Natural and Low-Toxicity Herbicide Options for St. Augustine Lawns

Managing weeds in a St. Augustine grass lawn can be challenging, especially in South Florida’s climate. Below are pre-emergent (preventive) and post-emergent (weed-killing) herbicide solutions that are safe for St. Augustine and have low toxicity (avoiding known carcinogens). Each option includes notes on effectiveness, how to apply, availability, and safety (with relevant studies or regulatory info where available).

Pre-Emergent Herbicides (Prevent Weeds from Germinating)

1. Corn Gluten Meal (Natural Pre-emergent)

What It Is: Corn gluten meal (CGM) is a by-product of corn processing that acts as a natural “weed and feed.” It inhibits root formation in germinating weed seeds, reducing weed establishment without harming mature turfgrass roots.

Effectiveness: When applied at sufficient rates (about 20 lbs per 1,000 sq ft), CGM can significantly reduce annual weed germination. It works on many broadleaf and grassy weeds by causing seedlings to dry out and die. It only affects sprouting seeds, so it doesn’t control established weeds.

Application: Apply before weed seeds germinate—typically in early spring (for summer weeds) and again in fall (for winter weeds) in Florida. Spread it evenly like a fertilizer (it also contains around 10% nitrogen to fertilize turf). If using a branded product, follow label rates; many recommend two applications per year. Avoid seeding or plugging new grass for a few weeks after application.

Safety: Corn gluten meal is a food byproduct—non-toxic to people, pets, and wildlife. Not a known carcinogen.

Availability: Widely available at garden centers and feed stores. Look for organic pre-emergent products or lawn fertilizers labeled with corn gluten. Lawn care companies sometimes offer it in organic lawn care packages.

2. Prodiamine or Dithiopyr (Synthetic Pre-emergents with Low Toxicity)

What They Are: Conventional pre-emergent herbicides sold under brand names like Barricade (prodiamine) or Dimension (dithiopyr). They form a barrier in the soil to stop weed seeds from emerging.

Effectiveness: Both are highly effective at preventing many weeds, especially crabgrass and other annual grasses, as well as some broadleaf weeds. A single application can prevent weeds for about 3–4 months. Dithiopyr also has early post-emergent activity on newly sprouted crabgrass.

Application: Available in granular or liquid formulations. Generally applied before weeds germinate (in South Florida, once in late fall and once in early spring). These products are safe on established St. Augustine grass at label rates but can stunt new grass seeding or rooting of fresh plugs.

Safety: Both prodiamine and dithiopyr have low acute toxicity and are not associated with significant long-term health risks when used properly. Dithiopyr in particular has shown no evidence of carcinogenicity in animal studies.

Availability: Common in “crabgrass preventer” products sold in garden centers under various brands (Scotts, Lesco, Vigoro, etc.). Always confirm the product is labeled for St. Augustine.

Post-Emergent Herbicides (Kill Existing Weeds)

3. Iron-Based Herbicide (FeHEDTA) – Selective Broadleaf Weed Killer

What It Is: Chelated iron (FeHEDTA) that targets many broadleaf weeds in lawns. The iron overload causes oxidative damage in the weed leaves, killing them, while grass is largely unharmed apart from possible temporary darkening.

Effectiveness: Works best on young broadleaf weeds (dandelion, clover, chickweed, dollarweed, and more). Weeds typically show blackening and shriveling within hours. Deep-rooted or perennial weeds might require repeat applications.

Application: Sold as ready-to-spray liquids or concentrates. Apply it as a foliar spray in weather that isn’t too hot or drought-stressed. Avoid rain or irrigation for a few hours after spraying.

Safety: Iron HEDTA is mineral-based and considered very safe for humans, pets, and the environment. It is not known to be carcinogenic. Mild irritation potential if the concentrate contacts skin or eyes.

Availability: Many brands offer this formulation (Scotts/Ortho “Weed B Gon with Iron,” Natria Lawn Weed Control, Fiesta, etc.).

