The Best Herbicide for St. Augustine Grass: What’s Safe to Use?

St. Augustine grass is known for its thick, carpet-like growth and excellent shade tolerance, making it a top choice for Southern lawns. But while it’s tough on foot traffic, it’s not so tough when it comes to herbicides. If you apply the wrong weed killer, you could end up damaging or even killing your lawn.

The best herbicides for St. Augustine grass are those that control weeds without damaging your lawn. Safe post-emergent options include Celsius WG (for broadleaf and some grassy weeds), Certainty (for sedges), and SedgeHammer® (for nutsedge). For pre-emergent weed control, use Prodiamine 65 WDG or Specticle FLO. Avoid herbicides like Quinclorac or 2,4-D, which can harm St. Augustine. Always follow label instructions, apply at the right time, and spot-treat when possible. Healthy lawns plus smart weed control = thick, green, weed-free turf.

In this guide, we’ll examine the safe weed killer options, when and how to apply them, and what to avoid. Whether you're battling broadleaf weeds, sedges, or unwanted grasses, this guide will help you win the war without sacrificing your lawn.

Why St. Augustine Grass Needs Special Care

St. Augustine (Stenotaphrum secundatum) is a warm-season grass common in southern U.S. states like Florida, Texas, and the Carolinas. It thrives in heat and humidity and spreads aggressively via stolons, creating a lush, dense lawn. But that lushness comes at a cost: St. Augustine is sensitive to many herbicides that are safe for other turf types like Bermuda or Zoysia.

Applying the wrong product — especially ones with 2,4-D or Quinclorac — can cause stunted growth, discoloration, or full-blown turf damage.

You’re in the right place if you’ve been wondering, “What herbicide is safe for St. Augustine?,” 

Common Weeds in St. Augustine Lawns

To choose the right herbicide, you must first know what you’re fighting. The most common weed types in St. Augustine lawns include:

• Broadleaf weeds: Dollarweed, spurge, chickweed, clover
  

• Grassy weeds: Crabgrass, annual bluegrass (Poa annua), goosegrass
  

• Sedges: Yellow nutsedge, purple nutsedge, kyllinga.

Each weed type requires a different control strategy, and not all herbicides are safe for St. Augustine grass. So let’s dive into the best options.

Related: The Best Crabgrass Weed Killers That Won’t Kill Your Turfgrass 

The Best Safe Herbicide Options for St. Augustine Grass

1. Celsius WG Herbicide

If you’re looking for a broad-spectrum broadleaf weed killer for St. Augustine grass, Celsius WG is your go-to. It controls over 100 weed species — including dollar weed, clover, spurge, and even some grassy weeds — without harming your St. Augustine turf.

• Safe at high temperatures
  

• Works well on southern lawns
  

• Apply with a non-ionic surfactant for better uptake

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2. Certainty Herbicide

Weeds like nutsedge and kyllinga are stubborn, but Certainty is designed to tackle them without damaging sensitive turfgrass. It’s highly selective and safe for St. Augustine grass, even when temperatures are rising.

• Targets sedges and some grassy weeds
  

• Effective in humid climates
  

• Pair with a surfactant for best results.

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3. Celsius and Certainty Herbicide Kit

Want full-spectrum control? This bundle gives you a powerful one-two punch for broadleaf weeds, sedges, and select grassy weeds while being safe for your warm-season turf.

• Trusted combination for pros and DIYers
  

• Surfactant and blue marker dye are included in the kit
  

• Spot spray for targeted treatment.

celsius-and-certainty-herbicide-kit-with-surfactant-and-dye

4. SedgeHammer® Herbicide

Need a safe weed killer for St. Augustine lawns that targets nutsedge only? SedgeHammer® is a gentle but effective choice.

• Controls yellow and purple nutsedge
  

• Safe for most warm-season grasses
  

• Use as a spot treatment for best results.

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🛑 Note: Avoid using Tenacity Herbicide on residential St. Augustine lawns — it's not labeled for use on this grass type.

5. Prodiamine 65 WDG (Pre-Emergent Herbicide)

Preventing weeds before they sprout is the most effective control method. Prodiamine is a pre-emergent herbicide that stops crabgrass, Poa annua, and other annual weeds from germinating.

