the potential for personal injury and contamination of ground water.
Pre-emergence herbicides containing dithiopyr control for cool-season grasses, such as fescue, crabgrass and bluegrass and small-seeded broadleaf weeds through interruption of the growth process when applied according to directions. Do not use pre-emergence herbicides on lawns over-seeded with annual ryegrass.
Manage difficult-to-control broadleaf weeds, such as dandelions, and perennial grasses with a post-emergence nonselective herbicide that kills any plant it touches. Removal of unwanted weeds and grasses with a nonselective herbicide requires special care to avoid damage to your St. Augustine lawn. Use a narrow stream of spray or apply with a paint brush to selected weeds or grasses to minimize damage to surrounding St. Augustinegrass. Nonselective herbicides for home and garden use contain glyphosate, such as Roundup, that attack plants systemically, or organic food-grade oil compounds, such as Pharm Solutions Organic Weed Killer, designed to kill all weeds and grasses on contact. Organic products sometimes require more than one application.
A thick turf of St. Augustine grass crowds out weeds. Shade out cool-season weed and grass seeds by raising the mowing height 1/2 inch in fall, to avoid germination of these seeds during Augustinegrass dormancy. Overseed with annual ryegrass to provide cover during the winter. Irrigate your lawn during dry spells in winter, whether overseeded or not. Take a soil sample in spring to determine lime and fertilizer needs, or apply 1/2 to 1 pound of nitrogen after the lawn greens up. To calculate the amount of fertilizer to use for application of the desired amount of nitrogen to 1,000 square feet of lawn, divide the percentage of nitrogen, represented by the first number in the fertilizer analysis, into the amount needed for the desired application. For example, to apply 1/2 pound of nitrogen using a fertilizer blend with analysis 4-1-2, divide .5 by .04, which equals 12.5 pounds per 1,000 square feet. Irrigate to a depth of 6 inches whenever your grass shows signs of wilt during spring and summer. Apply 1/2 to 1 pound of nitrogen every 4 to 8 weeks June through August using a balanced fertilizer blend.
- University of California Agriculture and Natural Resources: St. Augustinegrass
- Texas A&M AgriLife Extension: St. Augustine Grass
- Clemson Cooperative Extension: St. Agustinegrass Maintenance Calendar
- Monsanto: Frequently Asked Questions
- Pharm Solutions: Organic Weed Killer
- LSU Ag Center: Weed and Feed Damage
- Dow: Dithiopyr
- Colorado State University Extension: Calculating Fertilizer Application Rates
- Plant Breeding: Selection for Freezing Tolerance in St. Augustinegrass Through Somaclonal Variation and Germplasm Evaluation
- Jupiterimages/Photos.com/Getty Images