Maintaining a uniform St. Augustine grass lawn often involves battling the invasion of more resilient grasses. Bahia grass, a perennial species originally introduced for pasture and erosion control, is a common invader in residential turf. Its aggressive growth habit and deep root system make it difficult to remove once established. Eliminating this unwanted grass requires a targeted approach combining selective chemical treatments with cultural practices designed to favor the desired turf.
Identifying the Target and Host Grasses
Successful removal of Bahia grass relies entirely on correct identification, as St. Augustine grass is highly sensitive to many common herbicides. Bahia grass (Paspalum notatum) has thin, coarse, and light green blades that often give the lawn a patchy appearance. A defining characteristic is its prominent, thin seed stalks that stand much taller than the leaf blades, typically terminating in a distinctive V- or Y-shaped seed head. This structure houses the seeds that allow the grass to spread.
St. Augustine grass (Stenotaphrum secundatum) has wider, flat blades with a blue-green or darker green color. It spreads across the soil surface via thick, above-ground runners called stolons, which create a dense, carpet-like turf. Unlike Bahia, St. Augustine grass lacks the extensive, deep root system or the tall, Y-shaped seed heads. Knowing these differences helps ensure that only the weed is treated, preventing damage to the desired lawn.
Selective Herbicides for Post-Emergent Control
The chemical solution for eliminating Bahia grass without damaging St. Augustine turf lies in the use of specific, selective active ingredients. The most effective option is the systemic herbicide metsulfuron-methyl. This compound is absorbed through the foliage and roots, where it translocates throughout the system to inhibit the production of essential amino acids, eventually leading to plant death. It is one of the few chemicals that can selectively control Bahia grass in warm-season turf like St. Augustine.
Products containing metsulfuron-methyl are often sold under various trade names for turf use. Application must be done with precision, using a very low use rate, sometimes as little as 0.17 grams per gallon of water for spot treatments. It is necessary to include a non-ionic surfactant in the spray mixture, which helps the herbicide adhere to and penetrate the waxy leaf surface of the Bahia grass, increasing its effectiveness.
Timing and environmental conditions are important for a successful application and to avoid injuring the St. Augustine grass. The herbicide should not be applied to turf stressed by drought or high heat; avoid daytime temperatures exceeding 85°F. While St. Augustine grass is generally tolerant, temporary yellowing or chlorosis may occur, but it is short-lived. Because Bahia grass is tough to kill, a second application is usually necessary four to six weeks after the initial treatment to achieve complete control of established clumps.
Herbicides containing active ingredients like 2,4-D, Dicamba, or Quinclorac are often used for broadleaf weeds but should be avoided for Bahia grass control in St. Augustine. St. Augustine grass is sensitive to these compounds, and they can cause significant damage, stunting, or even death to the desirable turf. Spot treating only the Bahia clumps, rather than blanket spraying the entire lawn, is a recommended practice to limit chemical exposure to the host grass and surrounding ornamentals.
Cultural Practices and Long-Term Prevention
Chemical applications are best supported by cultural practices that improve the health of the St. Augustine turf and prevent future Bahia grass establishment. A primary non-chemical strategy is maintaining a high mowing height for the St. Augustine grass, ideally between 3.5 and 4 inches. This height allows the host grass to develop a dense canopy that shades the soil surface, naturally suppressing the germination and growth of new Bahia grass seedlings.
Since Bahia grass is a prolific seed producer, it is important to bag the clippings when seed heads are present to prevent them from scattering across the lawn. Established Bahia grass is difficult to remove by hand due to its deep root system. However, small, newly germinated seedlings can be manually pulled to stop them from becoming established clumps. This manual effort is most effective after a rain or irrigation event when the soil is moist.
For long-term prevention, a pre-emergent herbicide program is necessary to stop Bahia seeds from germinating. Products containing active ingredients like prodiamine are safe for St. Augustine turf and offer excellent pre-emergent control against many annual grassy weeds. Timing is crucial, typically in late winter or early spring when soil temperatures reach 65 to 70 degrees Fahrenheit for four to five consecutive days. This application window targets the beginning of the Bahia grass germination cycle, providing a protective barrier in the soil.