• Safe for St. Augustine when applied correctly
  

• Apply in early spring and again in fall
  

• Water in for activation.

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6. Specticle FLO (Advanced Pre-Emergent Option)

For elite-level St. Augustine grass weed control, Specticle FLO offers long-lasting pre-emergent protection with professional-grade results. Just be sure to follow label directions carefully — overapplication can injure turf.

• Excellent crabgrass and Poa annua prevention
  

• Can last 6+ months
  

• Use with precision sprayers.

specticle-flo-pre-emergent-herbicide

Removing Invasive Bermuda from St. Augustine Grass

Bermuda grass is a common invader in St. Augustine lawns. It’s aggressive spreading can choke out your turf, Fortunately, the Buy Recognition and Fusilade II Herbicide Kit offers a targeted solution designed specifically for removing Bermuda without harming your St. Augustine (or Zoysia) grass. 

What’s in the Kit?

• Recognition Herbicide, containing a turf safener (metcamifen) that allows safe application over St. Augustine.

• Fusilade II, a systemic post-emergent that kills grassy weeds like Bermuda.

• A non-ionic surfactant and blue marker dye for easy use and coverage monitoring. 

How It Works

• The safener in Recognition enables the use of Fusilade II on sensitive turfgrass without damage, especially crucial for St. Augustine.

• Tank mix the two herbicides plus surfactant and dye.

• Apply in mid-spring when Bermuda is active but not fully thick. This maximizes effectiveness and turf safety.

• Expect browning in 3-4 weeks, with follow-up sprays as needed, conducted 4-6 weeks apart.

Best Practices Before and After Application

• Mow lightly, then wait 3-7 days before spraying. Don’t mow again for another 4-7 days after application.

• Avoid spraying in extreme heat (above 95°F) or in windy conditions.

• Let treated areas rest, then apply a light liquid fertilizer or biostimulant to encourage St. Augustine to fill back in. 

• Spot spray regularly after initial clean-up to prevent Bermuda from returning.

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How to Safely Apply Herbicides on St. Augustine Lawns

Always Read the Label

Make sure the product specifically lists St. Augustine grass as a safe-use species. Follow mixing ratios, surfactant use, and reapplication intervals.

Apply at the Right Time

• Pre-emergents: Early spring and early fall
  

• Post-emergents: When weeds are actively growing (avoid drought stress or dormancy).

Spot-Treat When Possible

Minimize turf exposure and herbicide use by only targeting weedy areas.

Related: How to Get Rid of Bermudagrass In Your Zoysia or St. Augustine Lawn 

Boost Weed Control with Lawn Health Products

A healthy, dense lawn is your best natural defense against weeds. When your St. Augustine grass is thick, vigorous, and well-fed, it leaves little room for intruders like crabgrass, sedge, or spurge to sneak in. So once you’ve handled existing weeds with the right herbicides, the next step is to build resilience from the soil up.

Here are some top-rated products from the Golf Course Lawn store that work beautifully on St. Augustine grass to support recovery and promote strong, healthy growth:

Humic Max 16-0-8 Fertilizer

This premium granular fertilizer contains 16% nitrogen for lush green-up, 8% potassium for improved stress tolerance, and a generous dose of humic acid, which enhances soil biology and nutrient availability. It’s especially useful after weed control treatments when your lawn needs a recovery boost without risking burn.

• Encourages thick, green growth

• Supports root development and soil health

• Great for spoon-feeding St. Augustine lawns during the growing season.

country-club-16-0-8-humic-max-and-mesa%C2%AE-by-lebanonturf-sgn-150

CarbonizPN-G™ Soil Compost and Biochar

Weed control doesn’t stop at the surface — your soil plays a huge role in lawn health. CarbonizPN-G™ is a granular soil amendment packed with biochar, compost, humates, and beneficial microbes. It improves compaction, boosts microbial activity, and enhances water retention — all of which help your grass outcompete weeds naturally.

