Weed Killer That Won't Kill St Augustine Grass

Why St Augustine Grass Is So Sensitive to Weed Killers

St Augustine grass (Stenotaphrum secundatum) is a warm-season turfgrass prized for its thick, broad blades and lush green carpet. It thrives in southern climates and is a favorite for homeowners who want a low-maintenance, heat-tolerant lawn. However, St Augustine is also one of the most herbicide-sensitive turfgrasses available. Its broad leaves absorb chemicals more readily than fine-bladed grasses like Bermuda or fescue, making it vulnerable to non-selective or improperly applied products.

This sensitivity means you cannot simply grab any weed killer off the shelf. Using a non-selective herbicide like glyphosate will kill your St Augustine grass just as effectively as it kills the weeds. Even some selective products formulated for other grass types can cause yellowing, stunting, or death in St Augustine lawns. Understanding the difference between selective and non-selective herbicides is the first step toward protecting your lawn while eliminating weeds.

If you are also managing other lawn care tasks, such as maintaining a healthy watering schedule or choosing the right weed and feed for St Augustine grass, combining those practices with a safe herbicide strategy will give you the best results.

Selective vs. Non-Selective Weed Killers

Weed killers fall into two broad categories: selective and non-selective. Selective herbicides are designed to target specific types of plants (like broadleaf weeds or certain grasses) while leaving the desired turfgrass unharmed. Non-selective herbicides, such as glyphosate, kill virtually all plants they contact, including your lawn.

For St Augustine grass, you must use a selective herbicide that is explicitly labeled as safe for this grass type. Even within selective herbicides, not all products are safe for St Augustine. Some broadleaf weed killers are formulated for cool-season grasses and can damage warm-season turf. Always read the label and confirm that St Augustine grass appears in the list of tolerant turfgrasses.

If you are dealing with weeds in other areas of your landscape, such as flower beds or vegetable gardens, you may need different products. For example, a weed killer for flower beds that won't kill plants is formulated differently than a lawn herbicide. Similarly, if you have pets, consider a pet-safe weed killer that won't kill grass to minimize risks.

Safe Active Ingredients for St Augustine Grass

The following active ingredients are commonly found in selective herbicides labeled as safe for St Augustine grass. Each has a specific mode of action and target weed spectrum. Always confirm the individual product label before use.

1. 2,4-D (2,4-Dichlorophenoxyacetic acid)

2,4-D is one of the most widely used broadleaf herbicides in lawn care. It targets broadleaf weeds like dandelions, clover, and plantain while leaving grasses largely unaffected. Many three-way broadleaf herbicides combine 2,4-D with dicamba and MCPP for a broader spectrum of control. When formulated for St Augustine, 2,4-D products can be effective and safe if applied at the correct rate and timing.

2. Dicamba

Dicamba is another broadleaf herbicide often used in combination with 2,4-D and MCPP. It is effective against tough weeds like chickweed and knotweed. Dicamba can be absorbed by roots, so apply carefully to avoid runoff into desirable plants. Products labeled for St Augustine typically use lower concentrations to minimize turf injury.

3. MCPP (Mecoprop-p)

MCPP is a broadleaf herbicide that works well on clover, dandelion, and other common lawn weeds. It is often included in three-way herbicide blends. Like the other ingredients, MCPP should only be used in products specifically labeled for St Augustine grass.

4. Atrazine

Atrazine is a selective herbicide that controls both broadleaf and grassy weeds in St Augustine lawns. It is particularly effective against crabgrass and other annual grasses. However, atrazine is regulated in many areas and may not be available in all regions. Check local regulations before purchasing or applying atrazine-based products. It should not be used on newly sodded or sprigged lawns until the grass is well established.

5. Quinclorac

Quinclorac is effective against crabgrass and other grassy weeds, but it is not universally safe for St Augustine. Some formulations can cause injury, especially during hot weather or on stressed lawns. Only use quinclorac products that explicitly list St Augustine as tolerant, and follow application instructions precisely.

Comparison of Safe Herbicide Options

The table below summarizes the most common selective herbicide active ingredients and their suitability for St Augustine grass. Remember that product formulations vary, so always check the label.

Active Ingredient	Target Weeds	St Augustine Safety	Notes
2,4-D	Broadleaf weeds (dandelion, clover, plantain)	Safe when labeled for St Augustine	Often combined in three-way blends; avoid drift to sensitive plants
Dicamba	Broadleaf weeds (chickweed, knotweed)	Safe when labeled for St Augustine	Root uptake possible; avoid runoff
MCPP	Clover, dandelion, broadleaf weeds	Safe when labeled for St Augustine	Common in three-way herbicide blends
Atrazine	Broadleaf and grassy weeds (crabgrass)	Safe when labeled for St Augustine	Regulated in some areas; not for new sod
Quinclorac	Crabgrass and grassy weeds	Use only products labeled for St Augustine	Can cause injury in heat; apply with caution
Glyphosate (non-selective)	All plants	Not safe — will kill St Augustine	Only for spot treatment far from lawn or with shielded applicator

How to Apply Weed Killer Safely to St Augustine Grass

Even the right product can cause damage if applied incorrectly. Follow these best practices to protect your lawn while effectively controlling weeds.

