Sunniland St. Augustine Weed & Feed: Spot-Treating Surviving Weeds

The Broadcast Baseline: What Sunniland St. Augustine Weed & Feed Actually Does

Sunniland St. Augustine Weed & Feed is a staple for southern homeowners managing warm-season turf. The product typically utilizes a dual-action formula, combining a nitrogen-heavy fertilizer base (often a 16-4-8 or 15-0-15 NPK ratio) with the active herbicide ingredient Atrazine, usually at a 0.5% to 1.0% concentration. Atrazine is a photosystem II inhibitor that disrupts photosynthesis in susceptible broadleaf weeds and some annual grasses. Because St. Augustine grass (Stenotaphrum secundatum) possesses the specific enzymes required to metabolize Atrazine before it causes cellular damage, the chemical is highly effective as a broadcast treatment for common invaders like dollarweed, clover, chickweed, and henbit.

However, broadcast granular applications are inherently limited. They rely on the weed's foliage intercepting the granules, followed by moisture to dissolve the active ingredient into the soil or leaf tissue. When dealing with mature weeds, waxy cuticles, or species with deep rhizomatous networks, Sunniland's Atrazine-based formula will often suppress top growth without killing the root system, leading to rapid regrowth.

The Resistance Gap: Why Spot Treatment is Necessary

Relying solely on broadcast weed and feed creates a survival bottleneck in your lawn. The weeds that survive the initial Atrazine application are typically those with natural physiological defenses. These include thick waxy leaf cuticles that repel water-soluble granules, rapid vegetative reproduction via stolons or rhizomes, or complete genetic resistance to photosystem II inhibitors. According to turfgrass research from the University of Florida IFAS Extension, repeated use of Atrazine without rotating modes of action accelerates localized weed resistance, particularly in high-moisture environments where St. Augustine thrives.

To maintain a pristine lawn, you must transition from a broadcast-only mindset to an integrated approach. This means using Sunniland St. Augustine Weed & Feed for your spring and fall baseline fertility and broadleaf control, while deploying targeted liquid spot-treatments for the specific species that bypass the granular defense.

Precision Spot-Treatment Protocols for the 'Big Three' Survivors

The following protocols address the three most notorious weeds that survive Atrazine-based weed and feed applications in St. Augustine lawns. These require specific active ingredients, exact dilution rates, and careful application techniques to avoid phytotoxicity (chemical burn) to your turf.

1. Doveweed (Murdannia nudiflora)

Doveweed is a summer annual broadleaf weed that mimics grass, often escaping notice until it flowers with small purple blooms. Its thick, waxy cuticle makes it highly resistant to granular Atrazine absorption. Furthermore, doveweed thrives in the exact same wet, compacted soil conditions that St. Augustine tolerates.

• Active Ingredient: Sulfentrazone (e.g., Blindside or Dismiss). Sulfentrazone is a PPO inhibitor that causes rapid cell membrane disruption, bypassing the waxy cuticle barrier.
• Mix Rate: 0.25 fluid ounces of Sulfentrazone (40% active ingredient formulation) per 1 gallon of water to cover 1,000 square feet.
• Adjuvant: Add a Non-Ionic Surfactant (NIS) at a rate of 0.25% (roughly 1/2 teaspoon per gallon) to break the surface tension of the doveweed's waxy leaves.
• Timing: Spot-treat when doveweed is young and actively growing, ideally before it reaches the 3-leaf stage. Avoid application when soil temperatures exceed 85°F.

2. Torpedo Grass (Panicum repens)

Torpedo grass is a perennial grassy weed that spreads aggressively via underground rhizomes and above-ground stolons. Because it is biologically similar to St. Augustine, most broadleaf herbicides (including Atrazine) have zero effect on it. It is widely considered one of the most difficult weeds to eradicate in warm-season turf, a challenge frequently highlighted by the NC State Turfgrass Extension.

