Weed Killers That Are Safe for St. Augustine Grass (and Ones to Avoid)

Best weed killer for each weed

No single herbicide handles every weed in St. Augustine. Match the product to the weed:

Why St. Augustine is picky

St. Augustine is more sensitive to herbicides than Bermuda or zoysia. Several ingredients in popular lawn weed killers can yellow it, stunt it, or kill it outright, especially in summer heat. The bottle on the shelf that says "safe for lawns" doesn't mean safe for your lawn. The label's list of tolerant grasses is what matters.

Step one: stop weeds before they sprout

Pre-emergent herbicide forms a barrier that keeps weed seeds from germinating. It does nothing to weeds that are already up, so timing is everything. The Turf Method program applies it three times a year:

• Early February to stop spring and summer weeds
• Early May to extend protection through peak season
• Early October to stop winter weeds as it cools

For spring, the easy deadline is February 1 on the Gulf Coast, but the real signal is soil temperature: get it down before the soil holds around 55°F. Water it in as the label directs. A thick lawn mowed at the right height also crowds out weeds on its own. See how tall to mow.

Step two: spot-treat what gets through

Atrazine is the standard choice for broadleaf weeds and some grassy weeds in St. Augustine. It's labeled for St. Augustine when applied according to the label. The atrazine I use is Southern Ag Atrazine. A few things to know:

• Spot spray the weeds instead of blanketing the whole lawn.
• Mind the heat. Atrazine labels warn against applying when temperatures are high. Check yours for the exact limit, and plan treatments for mild weather.
• Skip new sod. Wait until newly laid sod or plugs are established, per the label.
• It works slowly. Give it a couple of weeks before deciding it didn't work.

Another option: Celsius WG

Celsius WG is another post-emergent many Gulf Coast homeowners use on St. Augustine. It's labeled for established St. Augustine and controls a wide range of broadleaf and some grassy weeds. Its label notes St. Augustine can be temporarily stunted in hot weather, so many people stick to cooler days. If you use it:

• Spot spray only, at the St. Augustine rate on the label.
• Be careful with surfactants. They help spray stick to weed leaves but raise the risk of injuring St. Augustine. Follow the label, and skip them for simple spot treatments.
• Only on healthy, established grass. Not on new sod, and not on drought- or heat-stressed turf.

Rules for any weed killer on St. Augustine

• Hit weeds while they're young. Small, actively growing weeds die much more easily than ones that have flowered and gone to seed. A weed you kill in March is one you aren't fighting in July.
• Try a test spot if you're unsure how your lawn will react. Treat a small area and wait a week.
• Some weeds need their own product. See nutsedge, dallisgrass and Virginia buttonweed, and Bermuda in St. Augustine.

What to be careful with

• Glyphosate (Roundup-type products). These kill any grass they touch, including St. Augustine. Only use them on weeds in beds or cracks, never on the lawn.
• Products with 2,4-D. Many common lawn weed killers and weed-and-feeds contain it, and St. Augustine is sensitive to it. Some products allow use at reduced rates; many simply aren't labeled for St. Augustine. Read the label.
• Quinclorac and MSMA. Popular for crabgrass in other lawns, but they can seriously damage St. Augustine.
• "Weed and feed" products. Even ones labeled for St. Augustine force you to put herbicide over the whole lawn on the fertilizer's schedule instead of spot treating weeds when they need it. Feeding and weeding work better as separate jobs.

A note on grassy weeds

Grassy weeds that come up inside St. Augustine are some of the hardest to remove selectively. Pre-emergent is your best defense. For stubborn clumps, spot-treating with a non-selective product and replugging the spot is sometimes the cleanest fix.

Common questions

What weed killer is safe for St. Augustine grass?

Atrazine is the standard post-emergent for broadleaf weeds in St. Augustine, and Celsius WG is another labeled option. For nutsedge, use halosulfuron (SedgeHammer). Whatever you buy, the label must list St. Augustine as a tolerant grass.

Will 2,4-D kill St. Augustine grass?

St. Augustine is sensitive to 2,4-D. Some products allow it at reduced rates, but many aren't labeled for St. Augustine at all, and injury is more likely in hot weather. Read the label before using any product that contains it.

Is Roundup safe for St. Augustine?

No. Glyphosate, the active ingredient in Roundup-type products, kills St. Augustine along with the weeds. Use it only in beds, cracks, and driveways, or to spot-kill a patch you plan to replug.

When is the best time to spray weeds in St. Augustine?

Spray young, actively growing weeds in mild weather. Most St. Augustine herbicide labels warn against spraying in high heat, so plan for spring and fall, and treat winter weeds like stickers from December through February.

Can I use weed and feed on St. Augustine?

Only one whose label lists St. Augustine, and even then it puts herbicide on the whole lawn on the fertilizer's schedule. Pre-emergent plus spot spraying works better. See weed and feed for St. Augustine.