4. Cinnamon Bark-Based Weed Killer (AgraLawn Crabgrass Killer – Natural Post-emergent)

What It Is: A natural herbicide made from cinnamon bark and other food-grade ingredients (corn meal, baking soda, cumin) for spot-treating weeds. It targets certain weeds without harming warm-season grasses like St. Augustine.

Effectiveness: Particularly useful for crabgrass, basketgrass, chickweed, and clover. It’s a powder shaken onto weed foliage when it’s wet, which kills weeds in a few days. Primarily for small patches rather than entire-lawn application.

Application: Spot treatment only. Moisten weed leaves and sprinkle the powder on top. Best done in warm, sunny weather so the product sticks and dehydrates the weed. May require a second application.

Safety: Extremely safe, using spices and baking soda as active ingredients. Non-toxic to people, pets, and pollinators. Just avoid inhaling the dust when applying.

Availability: Branded as AgraLawn Crabgrass Killer or sometimes Garden Weasel AG Crabgrass Killer. Sold online or in select nurseries/organic garden stores.

5. Natural Non-Selective Contact Herbicides (Vinegar, Citrus Oil, Clove Oil, Soap-Based)

What They Are: Non-selective sprays that kill plant foliage on contact. Common active ingredients include strong acetic acid (20% vinegar), citrus oil (d-Limonene), clove oil (eugenol), or fatty acid soaps (pelargonic acid, ammonium soaps).

Effectiveness: Quickly browns and wilts the top growth of weeds, often within hours. They do not typically kill the roots of perennial weeds, so regrowth may occur. Most effective on small, annual weeds.

Application: Sold as ready-to-use or concentrates under names like Avenger Weed Killer, BurnOut, or horticultural vinegar. For lawn use, take care because these products will burn grass too if sprayed carelessly. Shield or spot-treat weeds.

Safety: Low risk of serious health issues. High concentrations of vinegar can cause skin or eye burns; citrus and clove oils can irritate skin. Wear gloves and eye protection. These degrade quickly, posing minimal environmental impact.

Availability: Often found in the organic/natural section of garden centers, or sold online. Many are OMRI-listed for organic gardening.

6. Halosulfuron (SedgeHammer) – Low-Tox Synthetic for Sedges

What It Is: A selective post-emergent herbicide targeting sedges (yellow and purple nutsedge) and some broadleaf weeds without harming St. Augustine.

Effectiveness: Very effective on nutsedge by reaching underground tubers (nutlets). Often eliminates sedges fully after one or two treatments. Works slowly but thoroughly.

Application: Sold as water-soluble granules (under names like SedgeHammer and Empero). Spray onto actively growing sedges, then avoid mowing or watering for a day or two. A second application may be needed 6–10 weeks later.

Safety: Classified as “Not Likely To Be Carcinogenic to Humans” and breaks down quickly in soil. Minimal risk to people or pets once dried.

Availability: Commonly available in small packets at lawn/garden retailers or online. Follow label mixing instructions carefully.

Additional Tips for Safe Weed Management

Integrated Approach: Combine these products with good lawn care (proper mowing height, fertilization, and watering) to create a dense, healthy lawn that naturally suppresses weeds.
Spot Treatment vs. Broadcast: Reserve post-emergent herbicides for spot treatments when possible. Only apply pre-emergents to areas with a known weed problem.
Read Labels: Even low-toxicity products can irritate skin or eyes if mishandled. Wear gloves and follow label instructions exactly.
Protect Yourself: Basic precautions—long sleeves, gloves, washing skin after handling—greatly reduce any exposure. Keep children and pets off treated areas until sprays are dry or granules are watered in.
Regulatory Assessments: Most of these products have been reviewed by the EPA or university extensions. Iron-based herbicides are commonly used on school grounds and in parks due to their safety profile. The listed synthetic options (dithiopyr, halosulfuron, etc.) are typically considered low risk when applied correctly.

By combining a natural pre-emergent like corn gluten meal with careful use of selective, low-toxicity post-emergents for problem weeds, you can keep your St. Augustine lawn healthy and weed-free without resorting to harsher chemicals. Be patient with organic and reduced-risk approaches, as they sometimes require more diligence and repeat applications.