Grass Types
Can You Grow a Bermuda Grass St Augustine Mix? Expert Lawn Guide
The Horticultural Reality of Mixing Warm-Season Giants
When homeowners ask about establishing a bermuda grass st augustine mix, the short answer from turfgrass scientists is a resounding no. While both are aggressive, warm-season perennials dominating the Sun Belt, their cultural requirements and growth architectures are fundamentally incompatible. As of 2026, with climate shifts pushing warm-season grasses further into transition zones, the temptation to blend shade-tolerant St. Augustine (Stenotaphrum secundatum) with drought-hardy Bermuda (Cynodon dactylon) is increasing. However, attempting to grow them in the same turf canopy inevitably triggers a biological tug-of-war that leaves lawns patchy, uneven, and highly susceptible to disease.
The Core Conflict: Cultural Requirements
To understand why a bermuda grass st augustine mix fails, you must look at the agronomic data. According to the NC State Extension TurfFiles, these two species occupy entirely different ecological niches. What promotes vigorous growth in one will actively stunt or damage the other.
| Trait | Bermuda Grass (Hybrid/Common) | St. Augustine Grass |
|---|---|---|
| Ideal Mowing Height | 0.5" – 1.5" (Reel mower preferred) | 2.5" – 4.0" (Rotary mower required) |
| Sunlight Requirement | 8+ hours of direct, unfiltered sunlight | 4 – 6 hours of direct or dappled sunlight |
| Water Needs | Low to Moderate (Highly drought-tolerant) | Moderate to High (Wilts quickly without water) |
| Growth Architecture | Stolons (above) and Rhizomes (below) | Stolons only (above ground runners) |
| Soil pH Preference | 6.0 – 7.0 | 6.5 – 7.5 (More alkaline tolerant) |
Why the Bermuda Grass St Augustine Mix Happens
Rarely do professional landscapers intentionally seed or sod a bermuda grass st augustine mix. When this combination appears in residential lawns, it is usually the result of one of three scenarios:
- Accidental Invasion: Bermuda grass is notoriously invasive. Its underground rhizomes can easily bypass physical barriers, sneaking into established St. Augustine beds from neighboring properties or adjacent sports fields.
- The "Transition Zone" Compromise: Homeowners with yards featuring deep shade under oak trees and blazing sun near the street often try to plant St. Augustine in the shade and Bermuda in the sun, allowing them to "meet in the middle."
- Overseeding Mistakes: Misidentifying dormant winter grass or purchasing contaminated seed/sod mixes from low-quality suppliers can introduce Bermuda into a pure St. Augustine stand.
The Turf War: Rhizomes vs. Stolons
When these two grasses share the same soil, they do not coexist peacefully; they compete for resources. According to research published by the University of Florida IFAS Extension, St. Augustine spreads exclusively via thick, above-ground stolons. It relies on a broad leaf canopy to shade out competitors and retain soil moisture.
Bermuda grass, however, is a dual-threat. It spreads via above-ground stolons and deep, wire-like underground rhizomes. In a mixed canopy, Bermuda will aggressively push its rhizomes beneath the St. Augustine mat, stealing water and nutrients from the deeper soil profile. During peak summer heat, if the lawn is watered for the St. Augustine's needs, the Bermuda thrives. If the lawn is allowed to dry out to suit the Bermuda, the St. Augustine stolons desiccate and die back, leaving the Bermuda to fill the void.
Managing an Existing Mixed Lawn
If you have inherited a bermuda grass st augustine mix and cannot afford a complete lawn renovation, you must adopt a compromise maintenance strategy. Be warned: this approach will result in a lawn that looks perpetually uneven.
💡 The Mowing Compromise: You must set your mower deck to 2.5 inches. This is the absolute minimum height to prevent scalping the St. Augustine stolons, but it is far too high for Bermuda. Consequently, the Bermuda will develop a thick, spongy thatch layer and produce "stemmy" growth, requiring you to dethatch the lawn mechanically at least twice a year to prevent fungal diseases like brown patch.
