St. Augustine grass (Stenotaphrum secundatum) is a warm-season turfgrass commonly used in lawns, golf courses, and coastal landscapes due to its thick growth, shade tolerance, and ability to thrive in warm climates. However, maintaining a St. Augustine lawn can be challenging when broadleaf weeds invade, requiring selective herbicides like 2,4-D (2,4-Dichlorophenoxyacetic acid).

But will 2,4-D kill St. Augustine grass? The answer depends on the concentration, application rate, and grass health. This guide will explain how 2,4-D interacts with St. Augustine grass, when it is safe to use, and best practices for effective weed control without damaging your lawn.

Does 2,4-D Kill St. Augustine Grass?

How 2,4-D Affects St. Augustine Grass

✔ 2,4-D is a selective herbicide, primarily targeting broadleaf weeds, meaning it is not designed to kill grasses like St. Augustine.
✔ In low concentrations, St. Augustine grass can tolerate 2,4-D.
✖ High concentrations can damage or kill St. Augustine grass, causing yellowing, stunted growth, or even turf dieback.
✖ Stress factors such as heat, drought, or over-application increase the risk of grass injury.

When Can 2,4-D Kill or Harm St. Augustine Grass?

• Young or Newly Established Lawns – Immature St. Augustine grass is more sensitive to herbicides.
• High Application Rates – Using a strong concentration or reapplying too frequently can cause burning or thinning.
• Extreme Heat Conditions – Applying 2,4-D when temperatures exceed 85°F (29°C) increases the risk of grass injury.
• Weakened or Stressed Grass – If St. Augustine is drought-stressed, over-fertilized, or suffering from disease, it is more vulnerable to herbicide damage.

How to Use 2,4-D on St. Augustine Grass Without Killing It

Choosing the Right 2,4-D Product

• Use a 2,4-D formulation labeled as safe for St. Augustine grass. Some broadleaf weed killers contain other herbicides (Dicamba, MCPP, MSMA) that can be harmful to St. Augustine lawns.
• Avoid high-concentration herbicide mixes that combine 2,4-D with other powerful herbicides.

Safe Application Guidelines

✔ Apply during active growth (spring or early summer) when grass is not under stress.
✔ Use a low concentration – A diluted 2,4-D solution (0.5-1% mix) is generally safe for St. Augustine grass.
✔ Spot-treat weeds instead of broadcast spraying – This minimizes exposure to healthy turf.
✔ Avoid spraying in hot weather – Apply when temperatures are below 85°F (29°C) to prevent herbicide stress.
✔ Water the lawn 24 hours before application to reduce stress and improve tolerance.
✔ Reapply only if necessary – If weeds persist, wait at least 2-4 weeks before making a second application.

What Are Safer Alternatives to 2,4-D for St. Augustine Grass?

If you are concerned about herbicide damage, consider safer broadleaf weed killers specifically designed for St. Augustine grass:

Selective Herbicides Safe for St. Augustine Lawns

✔ Atrazine – Kills broadleaf weeds and some grassy weeds while being safe for St. Augustine.
✔ Metsulfuron-methyl – Effective against broadleaf weeds and sedges, with low impact on St. Augustine.
✔ Celsius WG Herbicide – A highly effective broadleaf weed and sedge control solution that is safe for St. Augustine.
✔ Simazine – A pre-emergent herbicide preventing weeds from germinating while being gentle on St. Augustine.

Best Practices for Weed Control in St. Augustine Lawns

✔ Mow at the Proper Height – Keeping St. Augustine grass at 3.5 to 4 inches shades out weeds and reduces herbicide need.
✔ Fertilize Properly – A well-fed lawn is more competitive against weeds. Avoid excessive nitrogen, which can weaken the grass.
✔ Improve Drainage and Soil Health – Healthy turf with good soil aeration is more resistant to weed competition.
✔ Use Pre-Emergent Herbicides – Apply pre-emergents in early spring to stop weeds before they appear.

Final Verdict: Will 2,4-D Kill St. Augustine Grass?

• 2,4-D will not directly kill healthy, mature St. Augustine grass, but high concentrations or repeated applications can cause significant damage.
• Young, stressed, or heat-exposed St. Augustine lawns are more susceptible to injury from 2,4-D.
• For safer broadleaf weed control, consider Atrazine, or Metsulfuron instead.
• Applying 2,4-D correctly—at the right dilution, temperature, and conditions—reduces the risk of harming your lawn.

By following best practices and using selective herbicides wisely, you can effectively manage weeds while keeping your St. Augustine lawn healthy and green.