• Active Ingredient: Quinclorac (e.g., Drive XLR8). Quinclorac mimics auxin plant hormones, causing uncontrolled, lethal growth in susceptible grassy weeds.
• Mix Rate: 0.367 fluid ounces per 1 gallon of water per 1,000 square feet.
• Adjuvant: Methylated Seed Oil (MSO) at 1% volume (1.25 oz per gallon) is strictly required for Quinclorac to penetrate torpedo grass foliage.
• St. Augustine Warning: Quinclorac can cause temporary stunting or yellowing in St. Augustine grass. Spot-treat only the specific patches of torpedo grass rather than broadcasting, and expect the surrounding St. Augustine to take 10-14 days to recover its color.

3. Yellow Nutsedge (Cyperus esculentus)

Nutsedge is neither a grass nor a broadleaf weed; it is a sedge. It features a triangular stem ('sedges have edges') and reproduces via underground tubers (nutlets). Atrazine does not translocate effectively into these tubers, meaning the weed will simply sprout new shoots weeks after a weed and feed application.

• Active Ingredient: Halosulfuron-methyl (e.g., SedgeHammer). This is an ALS (acetolactate synthase) inhibitor that halts amino acid production specifically in sedges.
• Mix Rate: 0.015 ounces (typically one pre-measured water-soluble packet) per 1 gallon of water.
• Adjuvant: Most commercial Halosulfuron formulations include the required surfactant in the packet. If using a generic liquid concentrate, add NIS at 0.25%.
• Application Technique: Spray the foliage until wet, but do not drench the soil. The chemical must be absorbed through the leaves and translocated down into the tubers. Do not mow for 48 hours before or after application to maximize leaf surface area.

Application Matrix: Integrating Granular and Liquid Treatments

Use the following decision matrix to coordinate your Sunniland broadcast applications with your liquid spot-treatments. Never apply liquid spot-treatments on the exact same day as granular weed and feed, as the compounding chemical load can severely burn St. Augustine stolons.

Weed Species	Sunniland W&F Efficacy	Spot-Treatment Active Ingredient	Exact Mix Rate (per 1 gal)	Wait Time After W&F
Dollarweed / Clover	High (Broadcast)	N/A (Handled by Atrazine)	N/A	N/A
Doveweed	Low (Mature Plants)	Sulfentrazone (PPO Inhibitor)	0.25 fl oz + 0.25% NIS	14 Days
Torpedo Grass	None (Grassy Weed)	Quinclorac (Synthetic Auxin)	0.367 fl oz + 1% MSO	21 Days
Yellow Nutsedge	None (Sedge Species)	Halosulfuron-methyl (ALS Inhibitor)	0.015 oz (packet)	7 Days

Calibrating Your Spot-Treatment Sprayer for St. Augustine Safety

The most common failure mode in spot-treating St. Augustine is improper sprayer calibration, leading to chemical overdose and severe turf burn. St. Augustine grass is highly sensitive to surfactant burn and herbicide accumulation. To ensure you are applying the exact spot-treatment rates listed above, perform a simple calibration test with plain water.

1. Measure the Area: Mark off a 100-square-foot section of your driveway or a sparse area of the lawn (e.g., a 10x10 foot square).
2. Spray with Water: Fill your 1-gallon pump sprayer with exactly 32 ounces of plain water. Spray the 100-square-foot area at your normal walking pace and sprayer pressure (usually 20-30 PSI).
3. Measure the Remainder: Check how much water is left in the tank. If you used 16 ounces to cover 100 square feet, you know your sprayer outputs 1 gallon per 800 square feet. You must adjust your chemical mix rates proportionally based on your specific output volume.

Temperature and Moisture Constraints for St. Augustine Safety

Environmental conditions dictate the success of your spot-treatments. St. Augustine grass enters a state of environmental stress when soil moisture is low and ambient temperatures are high. Applying Sulfentrazone or Quinclorac when daytime temperatures exceed 85°F drastically increases the risk of phytotoxicity. The herbicides volatilize and burn the grass blades before they can translocate into the weed.

Always schedule your spot-treatments for early morning (between 6:00 AM and 8:00 AM) when the stomata of the weeds are open and temperatures are cool. Ensure the lawn has been irrigated 24 hours prior to application; a drought-stressed weed will shut down its vascular system to conserve water, preventing the systemic herbicides (like Halosulfuron and Quinclorac) from traveling down to the roots and tubers. By respecting these thermal and moisture thresholds, you protect your St. Augustine turf while systematically eliminating the weeds that survive your baseline Sunniland weed and feed applications.