Safe Weed Control for St. Augustine Grass

Lawn Management

• Keeping your lawn in a good state of health is the safest way to prevent weeds in St. Augustine grass. Maintaining good soil and thatch condition means you avoid using potentially harmful herbicides. For example, when thatch reaches an inch in depth it can harbor pests and promote weed growth. Power raking the lawn in spring or summer helps break up the thatch. Pulling out single or small weed outbreaks should control non-serious weed problems safely.

Pre-emergence Herbicides

• Some pre-emergence herbicides combat goosegrass and crabgrass outbreaks on St. Augustine lawns, but safe application means treating the lawn before the beginning of March in most warm states, according to experts at the University of Arkansas Division of Agriculture. Both crabgrass and goosegrass don't suit treatment after they appear. When they do grow in a lawn, wait until the following spring before applying any chemicals.

Broadleaf Weeds and Moss

• Broadleaf weeds suit treatment with herbicides containing metsulfuron, such as the brand names Certainty, Blade or Manor. This chemical generally combats broadleaf weeds without damaging St. Augustine grass. Metsulfuron products are not available for home usage in many states. Gardeners can safely treat sphagnum moss weed problems with ferrous sulfate. The Texas A&M University AgriLife Extension suggests using 8 oz. for every 1,000 square feet of St. Augustine lawn.

Risks and Considerations

• St. Augustine grass will react badly to herbicides that contain MSMA or 2,4-D. All herbicides are potentially dangerous to either humans or the grass itself if used against package instructions. Some states have specific laws about herbicide use -- including local laws on permitted varieties. Spraying on windy days could mean the chemicals drift to neighboring plants. Anything used to apply herbicide should also be cleaned thoroughly with ammonia after use to remove all trace of harmful chemicals.