• Builds better soil structure

• Enhances nutrient uptake and root growth

• Improves water efficiency in sandy or compacted soils.

essential-g™-granular-carbon-free-shipping

Release 901C™ Biostimulant

This liquid biostimulant is like a multivitamin for your lawn. With 9% nitrogen plus carbon-rich compounds and micronutrients, it feeds your grass while energizing soil microbes. It’s ideal for use alongside your regular fertility program and perfect for stressed lawns after herbicide use.

• Fast green-up with minimal growth surge

• Improves nutrient efficiency and root activity

• Compatible with most herbicides and fertilizers.

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By combining smart weed control with a strong lawn care foundation, you’ll create a St. Augustine lawn that’s thick, vibrant, and far less likely to suffer from future weed outbreaks. Think of it as investing in long-term lawn insurance — the healthier your grass, the fewer weeds you’ll have to fight.

Seasonal Lawn Care Calendar for St. Augustine Grass

Want to keep weeds out before they start and make the most of your herbicide and fertilizer program? Here’s a simple season-by-season guide to keeping your St. Augustine lawn looking like a golf course.

Spring (March–May)

• Apply pre-emergent herbicide (like Prodiamine 65 WDG) once soil temps reach 55°F
  

• Spot-treat weeds with Celsius WG or Certainty as needed
  

• Begin fertilizing lightly with Humic Max 16-0-8
  

• Mow regularly at 3–4 inches.
  

• Apply CarbonizPN-G™ to improve soil structure.
  
  

Summer (June–August)

• Continue spot-treating summer weeds with herbicides.
  

• Apply biostimulants like Release 901C™ to support stress resistance
  

• Water deeply 2–3 times per week (1 inch total)
  

• Avoid applying herbicides during extreme drought or heat stress.
  
  

Fall (September–November)

• Apply fall pre-emergent (Prodiamine or Specticle FLO) to prevent winter weeds like Poa annua.
  

• Feed the lawn with a slow-release fertilizer or carbon-rich biostimulant.
  

• Continue mowing until growth slows
  

• Repair bare spots with plugs if needed — St. Augustine spreads by stolons, not seed.

Winter (December–February)

• Lawn goes dormant or semi-dormant in most regions.
  

• Limit foot traffic and mowing
  

• No need to fertilize or spray unless weeds are actively growing
  

• Plan for spring pre-emergent timing by tracking soil temps.

📌 Pro tip: Use a soil thermometer or local soil temp tracker to time your pre-emergent applications perfectly. A few degrees can make all the difference in weed prevention.

FAQs: St. Augustine Grass & Lawn Care

Q: How often should I water St. Augustine grass?
A: Water deeply 2–3 times per week, applying about 1 inch total. Adjust based on rainfall and soil type.

Q: What’s the best mowing height for St. Augustine?
A: Aim for 3–4 inches. Taller mowing helps shade out weeds and protect roots from heat stress.

Q: Can I overseed St. Augustine grass?
A: Not typically. St. Augustine doesn’t respond well to overseeding. It spreads via stolons and fills in bare spots naturally with good care.

Q: Why is my St. Augustine lawn turning yellow?
A: Could be due to iron deficiency, overwatering, chinch bugs, or herbicide damage. Consider applying a product like Golf Course Lawn Liquid Micronutrient Fertilizer for a quick iron boost. 

Q: How do I thicken up my St. Augustine lawn?
A: Regular mowing, spoon-feeding with liquid fertilizer, applying humic acids, and good watering habits will all help it spread.

The Last Word on Weed Control for St. Augustine Grass

Choosing the best herbicide for St. Augustine grass comes down to using effective and safe products. Whether you're targeting broadleaf weeds, sedges, or crabgrass, the Golf Course Lawn store has professional-grade solutions that protect your lawn while knocking out the competition.

Stick with safe, proven products like Celsius WG, Certainty, and Prodiamine 65 WDG and pair them with smart cultural practices, and you'll be well on your way to a thick, weed-free, and vibrant St. Augustine lawn.

Shop safe, professional-grade products for St. Augustine lawns today and get one step closer to the lawn you’ve always wanted. Check out our YouTube channel for more lawn care tips and tricks.