A warm season perennial grass that flourishes in tropical locations, St Augustine (Stenotaphrum secundatum) grows throughout the southern United States and in Hawaii. The grass is also cultivated in the South Pacific, Africa, South America, southern Mexico and the Caribbean. When maintained well, the grass suffers from few weeds during its active growing season but is prone to annual weeds during its winter dormancy.
Beginning in the 1950s the primary herbicide used for weed control in St. Augustine grass was atrazine. A post-emergence herbicide, atrazine appeared to cause no damage in the sensitive grass, unlike the more common herbicide combinations of the era which contained 2,4-D and mecoprop (MCCP). Use of atrazine in other turf grasses caused damage, but St. Augustine grass was immune to the herbicide, according to the University of Florida. Atrazine remains one of the leading post emergence herbicides for use in St. Augustine grass by professionals.
Atrazine does have a few drawbacks when used on St. Augustine grass. The herbicide has been found in groundwater when used in the Midwest, where the water table is high. some St. Augustine grass cultivars, such as Bitterblue and FX-10, often suffer summer damage if atrazine is used. The Floratam cultivar of St. Augustine grass tolerates atrazine herbicides well. In some parts of the United States, such as Florida, atrazine can only be used by a licensed professional.
Homeowners can purchase herbicides containing clopyralid at most garden supply stores. The herbicide does not damage St. Augustine grass and is highly effective at ridding St. Augustine lawns of the most common weeds. Metsulfuron has also been shown to be an effective weed killer in St. Augustine grass. Care must be taken when using metsulfuron on St. Augustine lawns that have been mixed with Bahia grass because the herbicide will kill all the Bahia grass.
A relatively new herbicide, carfentrazone-ethyl, kills difficult weeds in St. Augustine grass, such as alligatorweed, clover, spurge and diamondflower. The herbicide is fast acting and usually kills the weeds within four days of application, according to the University of Florida. Carfentrazone-ethyl herbicides do contain 2,4-D but they do not contain mecoprop (MCCP). The combination of 2,4-D and mecoprop (MCCP) is believed to be what damages St. Augustine grass.
Maintaining a healthy St. Augustine lawn through regular watering and a diligent fertilizing regime will help reduce weed growth. Despite the efforts of the gardener, weeds do occur and using a herbicide can be quite successful. Purchase only herbicides that clearly state they are for St. Augustine grass and will not harm it. Follow all directions on the pesticide label prior to application.