Timing Is Critical

Apply selective herbicides when weeds are actively growing and temperatures are between 65°F and 85°F. Avoid application during heat waves, drought, or when the lawn is dormant. St Augustine grass is most vulnerable to herbicide injury during transition periods (spring green-up and fall dormancy). Early morning or late afternoon application reduces evaporation and drift.

Do Not Apply to Stressed Grass

If your lawn is suffering from drought, disease, insect damage, or recent mowing, wait until it recovers before applying weed killer. Stressed grass is more susceptible to herbicide injury. Similarly, do not apply to newly sodded or sprigged lawns until the grass is well rooted and established.

Calibrate Your Sprayer

Over-application is a leading cause of turf damage. Calibrate your sprayer to deliver the correct rate specified on the product label. Use a dedicated sprayer for herbicides and never reuse it for other purposes without thorough cleaning. If you are using a granular product, use a spreader calibrated for the correct setting.

Watering After Application

Some products require watering after application to activate the herbicide, while others should be left dry for a period. Follow the label instructions exactly. Overwatering immediately after application can leach the product into the root zone and cause injury.

Spot Treat When Possible

For small weed infestations, spot treatment with a hand sprayer is safer than broadcasting herbicide over the entire lawn. This minimizes the total amount of chemical used and reduces the risk of turf injury. If you prefer a more targeted approach, consider using weeding tools for manual removal of small patches.

Common Weeds in St Augustine Lawns and How to Control Them

St Augustine lawns face a variety of weed pressures depending on region, season, and lawn health. Here are some of the most common weeds and the selective herbicide options that target them.

Broadleaf Weeds

Dandelions, clover, plantain, chickweed, and knotweed are common broadleaf weeds in St Augustine lawns. Three-way broadleaf herbicides containing 2,4-D, dicamba, and MCPP are effective against most of these weeds when applied according to label directions. For clover specifically, MCPP and dicamba are particularly effective.

Grassy Weeds

Crabgrass, goosegrass, and annual bluegrass can invade St Augustine lawns, especially in thin or stressed areas. Atrazine is effective against crabgrass and other annual grasses in St Augustine, but it must be applied before weeds emerge or when they are very small. Quinclorac can also control crabgrass, but only use products labeled for St Augustine and avoid application during hot weather.

Perennial Weeds

Weeds like nutsedge and pennywort can be challenging to control in St Augustine. Nutsedge requires specialized herbicides (like halosulfuron) that are safe for St Augustine but not always available to homeowners. Pennywort is a broadleaf weed that responds to three-way herbicides. For persistent perennial weeds, repeated applications may be necessary, and improving lawn health through proper fertilization and watering will help the grass outcompete weeds.

If you are also dealing with weeds in other areas, such as your vegetable garden, be sure to use a weed and grass killer safe for vegetable gardens to avoid contaminating your edible crops.

Preventing Weeds in St Augustine Grass

The best weed control is a healthy, dense lawn. St Augustine grass that is properly watered, fertilized, and mowed will naturally crowd out many weeds. Here are key preventive practices:

• Maintain proper mowing height: St Augustine should be mowed at 3.5 to 4 inches. Cutting too short weakens the grass and allows weeds to establish.
• Water deeply and infrequently: Water once or twice per week deeply to encourage deep roots. Avoid daily shallow watering, which promotes shallow roots and weed growth.
• Fertilize appropriately: Use a fertilizer formulated for St Augustine grass and follow the recommended schedule. Avoid over-fertilizing, which can promote weed growth and thatch buildup.
• Apply pre-emergent herbicides: Pre-emergent products prevent weed seeds from germinating. Apply in early spring before soil temperatures reach 55°F for crabgrass control. Always choose pre-emergents labeled for St Augustine.
• Aerate and dethatch: Compacted soil and thatch buildup weaken the lawn and create opportunities for weeds. Aerate annually and dethatch as needed.
• Overseed bare spots: Fill in thin or bare areas with St Augustine sod or plugs to prevent weeds from taking over.

For more detailed guidance on maintaining a healthy lawn, explore our lawn care guides and learn about proper sprinkler systems to keep your grass consistently hydrated.

Frequently Asked Questions