The invasion of a St. Augustine grass lawn by Bermuda grass is a frustrating challenge for warm-season turf management. St. Augustine grass has broad, coarse leaf blades and a dense growth habit. Bermuda grass, conversely, features fine blades and an aggressive growth pattern that quickly infiltrates and dominates other turf varieties. This weed issue is highly challenging because both are warm-season grasses, thriving during the same periods. Their similarity means many common herbicides cannot distinguish between them, making selective removal difficult without damaging the desirable St. Augustine turf.
Why Bermuda Grass is Difficult to Eradicate
Bermuda grass is persistent due to its dual system of reproductive structures. The plant spreads rapidly above ground using stolons, which are horizontal stems that root at the nodes and produce new shoots. These runners allow it to colonize large, open areas quickly, especially in full sun.
It also spreads underground via rhizomes, which are subterranean stems that store energy. These rhizomes penetrate deep into the soil and contain stored carbohydrates that support rapid regrowth after the above-ground portion is damaged. Simply pulling the visible grass or mowing it short is ineffective because the extensive network of rhizomes quickly regenerates the entire plant. This aggressive growth rate and complex root structure make Bermuda grass a tenacious weed.
Non-Chemical Methods for Suppression
Cultural practices that promote the health and density of St. Augustine grass can naturally suppress Bermuda grass. St. Augustine turf performs best when mowed at a height between 3 and 4 inches. This taller height allows the St. Augustine blades to shade the soil surface, blocking the sunlight that Bermuda grass needs to thrive. Since Bermuda grass favors a short cut and full sun, the shade cast by the taller St. Augustine starves the competitor of light energy.
Adjusting irrigation habits is another effective method to discourage the shallow-rooted Bermuda grass. St. Augustine grass should be watered deeply and infrequently, wetting the soil to a depth of about six inches. This practice encourages St. Augustine to develop a deeper, more robust root system, increasing its competitive advantage. Frequent, shallow watering, in contrast, encourages the Bermuda grass’s shallow stolons to root aggressively and spread.
Manual removal of Bermuda grass is physically demanding and must be thorough to be successful. When hand-pulling, remove the entire stolon and as much of the attached rhizome network as possible to prevent immediate regrowth. If any portion of the rhizome remains, the plant will regenerate using its stored energy reserves. This method is most practical for small, newly developing patches.
For larger, localized infestations, solarization or shading is a temporary non-chemical solution. Covering the infested area with black plastic sheeting or a thick layer of mulch completely blocks sunlight, which eventually kills both the Bermuda grass and the St. Augustine grass beneath it. The plastic traps heat and moisture, which cooks the plant tissue. This technique requires leaving the area covered for several weeks during the hottest part of the year, after which the area must be replanted with healthy St. Augustine sod.
Selective Herbicide Treatments
Controlling Bermuda grass within an established St. Augustine lawn often requires the precise application of selective post-emergent herbicides. The difficulty is finding a product that targets the fine-bladed Bermuda grass while leaving the broad-bladed St. Augustine turf unharmed. An effective solution involves combining two specific active ingredients: a safener-containing herbicide and a grass-targeting herbicide.
The herbicide combination typically involves a product containing a safener, like metcamifen, tank-mixed with a grass-selective herbicide, such as fluazifop-p-butyl. The safener increases the St. Augustine grass’s tolerance to the herbicide, allowing the mixture to be applied without major damage to the desirable turf. The fluazifop component is absorbed through the leaves of the Bermuda grass and translocated throughout the plant to suppress growth.
Another active ingredient often paired with metcamifen is trifloxysulfuron. While trifloxysulfuron alone does not control Bermuda grass, its inclusion allows for a broader spectrum of weed control, targeting sedges and other grassy weeds. These specialized herbicides are systemic, meaning they move through the plant’s vascular system to reach the underground rhizomes, which is necessary for effective long-term control.
The application of selective herbicides requires a precise, multi-step strategy and careful timing. Treatment is most successful when the Bermuda grass is actively growing and healthy, typically during late spring and summer. Multiple sequential applications are necessary because the extensive rhizome network will attempt to regenerate after the first application. Follow-up sprays are often recommended every four to six weeks until the infestation is under control.
Strict adherence to label instructions regarding temperature is necessary to prevent injury to the St. Augustine grass. Herbicides are generally not applied when temperatures exceed 90 degrees Fahrenheit, as the turf is already under heat stress and more susceptible to injury. Applying herbicides in the early morning ensures the product is absorbed before the hottest part of the day. Also, avoid application when St. Augustine is recovering from winter dormancy or drought stress to minimize potential damage.