Grass Types
Weed That Looks Like St. Augustine Grass: Identification and Control
The St. Augustine Imposters: Why Misidentification Ruins Lawns
St. Augustine grass (Stenotaphrum secundatum) is a staple in USDA zones 8 through 10, prized for its broad, flat blades and aggressive stoloniferous growth. However, these exact morphological traits make it a magnet for mimic weeds. When a homeowner spots a weed that looks like St. Augustine grass, the immediate instinct is often to grab a standard grassy weed killer. This is a fatal error.
St. Augustine is highly sensitive to specific herbicide active ingredients—most notably quinclorac and fluazifop. Applying these chemicals to kill a crabgrass or nimblewill invader will severely injure or completely melt your St. Augustine turf while barely scratching the imposter. Accurate botanical identification at the collar region is the only way to protect your lawn while eliminating the threat.
The 'Opposite vs. Alternate' Leaf Test: Your Primary Diagnostic Tool
Before purchasing any herbicide, you must examine the collar region of the suspect weed. The collar is the area where the grass blade meets the stem (culm). This region holds the definitive genetic markers required to separate true St. Augustine from its mimics.
Expert Identification Tip: Gently pull a blade downward to expose the collar. St. Augustine grass features an opposite leaf arrangement (blades emerge directly across from one another on the stolon) and a ligule consisting of a short fringe of hairs. If the leaves emerge in an alternate pattern (staggered along the stem) or feature a large, membranous (paper-like) ligule, you are dealing with an imposter.
Top 4 Weeds That Mimic St. Augustine Grass
1. Nimblewill (Muhlenbergia schreberi)
Nimblewill is the most frequent culprit when homeowners search for a weed that looks like St. Augustine grass. It spreads via stolons, creating a mat-like appearance that blends into the surrounding turf from a distance. However, nimblewill stolons are thin, wiry, and light green, lacking the thick, fleshy, purple-tinged stolons of true St. Augustine. Nimblewill leaves are alternate, shorter (typically under 2 inches), and feature a short, membranous ligule. It thrives in the shaded, damp areas where St. Augustine often thins out.
2. Hairy Crabgrass (Digitaria sanguinalis)
While smooth crabgrass is easily identified by its thin blades, hairy crabgrass produces broad, flat blades that closely match the width of St. Augustine. The key differentiator is texture and ligule. Hairy crabgrass blades and sheaths are covered in distinct trichomes (hairs), and the ligule is a prominent, jagged, paper-like membrane. Furthermore, crabgrass is a summer annual that germinates from seed, meaning it lacks the established perennial stolon network of St. Augustine, though it does root at the nodes.
3. Dallisgrass (Paspalum dilatatum)
Dallisgrass does not spread via stolons; it is a clumping perennial. However, its individual blades are thick, wide, and feature a prominent midrib, making isolated clumps look remarkably like St. Augustine. The giveaway is the seed head: dallisgrass produces massive, finger-like racemes that shoot up rapidly, often within 48 hours of mowing. Its collar features a long, membranous ligule and alternate leaf arrangement.
4. Centipedegrass (Eremochloa ophiuroides)
Often intentionally planted in the Southeast, centipedegrass becomes a severe weed when it invades an established St. Augustine lawn. Both are stoloniferous, but centipedegrass stolons are thinner and feature an alternating leaf pattern. The blades are distinctly folded in the bud (unlike St. Augustine, which is rolled) and possess a distinct apple-green coloration compared to the deeper blue-green of healthy St. Augustine.
Side-by-Side Identification Matrix
Use this diagnostic matrix in the field to confirm the identity of the invading species before selecting a control protocol.
| Trait | St. Augustine | Nimblewill | Hairy Crabgrass | Dallisgrass |
|---|---|---|---|---|
| Leaf Arrangement | Opposite | Alternate | Alternate | Alternate (Clumping) |
| Ligule Type | Fringe of hairs | Short membranous | Large, jagged membrane | Long membranous |
| Stolon Structure | Thick, fleshy, robust | Thin, wiry, delicate | Roots at nodes, no true stolon | None (Bunch-type) |
| Blade Texture | Smooth, flat, broad | Slightly rough, narrow | Hairy (trichomes present) | Smooth, prominent midrib |
Targeted Eradication Protocols Safe for St. Augustine
Controlling grassy weeds in a St. Augustine lawn is notoriously difficult because the chemical mechanisms that kill grassy weeds also kill St. Augustine turf. According to NC State TurfFiles, selective post-emergent control of grassy weeds in St. Augustine is severely limited, making pre-emergent strategies and manual removal the primary lines of defense.
