St. Augustine grass (Stenotaphrum secundatum) is a warm-season turf species favored for producing a dense, lush lawn in tropical and subtropical climates. Characterized by broad, coarse leaf blades, this grass spreads aggressively via above-ground runners called stolons. Although it offers superior shade tolerance, mixing St. Augustine with other grasses is generally not recommended. Fundamental differences in growth structure and resource requirements create a competitive and aesthetically inconsistent environment.
Fundamental Reasons Against Mixing Grass Types
The primary issue with mixing St. Augustine with other grasses lies in its unique and aggressive growth habit. St. Augustine spreads rapidly using robust stolons, which quickly form a thick, carpet-like sod that physically crowds out finer-bladed species. This vigorous lateral growth leads to a fierce competition for sunlight, water, and nutrients at the soil level.
Another major incompatibility stems from the vastly different needs for essential resources. St. Augustine is a high-input grass, demanding significant amounts of nitrogen fertilizer, typically between 2 to 4 pounds of nitrogen per 1,000 square feet annually, and regular watering to maintain its deep green color. Most other common turfgrasses, particularly those with a finer texture, require less fertilizer and less frequent irrigation, creating an impossible maintenance schedule for a mixed lawn.
Aesthetically, blending coarse, wide St. Augustine blades with narrow-bladed grasses results in a non-uniform and patchy appearance. The dark, often blue-green color of St. Augustine contrasts sharply with the lighter green of many other varieties. Differing blade widths create an uneven texture. Maintaining a consistent look requires favoring one grass’s needs, which inevitably compromises the health of the second species.
Specific Outcomes When Mixed With Common Turfs
Mixing St. Augustine with other warm-season competitors, such as Bermuda grass or Zoysia grass, often results in a perpetual turf war. In areas of full, intense sunlight, the finer-textured, drought-tolerant Bermuda grass may aggressively encroach and overtake St. Augustine. Conversely, St. Augustine will almost always dominate and choke out Bermuda in partially shaded areas due to its higher shade tolerance, leading to clumpy invasion patterns across the yard.
When St. Augustine is mixed with cool-season grasses like Tall Fescue or Ryegrass, the outcome is highly temporary and problematic. These cool-season grasses are sometimes used for temporary winter color, but they are not built to survive the high temperatures and humidity required by St. Augustine during its peak growing season. The cool-season grass will thin out and die once summer heat arrives, leaving large, dead patches that weaken the St. Augustine underneath.
Cultural Practices for Maintaining a Mixed Lawn
Homeowners with an existing mixed turf face unique management challenges that prioritize the St. Augustine’s specific needs. Mowing height is particularly important, as St. Augustine must be maintained at a high setting, generally between 3 and 4 inches, to promote deep root growth and shade out weeds. This high cut is detrimental to finer-bladed grasses, which prefer to be cut much shorter, forcing the homeowner to sacrifice the health of the secondary species.
Weed and pest control becomes complex because St. Augustine is highly sensitive to many common herbicides. Many broadleaf weed killers containing 2,4-D, which are safe for Fescue or Bermuda, can severely injure or kill St. Augustine grass. Safe alternatives like Atrazine must be used, which may not be effective against all weeds or may harm the secondary grass in the mixture.
Watering practices must also cater to the water-intensive nature of St. Augustine. While St. Augustine is moderately drought-tolerant, it requires regular, deep watering to stay lush during the growing season. This high moisture regimen can create conditions that are too wet for drought-tolerant species, encouraging disease or simply favoring the St. Augustine’s dominance.