St Augustine Grass Mixed With Bermuda: Lawn Care & Compatibility

The Reality of Mixing St. Augustine and Bermudagrass

Finding St Augustine grass mixed with Bermuda in your lawn is rarely the result of intentional landscaping. More often, it is the consequence of seed contamination, encroachment from a neighbor's property, or shifting microclimates in your yard where shade and sun patterns have evolved. While both are aggressive warm-season turfgrasses, their fundamental biological differences create a highly competitive environment. According to Texas A&M AgriLife Extension, St. Augustine (Stenotaphrum secundatum) thrives in coastal, humid environments and relies on above-ground stolons, whereas Bermudagrass (Cynodon dactylon) utilizes both stolons and deep rhizomes, making it exceptionally drought-tolerant and sun-dependent.

When these two species occupy the same soil, they do not blend into a uniform carpet. Instead, they form distinct, uneven patches with conflicting cultural requirements. Managing this mixture requires a strategic decision: either implement a compromised maintenance routine to force coexistence, or deploy targeted eradication tactics to reclaim the lawn for a single species.

Biological and Cultural Conflicts

The primary challenge in managing a mixed lawn lies in the opposing physiological needs of the two grasses. Bermudagrass is an allelopathic species, meaning its roots exude biochemicals that actively inhibit the growth of competing plants, including St. Augustine. Conversely, St. Augustine forms a dense, broad-leafed canopy that shades out the sun-loving Bermuda if allowed to grow at its preferred height.

Direct Comparison of Cultural Requirements

The Coexistence Strategy: Compromise Maintenance

If you choose to maintain the mixed lawn rather than eradicate one species, you must adopt a 'middle-ground' maintenance protocol. This approach prevents either grass from completely dominating but requires vigilant monitoring to prevent stress-induced diseases like brown patch or gray leaf spot.

1. The Mowing Compromise

Set your mower deck to exactly 2.0 to 2.5 inches. This height is slightly lower than the ideal range for St. Augustine, meaning you must ensure your lawn is perfectly graded to avoid scalping the broad stolons. For the Bermudagrass, this height is higher than ideal, which will cause it to grow 'stemmy' and produce excessive thatch. To counteract this, you must dethatch the Bermuda-dominant zones annually in late spring using a power rake or vertical mower.

2. Split-Zone Fertilization

Because Bermudagrass requires up to 25% more nitrogen than St. Augustine, applying a uniform fertilizer will either starve the Bermuda or burn the St. Augustine. Use a slow-release, polymer-coated nitrogen fertilizer (such as a 24-0-6 NPK blend) at a rate of 3 lbs of actual nitrogen per 1,000 square feet annually. Apply it in three split applications: late April, early July, and early September. This steady release prevents the rapid flush of growth that leads to fungal outbreaks in St. Augustine.

3. Managing Iron Chlorosis

St. Augustine is highly susceptible to iron chlorosis in alkaline soils (pH above 7.0), manifesting as bright yellow blades with green veins. Bermudagrass tolerates higher pH levels much better. If your soil test indicates a pH above 7.0, do not rely on standard iron sulfate, which quickly binds to the soil and becomes unavailable. Instead, apply a chelated iron product containing Fe-EDDHA (such as Sequestrene 138) at a rate of 2 ounces per 1,000 square feet. This specific chelate remains bioavailable to the St. Augustine even in highly alkaline conditions without affecting the Bermuda.

The Eradication Strategy: Reclaiming Your Lawn

If the uneven texture and conflicting maintenance requirements are unacceptable, you must selectively remove one species. As noted by NC State Extension, selective removal of grassy weeds from a turfgrass canopy is one of the most difficult challenges in turf management because the chemical pathways of the two plants are nearly identical.

Removing Bermudagrass from St. Augustine

• Chemical Approach: Products containing fluazifop-butyl (e.g., Ortho Grass B Gon) are labeled for selective grassy weed control in St. Augustine. However, fluazifop will cause temporary stunting and yellowing of the St. Augustine. Apply at the lowest labeled rate (typically 0.5 fl oz per gallon per 1,000 sq ft) and add a non-ionic surfactant to ensure the chemical adheres to the fine Bermuda blades.
• Mechanical Approach: For isolated patches, use a glyphosate wick applicator. Because Bermuda grows lower to the ground than St. Augustine when unmowed, you can wipe the glyphosate directly onto the Bermuda foliage without touching the taller St. Augustine canopy.

Removing St. Augustine from Bermudagrass

This is significantly easier. St. Augustine is highly sensitive to broadleaf and selective grass herbicides that Bermudagrass can tolerate. Applying a product containing atrazine or metribuzin during the active spring green-up phase will severely damage or kill the St. Augustine stolons while the Bermudagrass remains largely unaffected. Alternatively, simply drop your mowing height to 0.75 inches for three consecutive weeks; the St. Augustine will scalp down to its stolons and desiccate, while the Bermuda recovers rapidly.

Frequently Asked Questions

Will St. Augustine eventually choke out Bermudagrass?

Only in shaded environments. If the mixed area receives less than six hours of direct sunlight, the St. Augustine's broad leaf blades will form a dense canopy that starves the Bermudagrass of the UV radiation it requires for photosynthesis. In full sun, however, the Bermuda's deep rhizome system and allelopathic root exudates will typically outcompete and degrade the St. Augustine over a 3-to-5-year period.

Can I overseed a mixed lawn with ryegrass in the winter?

Yes, but you must adjust your seeding rate. Perennial ryegrass establishes easily in the open, low-growing Bermuda canopy, but struggles to penetrate the thick thatch and stolon network of St. Augustine. Before overseeding in late October, heavily aerate the St. Augustine zones and increase the seed rate in those specific areas to 10 lbs per 1,000 square feet, while using a standard 5 lbs per 1,000 square feet in the Bermuda zones.