CRITICAL HERBICIDE WARNING: Never apply products containing Quinclorac (e.g., Drive XLR8), Fluazifop (e.g., Fusilade), or MSMA to St. Augustine grass. These active ingredients will cause severe phytotoxicity, resulting in massive turf necrosis. Always verify the active ingredient on the label, not just the brand name.
Pre-Emergent Defense (Winter/Early Spring)
Because post-emergent options for grassy mimics are limited, stopping germination is paramount. Apply a pre-emergent when soil temperatures at a 2-inch depth consistently reach 55°F for three consecutive days (typically late February to early March in zone 8).
- Prodiamine 65 WDG: Apply at 0.5 to 1.15 oz per 1,000 sq ft. Provides excellent crabgrass and nimblewill seed suppression. Water in with 0.25 inches of irrigation immediately.
- S-metolachlor (Cavalcade): Highly effective against tough grassy weeds and safe for St. Augustine. Apply at 1.5 to 2.0 oz per 1,000 sq ft.
Post-Emergent Rescue (Active Infestations)
If the weed that looks like St. Augustine grass has already established, your chemical options depend heavily on the ambient temperature and the specific weed species.
- Atrazine: The safest post-emergent for St. Augustine, but it is highly temperature-dependent. Apply only when daytime highs are below 85°F (typically late fall or early spring). Atrazine provides moderate control of nimblewill and suppresses crabgrass. Rate: 2.5 oz per 1,000 sq ft.
- Penoxsulam + Sulfentrazone (e.g., Blindside): While primarily a broadleaf and sedge killer, this combination safely suppresses certain invasive grassy weeds and centipedegrass in St. Augustine lawns during warm weather. Apply at 0.25 oz per 1,000 sq ft.
- Manual Extraction: For dallisgrass and isolated crabgrass plants, chemical control in St. Augustine is practically non-existent without turf injury. Use a serrated weeding knife to extract the deep, bulbous root system of dallisgrass, ensuring you remove the entire basal crown to prevent regeneration.
Cultural Suppression via Mowing Heights
St. Augustine grass thrives at a mowing height of 3.5 to 4.0 inches. Maintaining this higher canopy creates dense shade at the soil level, which drastically reduces the photosynthetic capacity of low-growing mimics like nimblewill and crabgrass. Lowering your mowing height to 2.5 inches to 'scalp' the weeds will shock and thin your St. Augustine, opening the canopy for further weed invasion.
Frequently Asked Questions
Can I use Celsius WG on weeds that look like St. Augustine?
Celsius WG (thiencarbazone + iodosulfuron + dicamba) is an excellent, safe herbicide for St. Augustine grass. However, it is formulated primarily for broadleaf weeds and sedges (like doveweed and yellow nutsedge). It will not effectively kill grassy mimics like hairy crabgrass or nimblewill.
Why does my St. Augustine grass look wiry and thin in the shade?
If the turf in your shaded areas looks thin, wiry, and light green, it is likely not your St. Augustine grass at all, but rather a nimblewill invasion. Nimblewill thrives in the exact low-light, damp conditions where St. Augustine struggles, allowing it to quietly overtake the canopy.
Is there a selective herbicide that kills crabgrass but not St. Augustine?
Currently, there is no highly effective, fully selective post-emergent herbicide that will kill mature crabgrass without causing significant injury to St. Augustine grass. As noted by Texas A&M AgriLife Extension, managing crabgrass in St. Augustine relies almost entirely on robust pre-emergent applications and maintaining a thick, healthy turf canopy to outcompete seedlings.