Fertilization and Irrigation Adjustments
You must water to the needs of the most demanding grass (St. Augustine). Apply 1 to 1.5 inches of water per week, split into two deep waterings. For fertilization, stick to a moderate nitrogen schedule (3–4 lbs of nitrogen per 1,000 sq ft annually). Over-fertilizing will cause the Bermuda to aggressively outpace the St. Augustine, accelerating the takeover.
Chemical Eradication: The Grass-on-Grass Nightmare
Many homeowners attempt to use selective herbicides to kill one grass while saving the other. This is where the bermuda grass st augustine mix becomes a massive financial and chemical headache.
"Attempting to chemically eradicate Bermuda grass from a St. Augustine lawn is one of the most difficult tasks in turf management. The genetic similarity between the two monocots means most herbicides that kill Bermuda will also severely damage St. Augustine." — Turfgrass Management Principle
Suppressing Bermuda in St. Augustine
The only viable chemical option is Fluazifop-butyl (sold under brand names like Ornamec or Fusilade II). This herbicide targets grassy weeds and can suppress Bermuda grass. However, it is not a silver bullet. It requires 3 to 4 carefully timed applications spaced 21 days apart. Furthermore, applying Fluazifop when temperatures exceed 85°F will cause severe phytotoxicity to the St. Augustine, resulting in yellowing and stunted growth. It suppresses Bermuda; it rarely eradicates it entirely.
Eradicating St. Augustine in Bermuda
There are currently no selective herbicides on the consumer market that will kill St. Augustine without also killing Bermuda grass. If Bermuda is your desired turf and St. Augustine is the invader, your only option is spot-treatment with a non-selective herbicide like Glyphosate, followed by immediate resodding of the dead patches with Bermuda plugs.
The Professional Solution: Micro-Zoning Your Yard
Instead of fighting a losing battle with a bermuda grass st augustine mix, modern landscape architecture dictates micro-zoning. Assess your yard's solar exposure using a sun-tracking app over a 48-hour period in mid-summer.
- Zone A (8+ Hours Sun): Kill all existing turf, amend the soil, and plant a modern, drought-tolerant Bermuda cultivar like TifTuf or Celebration.
- Zone B (4–6 Hours Sun): Transition to a shade-tolerant St. Augustine cultivar like Palmetto or CitraBlue.
- The Buffer: Install a physical barrier between the zones. A 6-inch deep steel edging or a decorative river-rock trench will prevent the Bermuda's rhizomes from invading the St. Augustine zone.
For areas receiving less than 4 hours of sunlight, abandon warm-season grasses entirely and transition to shade-loving groundcovers like Asiatic jasmine or mondo grass, or apply high-quality mulch.
Frequently Asked Questions
Can I overseed a Bermuda grass st augustine mix with winter ryegrass?
Yes, but it complicates spring transition. Perennial ryegrass will compete with both warm-season grasses as soil temperatures rise above 65°F in the spring. Because St. Augustine breaks dormancy slower than Bermuda, the ryegrass can choke out emerging St. Augustine stolons, giving the Bermuda a competitive head start.
Will a pre-emergent stop Bermuda from spreading into my St. Augustine?
No. Pre-emergents like Prodiamine 65 WDG or Dithiopyr only stop seeds from germinating. Bermuda grass primarily spreads into neighboring lawns via underground rhizomes and above-ground stolons, completely bypassing the pre-emergent chemical barrier in the soil.
Is Zoysia a better alternative for a mixed sun/shade yard?
According to Texas A&M AgriLife Extension, Zoysia (specifically cultivars like Zeon or Geo) offers a much better middle-ground than a bermuda grass st augustine mix. Zoysia tolerates moderate shade (5-6 hours of sun) while maintaining excellent drought tolerance and a uniform, dense canopy that naturally resists weed invasion.
