Grass Types
A Spot-Treatment Guide for Weeds in St. Augustine Grass
The Unique Vulnerability of St. Augustine Turf
St. Augustine grass (Stenotaphrum secundatum) is the dominant warm-season turf across the Gulf Coast and Southeastern United States, prized for its shade tolerance and coarse, dark green canopy. However, its aggressive stoloniferous growth habit and tendency to accumulate thick thatch layers create a specific microenvironment that invites specialized weed invasions. When managing weeds in St. Augustine grass, broadcast spraying is often unnecessary and increases the risk of phytotoxicity. Spot-treating specific weed species with calibrated, targeted herbicides preserves the surrounding turf while eliminating the invaders at the root.
Unlike Zoysia or Bermuda grass, St. Augustine is notoriously sensitive to standard broadleaf herbicides, particularly ester-based 2,4-D formulations. Misapplication can cause severe leaf curling, chlorosis, and stolon dieback. This guide provides a species-specific, spot-treatment decision framework designed for residential and commercial turf managers.
The Spot-Treatment Decision Matrix
Identifying the weed correctly is the first step in selecting an active ingredient that will not harm your St. Augustine lawn. Use the matrix below to match the weed species to the correct chemical protocol.
| Weed Species | Classification | Key Identification Feature | Recommended Active Ingredient | Spot-Treat Rate (per 1,000 sq ft) |
|---|---|---|---|---|
| Dollarweed | Broadleaf | Round, scalloped leaves with center stem | Atrazine or Sulfentrazone | 1.5 oz Atrazine / 0.5 oz Sulfentrazone |
| Doveweed | Broadleaf (Grass-like) | Fleshy stems, purple flowers, rubbery texture | Sulfentrazone + Metsulfuron | 0.75 oz (Blindside blend) |
| Dallisgrass | Grassy (Perennial) | Clumping growth, prominent seed heads | Glyphosate (Non-selective) | 2% solution (Paint or shielded spray) |
| Crabgrass | Grassy (Annual) | Prostrate growth, crab-like tillering | Quinclorac | 0.37 oz (with 1 tsp NIS) |
Broadleaf Weed Spot-Treatment Protocols
Dollarweed (Hydrocotyle umbellata)
Dollarweed thrives in over-irrigated, poorly drained soils. Before reaching for an herbicide, audit your irrigation system. If the St. Augustine is receiving more than 1 inch of water per week, reduce it immediately. For chemical control, Atrazine remains the gold standard for St. Augustine lawns. Apply a spot-treatment mixture of 1.5 fluid ounces of Atrazine per 1,000 square feet, mixed into 1 gallon of water.
Temperature Warning: Never apply Atrazine when daytime temperatures exceed 85°F. High heat combined with Atrazine will cause severe phytotoxicity, resulting in yellowing and necrosis of the St. Augustine stolons.
Doveweed (Murdannia nudiflora)
Doveweed is a deceptive invader. It mimics St. Augustine grass in its early vegetative stages, often going unnoticed until it produces distinct purple flowers and emits a rubbery odor when mowed. Standard 2,4-D blends will not control it. The most effective spot-treatment relies on a combination of sulfentrazone and metsulfuron-methyl (commonly sold under professional brands like Blindside). Mix at a rate of 0.75 oz per 1,000 square feet. Because Doveweed propagates via both seeds and creeping stems, expect to perform a second spot-application 14 days after the initial treatment to catch late-germinating seedlings.
Grassy Weed Spot-Treatment Protocols
Dallisgrass (Paspalum dilatatum)
Dallisgrass is widely considered the most difficult grassy weed to manage in St. Augustine turf. Because it is a perennial grass closely related to the desirable turf, selective post-emergent herbicides that kill Dallisgrass will also kill St. Augustine. Following the residential ban on MSMA, the only reliable spot-treatment method is the careful application of a non-selective herbicide like glyphosate.
