St. Augustine grass is a warm-season turf common across the southern United States, prized for its thick, blue-green texture and ability to tolerate heat and moderate shade. Maintaining this lawn requires careful timing of fertilization and weed control, often called “weed and feed.” Success depends on aligning products with the grass’s active growth cycle and specific vulnerabilities. Because St. Augustine grass is sensitive to certain chemicals and easily damaged if treated incorrectly, understanding its needs is fundamental to achieving a healthy, dense lawn.
Specific Needs of St. Augustine Grass
St. Augustine grass actively grows when temperatures are high (80°F to 95°F) and goes dormant when cold. Applying fertilizer during dormancy can stimulate tender growth easily killed by frost, weakening the turf. The grass requires high nitrogen levels to promote its characteristic dense, horizontal growth. It is particularly sensitive to phenoxy compounds, such as 2,4-D, common in general weed and feed products, which can severely injure or kill the grass. For safe weed control, specialized products containing Atrazine or metribuzin are recommended.
Spring Application: Waking Up the Lawn
The first application occurs in the spring once the grass emerges from winter dormancy. Timing is governed by soil temperature, starting when the soil consistently reaches 55°F at a four-inch depth, which is the ideal window for pre-emergent herbicide application. It is best to wait until the grass has been mowed at least twice, indicating active growth and reduced frost risk. The first fertilizer should be a slow-release, high-nitrogen formula (e.g., 15-5-10) to encourage deep green color and recovery. Slow-release nitrogen provides a steady food source over eight to twelve weeks, preventing a sudden growth surge that can lead to thatch or disease issues.
Summer Maintenance and Growth Support
Summer (late May through August) is the peak growing season, requiring continued nutrient support through additional feedings, usually a second or third application of high-nitrogen fertilizer every eight to ten weeks. A high-nitrogen, low-phosphorus analysis (e.g., 21-0-0 or 15-0-15) is appropriate for established lawns. Weeds that break through the pre-emergent barrier require post-emergent herbicides. Use a product specifically labeled safe for St. Augustine grass, such as those containing Atrazine, and follow instructions strictly. Post-emergent herbicides should never be applied when air temperatures exceed 90°F, as this can severely burn the turf.
Fall Preparation for Dormancy
The final application, often called a “winterizer,” occurs in early to mid-fall, four to six weeks before the first expected frost. The purpose is to strengthen the root system for winter survival and encourage earlier spring green-up, not to promote top growth. Therefore, the fertilizer must be high in potassium (K) and low in nitrogen (N), with analyses like 15-0-15 being common. Applying high-nitrogen fertilizer too late stimulates tender leaf growth easily damaged by cold temperatures. This late-season application is also the last opportunity for necessary broadleaf weed control before the grass enters full dormancy.