Grass Types
St Augustine Weeds: Identification and Spot-Treatment Guide
The St. Augustine Herbicide Sensitivity Problem
St. Augustine grass (Stenotaphrum secundatum) is a premier warm-season turfgrass for coastal and southern lawns, but its broad leaf blades and stoloniferous growth habit make it uniquely vulnerable to chemical burn. The most critical mistake homeowners make is applying standard 'Weed and Feed' formulations designed for cool-season grasses. Products containing high concentrations of 2,4-D, MCPP, and dicamba will severely stunt St. Augustine, causing permanent chlorosis and stolon death, particularly in the widely planted 'Floratam' cultivar.
Furthermore, MSMA (monosodium methanearsonate), once the go-to post-emergent for grassy weeds in warm-season turfs, has been federally restricted for residential use since 2009 and remains unavailable in 2026. Effective St Augustine weeds management requires a surgical approach using specific active ingredients that target weed cellular processes without disrupting the grass's photosynthetic pathways.
⚠️ Floratam Phytotoxicity Warning: Never apply herbicides containing 2,4-D, dicamba, or MCPP to 'Floratam' St. Augustine when ambient temperatures exceed 85°F. Even at reduced rates, these chemicals will cause severe leaf curling and root inhibition. Always check the product label for the specific St. Augustine cultivar warnings.
Broadleaf Weed Spot-Treatments
Broadleaf weeds compete aggressively with St. Augustine for water and soil nutrients. Because St. Augustine forms a dense, interlocking canopy via above-ground stolons, broadleaf weeds usually exploit thin, shaded, or overly wet areas of the lawn. According to turfgrass pathology data from the UF/IFAS Extension, maintaining proper soil drainage is the first line of defense against moisture-loving broadleaves like dollarweed.
| Weed Species | Visual Identifier | Safe Active Ingredient | Product Example (2026) | Est. Cost |
|---|---|---|---|---|
| Dollarweed (Hydrocotyle umbellata) | Round, scalloped leaves; stem attaches to center of leaf | Atrazine | Hi-Yield Atrazine Weed Killer | $22 (1 qt) |
| Doveweed (Murdannia nudiflora) | Thick, fleshy leaves; emits dog-urine odor when mowed | Sulfentrazone + Metribuzin | Blindside Herbicide | $68 (5 oz) |
| White Clover (Trifolium repens) | Trifoliate leaves with white chevron markings | Carfentrazone-ethyl | SpeedZone Southern | $45 (1 pt) |
The Doveweed Challenge
Doveweed has become the most problematic broadleaf weed in St. Augustine lawns across the Gulf Coast. It mimics the grass blade but feels rubbery to the touch. Standard Atrazine applications will only suppress it. For complete eradication, professional-grade Blindside (sulfentrazone + metribuzin) is required. Mix 0.11 oz of Blindside per gallon of water to treat 1,000 square feet. Always include a non-ionic surfactant at 0.25% v/v to penetrate the waxy cuticle of the doveweed leaf.
Grassy Weed Eradication Protocol
Grassy weeds like Crabgrass (Digitaria sanguinalis) and Dallisgrass (Paspalum dilatatum) are notoriously difficult to treat in St. Augustine because they share the same monocot biological pathways. However, the NCSU TurfFiles database confirms that ACCase-inhibiting herbicides selectively target grassy weeds without harming broadleaf turf or St. Augustine.
- Select the Herbicide: Purchase a product containing Sethoxydim (e.g., Monterey Poast or Bonide Grass Beater).
- Calculate the Rate: Mix 1.5 fluid ounces of Sethoxydim per gallon of water. This covers approximately 1,000 square feet.
- Add Surfactant: Add 1 teaspoon (approx. 0.16 oz) of a high-quality non-ionic surfactant to the tank. Sethoxydim will bead up and roll off the leaf without it.
- Spot Spray: Apply only to the grassy weed patches. Avoid broadcast spraying the entire lawn to prevent unnecessary chemical stress.
- Repeat Application: Sethoxydim is a slow-acting systemic. Expect to see yellowing in 7-10 days. A second application 14 days later is mandatory for mature Dallisgrass.
Sedge Control: Targeting the Tuber
Sedges are not grasses or broadleaves; they are in a separate botanical family characterized by solid, triangular stems ('sedges have edges'). Purple nutsedge (Cyperus rotundus) is the most aggressive, producing chains of underground tubers (nutlets) that survive most standard herbicides.
'Pulling nutsedge by hand is counterproductive. The tension snaps the shoot, leaving the dormant tubers in the soil, which stimulates the activation of secondary nutlets, resulting in three new plants for every one you pulled.' — Texas A&M Agrilife Extension
To break the tuber chain, you must use Halosulfuron-methyl (e.g., SedgeHammer). Mix one 13.5-gram water-soluble packet into 1 to 2 gallons of water. Apply directly to the sedge foliage. The chemical translocates down the stem and sterilizes the underground nutlets, preventing regrowth. Expect to pay around $32 for a box of 5 packets, which is highly cost-effective for spot treatments.
Pre-Emergent Defense Matrix
Post-emergent spot treatments are reactive. A proactive pre-emergent strategy prevents weed seeds from germinating, saving you time and chemical costs. However, St. Augustine has a shallow, sensitive root system. Root-inhibiting pre-emergents like Dithiopyr (Dimension) can cause severe root pruning if applied during the spring 'root flush' (when soil temperatures hit 65°F-70°F).
Optimal Pre-Emergent Selection for St. Augustine:
- Fall Application (Sept 1-15): Prodiamine (Barricade) at 0.33 oz per 1,000 sq ft. Provides 4-6 months of residual control against winter annuals like Poa annua and henbit.
- Spring Application (Feb 15-28): Indaziflam (Specticle Flo) at 0.38 oz per 1,000 sq ft. Offers superior crabgrass and doveweed prevention with a much safer margin for St. Augustine root development compared to older chemistries.
Post-Treatment Lawn Recovery
After successfully killing weeds, you will be left with bare spots in the turf canopy. St. Augustine does not produce seed that establishes true-to-type, so you must rely on stolon creep to fill the gaps. Ten days after your final herbicide application, apply 0.5 lbs of Nitrogen per 1,000 square feet using a fast-release urea or ammonium sulfate fertilizer. Water deeply to push the surrounding stolons laterally across the bare soil, closing the canopy before new weed seeds can find sunlight.
Frequently Asked Questions
Can I use Celsius WG on St. Augustine grass?
Yes, Celsius WG (thiencarbazone + iodosulfuron + dicamba) is labeled for St. Augustine, but it must be used with extreme caution on the 'Floratam' cultivar. It is highly effective against tough broadleaves like Virginia buttonweed, but you must avoid spraying during summer heat stress and never exceed the labeled rate, or you will induce severe yellowing.
Why is my St. Augustine turning purple after spraying weeds?
Purple discoloration is a classic symptom of herbicide phytotoxicity or severe phosphorus deficiency induced by root stress. If this happens within 3-5 days of spraying a broadleaf herbicide, you have likely applied a product containing 2,4-D during high heat, or you overlapped your spray passes, doubling the chemical dose on the grass.
Is vinegar a safe spot-treatment for St. Augustine weeds?
Horticultural vinegar (20% acetic acid) is a non-selective contact burn-down herbicide. It will destroy the weed, but it will also instantly burn the St. Augustine grass it touches. Because St. Augustine relies on above-ground stolons, burning the stolon with vinegar will kill the entire connected section of the lawn. Stick to selective systemic herbicides for spot treatments.
