Weeds in St. Augustine Grass: How to Identify the Lookalikes and Kill Them Safely

Doveweed is the classic fooler. I tell homeowners to look at color first: if a patch is a lime-green shade your lawn never is, pull a runner and check for those little purple flowers. Our doveweed guide goes deeper. If you're still stuck, run a photo through the weed identifier. It's faster than squinting at ligules.

Broadleaf Weeds in St. Augustine

• Dollarweed (pennywort): shiny, round, silver-dollar leaves with the stem attached in the center, per LSU AgCenter. It's a moisture signal. See our dollarweed guide.
• Virginia buttonweed: opposite, linear to oval leaves, small white four-petal flowers, and a low mat. LSU calls it the most troublesome lawn weed in Louisiana because mowing spreads its stem fragments.
• Chamberbitter: looks like a tiny mimosa sprout with small round fruits tucked under the stem. The University of Florida lists it among the annual broadleaves a preemergence herbicide can stop.
• Spotted spurge: small oval leaves with a red to purple spot, a flat mat from one center, and milky sap when you snap a stem (LSU). More on our spotted spurge page.

Grassy Weeds and Sedges

Crabgrass germinates when soil warms to about 60°F and shows up where turf is thin, says LSU AgCenter. Postemergence products work better on young plants with fewer than two tillers. Our crabgrass page covers it in more detail.

Goosegrass grows in a flat, silver-white "wagon wheel" and flags compacted, high-traffic soil (LSU). Aeration helps. See goosegrass.

Nutsedge isn't a grass at all. Roll the stem between your fingers: sedges are triangular and solid, per the University of Florida. LSU notes yellow nutsedge spreads by tubers and points to poor drainage. Our nutsedge guide and yellow nutsedge page cover it.

Can St. Augustine Choke Out Crabgrass?

Partly, if it's thick. The University of Florida says dense turf shades the soil so weed seeds can't get the light they need, and that weeds fill voids rather than create them. But crabgrass tolerates low mowing and moves into thin spots, so a scalped or shaded St. Augustine lawn won't win on its own. UF's St. Augustinegrass guide notes the CitraBlue cultivar's flatter, denser growth competes with weeds better than Floratam.

What Kills Weeds but Not St. Augustine

St. Augustine is picky about herbicides. UGA Extension calls it "sensitive to many herbicides." This matrix pulls ingredients from the University of Florida's homeowner herbicide tolerance table, UGA Extension's home lawn guide, and LSU AgCenter's weed pages. It's a starting list, not a prescription.

Matching an ingredient to a bottle: ignore the name on the front. Turn the bottle over, read the active-ingredients panel, and confirm St. Augustinegrass appears in the list of grasses the label says it's safe on. If St. Augustine isn't listed, it isn't a St. Augustine product.

What to Avoid

• Quinclorac. The University of Florida rates every quinclorac mix on its homeowner list either "potentially damaging, do not use" or not recommended on St. Augustine, and UGA marks quinclorac sensitive. That rules out many "weed and crabgrass killer" bottles.
• Standard 2,4-D + MCPP + dicamba. The University of Florida rates most three-way products "some injury may occur" on St. Augustine, and rates at least one "do not use." UGA calls standard mixes injury-prone, so use only low rates as spot treatments. It says southern formulas with less 2,4-D improve tolerance, but never at or above 90°F with high humidity. That's why the buttonweed row above points to St. Augustine-labeled southern formulas only.
• Glyphosate (Roundup). Nonselective. UGA rates St. Augustine sensitive, and the University of Florida says treated spots usually need resodding. One exception: UF notes the ProVista cultivar is glyphosate-tolerant.

If you'd rather feed and weed in one pass, compare options in our St. Augustine weed and feed roundup. The University of Florida cautions these make sense only when weeds are spread evenly across the lawn.

Temperature and Timing Rules

The heat rule: stay below 85°F to be safe. The University of Florida says not to apply postemergence products like atrazine when turf is drought-stressed or air temperatures exceed 85°F. UGA Extension puts its line at 90°F, so 85°F covers both. UGA also says not to apply atrazine while warm-season grass is coming out of winter dormancy.

How long atrazine takes: it's slow, so plan on a week or more and a repeat at the label interval. The University of Florida warns that mowing within several days of spraying cuts a postemergence herbicide's effect.

