The Best Weed Killer for Saint Augustine Lawns

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, in which it makes a thick green lawn when its basic cultural conditions for sunlight, nutrients, moisture and warm temperatures are satisfied. A lush turf offers 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 on your St. Augustine lawn.

Broadleaf Weeds

Opportunistic cool-season weeds such as chickweed, clover and henbit may take hold on your St. Augustine lawn during the winter dormancy period. A selective hormone-based herbicide that kills broadleaf weeds rids your St. Augustine lawn of the 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 very likely to happen on this particular grass. Merchandise formulas designed specifically for St. Augustinegrass can include atrazine, but follow cautions for use carefully to avoid the potential for personal injury and contamination of ground water.

Cool-Season Herbs

Pre-emergence Sensors 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 in accordance with 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 a St. Augustine lawn. Utilize 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 usage contain glyphosate, such as Roundup, that attack plants systemically, or natural food-grade petroleum compounds, such as Pharm Solutions Organic Weed Killer, designed to destroy all weeds and grasses on contact. Organic products sometimes need more than one application.


A thick turf of St. Augustine grass crowds out weeds. Shade out cool-season grass and grass seeds by increasing the mowing height 1/2 inch in autumn, to avoid germination of the seeds during Augustinegrass dormancy. Overseed with annual ryegrass to offer cover during the winter. Irrigate your lawn during dry spells in winter, whether overseeded or not. Have a soil sample in spring to determine lime and fertilizer requirements, or apply 1/2 to 1 pound of nitrogen after the lawn greens up. To compute the amount of fertilizer to use for application of the desired quantity of nitrogen to 1,000 square feet of lawn, split the percentage of nitrogen, represented by the first number in the fertilizer analysis, in the amount required for the desired application. For instance, to apply 1/2 pound of nitrogen by means of a fertilizer mix with evaluation 4-1-2, split .5 by .04, which equals 12.5 lbs per 1,000 square feet. Irrigate to a depth of 6 inches when 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 mix.

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