Do not broadcast spray. Instead, use a foam brush to paint a 2% glyphosate solution directly onto the Dallisgrass leaves, or use a specialized shielded spray nozzle. Once the Dallisgrass turns brown (usually within 7 to 10 days), rake out the dead clump and immediately plant a St. Augustine plug to prevent crabgrass from colonizing the bare soil.
Crabgrass (Digitaria spp.)
While pre-emergent herbicides are the best defense against crabgrass, late-summer breakthroughs require post-emergent spot-treatment. Quinclorac is highly effective and safe for St. Augustine when applied correctly. Mix 0.37 oz of Quinclorac per 1,000 square feet. Crucially, Quinclorac requires a methylated seed oil (MSO) or a high-quality non-ionic surfactant to penetrate the waxy cuticle of the crabgrass leaf. Add 1 teaspoon of surfactant per gallon of spray solution.
Safe Herbicide Formulations for St. Augustine
When purchasing off-the-shelf broadleaf herbicides for general weeds like clover or chickweed, you must read the active ingredient label carefully. St. Augustine grass is highly susceptible to damage from standard 2,4-D esters.
The Amine Rule: If you must use 2,4-D, ensure the label specifies an amine formulation, not an ester. Furthermore, look for products explicitly branded for 'Southern Lawns' or 'St. Augustine,' which typically dilute the 2,4-D concentration and buffer it with MCPP and Dicamba at lower, safer thresholds. According to turf management guidelines from UF/IFAS Turfgrass Science, applying these reduced-rate blends during the cooler morning hours minimizes volatilization and turf stress.
Step-by-Step Spot Spraying Technique
Precision is paramount. Drift onto healthy St. Augustine stolons can cause localized dieback. Follow this calibrated procedure for every spot-treatment session:
- Calibrate the Sprayer: Use a dedicated 1-gallon or 2-gallon pump sprayer. Do not use a sprayer previously used for non-selective herbicides like Roundup.
- Upgrade the Nozzle: Replace the standard cone nozzle with a TeeJet 8002VS flat fan nozzle. This produces a precise, even spray pattern that minimizes off-target drift compared to aerosolized cone sprays.
- Mix the Surfactant First: Fill the tank halfway with water, add the required non-ionic surfactant (0.25% v/v ratio, roughly 1/2 teaspoon per gallon), and agitate. Surfactants reduce the surface tension of the water, allowing the herbicide to spread evenly across the waxy leaves of weeds like Doveweed.
- Add the Herbicide: Measure the active ingredient using a dedicated syringe or graduated cylinder for exact accuracy. Add to the tank and fill the remaining water.
- Apply to the Point of Drip: Spray the target weed until the leaves are uniformly coated and just beginning to drip. Do not drench the soil; most post-emergent broadleaf herbicides are foliar-absorbed.
- Observe the 48-Hour Rule: Do not mow the lawn for 48 hours before or 48 hours after application. This ensures the weed has maximum leaf surface area to absorb the chemical and translocate it to the root system.
Post-Treatment Lawn Recovery
Herbicide applications, even when perfectly targeted, induce mild metabolic stress on the surrounding St. Augustine grass. To accelerate recovery and encourage the turf to fill in the voids left by dying weeds, apply a light dose of liquid fertilizer 10 to 14 days after the herbicide application.
Use a fertilizer with a 3-1-2 NPK ratio (such as a 15-5-10 blend), applying exactly 0.5 lbs of actual Nitrogen per 1,000 square feet. Water the lawn deeply the morning after fertilization to push the nutrients into the root zone. For comprehensive regional maintenance schedules and cultivar-specific fertilizer thresholds, consult the Texas A&M AgriLife Extension St. Augustinegrass guide. By combining precise chemical spot-treatments with proactive cultural recovery, you can maintain a dense, weed-free St. Augustine canopy without risking the chemical burn associated with aggressive broadcast applications.
