Fertilizers & Weed Killers for St. Augustine Grass

Pre-Emergent Herbicides

• Apply a pre-emergent herbicide to kill annual grassy and broadleaf weeds in the early spring before soil temperatures favor their germination. For example, crabgrass germinates when soil temperatures reach 60 degrees Fahrenheit. Spray your lawn with a pre-emergent herbicide before soil temperatures reach 60 degrees, so that the chemicals effectively kill crabgrass as soon as it emerges from seeds. Use a pre-emergent herbicide that contains active ingredients such as benefin, trifluralin and dithiopyr. Water your St. Augustine lawn after applying the pre-emergent herbicide.

Spring Fertilizing

• Fertilize St. Augustine grass when 50 percent of the lawn has greened up or when soil temperatures are above 60 degrees Fahrenheit, as suggested by gardening writer Walter Reeves. Apply 1/2 pound of nitrogen per 1,000 square feet when your grass has started growing for the year. Use a fertilizer that has an NPK amount of 16-4-8 or 12-4-8. The NPK amount represents the percentage of nitrogen, phosphorous and potassium in the fertilizer.

Fertilizing Throughout the Growing Season

• St. Augustine lawns need 2 pounds of nitrogen per 1,000 square feet applied every growing season. Break up the nitrogen into 1/2 pound increments per 1,000 square feet to prevent burning the grass. Excessive amounts of nitrogen dehydrate grass and cause brown spots. Fertilize once a month until September. Use a fertilizer with a high percentage of potassium in the fall. Your fall fertilizer should have an NPK amount of 10-20-20 to help the grass survive winter stress.

Post-Emergent Herbicide

• Use a post-emergent herbicide to kill weeds popping up throughout the growing season. Choose a post-emergent herbicide to kill broadleaf weeds and annual grassy weeds that have active ingredients such as dicamba or fluazifop. Avoid using post-emergent herbicides that contain 2,4-D or MSMA, because they injure St. Augustine grass. Spot-treat weeds rather than applying the herbicide over the entire lawn. Use post-emergent herbicides when you have a 24-hour window of dry weather, so that the chemicals can absorb into the weeds' foliage.