Bermuda grass and St. Augustine grass dominate lawns across the southern United States due to their heat tolerance. While planting them together is physically possible, it is generally not recommended for homeowners. The core issue is the fundamental biological incompatibility between the two species, making it nearly impossible to maintain a uniform, healthy, and visually appealing lawn. This incompatibility stems from their contrasting growth habits, light requirements, and specific maintenance needs.
Contrasting Growth Habits and Light Requirements
A mixed lawn is problematic due to the difference in how each grass spreads and its tolerance for shade. Bermuda grass has a fine texture and highly aggressive growth, spreading through both above-ground runners (stolons) and underground stems (rhizomes). This dual-spreading mechanism allows it to fill in bare spots quickly. Bermuda grass demands full, direct sunlight, requiring at least six to eight hours daily; its growth will thin out and eventually die in shaded conditions.
St. Augustine grass features a coarser texture with wide leaf blades and spreads exclusively via thick, above-ground stolons. This growth habit forms a dense, carpet-like turf, but it is less aggressive than Bermuda grass underground. St. Augustine grass exhibits a moderate tolerance for shade, making it a better choice for yards with trees or structures that block sunlight. In a mixed lawn, sun-loving Bermuda will quickly dominate open areas, while St. Augustine will prevail only in partial shade, creating a patchwork of turf.
The Aesthetic and Competitive Outcome of Mixing
When these two species are planted together, the result is a visually inconsistent and patchy lawn. The difference in leaf texture is immediately noticeable, with the fine blades of Bermuda grass (about 1 to 2 millimeters wide) contrasting sharply with the broad blades of St. Augustine grass (up to 9 millimeters wide). This variation in leaf structure and color, ranging from deep emerald green for Bermuda to a bright blue-green for St. Augustine, ensures the lawn looks uneven.
The competitive dynamics further contribute to the lawn’s inconsistency. In sunny sections, aggressive Bermuda grass will rapidly outcompete and choke out the St. Augustine grass. Conversely, St. Augustine will prevail in shaded areas, weakening the Bermuda grass, which cannot tolerate low light. Additionally, the grasses have different dormancy cycles. Bermuda grass tends to go dormant earlier in the fall and awaken later in the spring than St. Augustine, resulting in brown patches appearing at different times during cooler months.
Adjusting Maintenance Practices for Mixed Turf
Different maintenance protocols make a mixed lawn impractical to care for, especially regarding mowing and fertilization. Bermuda grass is maintained at a low height, typically between 0.5 to 2.5 inches, to encourage dense growth. St. Augustine grass requires a much higher cut, ideally between 2.5 and 4 inches, to maintain enough leaf surface and prevent stress. If the lawn is mowed high for St. Augustine, the Bermuda grass becomes leggy and thin. If mowed low for Bermuda, the St. Augustine will be severely damaged.
Fertilization needs also conflict, making it difficult to feed both species optimally. Bermuda grass is a heavy feeder, requiring 3 to 5 pounds of nitrogen per 1,000 square feet annually to support its vigorous growth. St. Augustine grass has lower nitrogen requirements, typically thriving on 2 to 3 applications per year. Applying the high nitrogen rate needed for Bermuda can cause excessive, disease-prone growth in St. Augustine, while the lower rate will cause Bermuda grass to thin out.
Weed control presents another challenge due to the sensitivity of St. Augustine grass to many common herbicides. Many selective broadleaf herbicides safe for Bermuda grass, such as those containing 2,4-D, are harmful to St. Augustine grass and can cause severe damage. Conversely, herbicides used to suppress grassy weeds in St. Augustine, like Atrazine, can injure or kill Bermuda grass. This chemical incompatibility means that controlling weeds requires a delicate balancing act.
Strategies for Establishing a Single Species Lawn
For homeowners dealing with a mixed lawn, the most effective approach is to transition to a single, uniform species suited to the yard’s specific conditions. The first step involves choosing the dominant grass based on the lawn’s microclimates. St. Augustine is the better choice if shade is present, while Bermuda is ideal for full-sun, high-traffic areas. Addressing the light conditions first determines which grass is most likely to thrive long term.
Complete eradication of the unwanted species is the most reliable method. This can be accomplished by applying a non-selective herbicide, such as glyphosate, to the entire area to kill both grasses before re-establishing the desired species via sod or sprigs. A less aggressive, more labor-intensive approach involves selective suppression, where maintenance practices are continually adjusted to favor one grass. For instance, consistently mowing high will slowly suppress the low-growing Bermuda grass, or reducing water and fertilizer can stress the St. Augustine, allowing the Bermuda to take over.