Here's the calendar by region, straight from extension dates.

Texas: time pre-emergent by soil temperature (crabgrass starts near 60°F, per LSU) and confirm local dates with your county AgriLife Extension office. The herbicide timing calculator gives pre-emergent dates by ZIP code.

If you'd rather not track this yourself, WGIS Premium sends reminders timed to your ZIP and grass type for pre-emergent windows, inside a 12-month St. Augustine care plan, so the Feb 15 window doesn't slip past you.

Prevention: Fix What Let the Weeds In

The University of Florida says it plainly: weeds are a result of weak turf, not the cause. The usual culprits in St. Augustine:

• Mowing too low. Mow standard cultivars like Floratam at 3.5 to 4 inches and dwarf cultivars at about 2.5 inches, per the University of Florida. LSU AgCenter warns that mowing low hands bermudagrass the advantage.
• Overwatering. UF says dollarweed and sedges thrive in constantly wet soil. Water when blades fold, about 1/2 to 3/4 inch at a time.
• Shade. Floratam doesn't persist well with less than 6 hours of sun, per UF.
• Pests and disease. Nematodes thin turf and invite spurge and Florida pusley. If brown patches came first, check our St. Augustine disease guide.

Regional Notes: Florida, Texas, Louisiana

Florida: St. Augustine is the state's most common lawn grass (UF/IFAS), and wet sites breed dollarweed, doveweed, and sedges. Torpedograss often arrives in fill soil, says LSU, so inspect soil before a renovation. Louisiana: Virginia buttonweed is LSU's most troublesome lawn weed, and doveweed signals poor drainage. Texas: bermudagrass creeping in from neighbors and alleys is the headache I hear about most. UGA's bulletin notes ethofumesate with atrazine can control bermuda, but only in nonresidential St. Augustine, so most homeowners are left with spot-kill and resod.

Where the Extension Services Disagree

The extension services don't agree on everything. UF/IFAS rates sulfentrazone alone as tolerant on St. Augustine. UGA's table marks it sensitive. Imazaquin flips the other way. Same with atrazine on crabgrass: UF says it helps on young grassy weeds, while UGA calls its crabgrass activity generally poor. Any guide that hands you one "safe" product list is skipping that nuance. The label on the specific bottle, for your specific grass, is the tiebreaker.

Your cultivar matters. ProVista tolerates glyphosate, CitraBlue out-competes weeds better than Floratam, and Raleigh tends to yellow in peak summer heat, per UF. Know which one you have before you shop.

Worked Example: Spot-Spraying a 5,000 Sq Ft Lawn

Rates vary by bottle, so here's the math you do with your label.

1. Measure the weedy area, not the lawn. Say you find three doveweed patches: 10 x 12 ft (120 sq ft), 15 x 20 ft (300 sq ft), and 8 x 10 ft (80 sq ft). That's 500 sq ft, a tenth of the lawn.
2. Set your water volume. The University of Florida says to apply herbicides in 1/2 to 1 gallon of water per 1,000 sq ft. For 500 sq ft, mix 1/4 to 1/2 gallon. Spraying the whole 5,000 sq ft would take 2.5 to 5 gallons.
3. Calibrate. Mark off 1,000 sq ft (UF's unit), spray it with plain water at your normal pace, and measure what you used.
4. Add product. Read the label's rate per 1,000 sq ft or per gallon and scale it to your 500 sq ft. The tank mix calculator does the conversion.

Spot-treating that 500 sq ft instead of the whole yard uses a tenth of the chemical and exposes far less turf to injury.

Your St. Augustine Weed Action Plan

1. Identify first. Upload a photo for a free diagnosis or use the table above.
2. Find the cause: mowing height, wet spots, shade, or compaction.
3. Check the thermometer. No post-emergent sprays above 85°F.
4. Pick a product whose label lists St. Augustine and contains no quinclorac.
5. Spot-spray measured patches with the tank mix calculator.
6. Put pre-emergent dates on the calendar or let a Premium care plan handle reminders.
7. Bermuda or torpedograss? Plan a spot-kill and resod, and confirm the ID before you use glyphosate.

Weeds in St. Augustine are fixable. Name the weed, fix the condition that invited it, and spray only in the right window. For the broader list beyond St. Augustine, see our common lawn weeds guide.