Lawn Care · Gulf Coast and Southern Lawns
In St. Augustine, crabgrass is a prevention job, not a spray job.
The crabgrass killers most people buy, including quinclorac (Drive XLR8, Quinclorac 75 DF), Q4 Plus, fenoxaprop (Acclaim Extra) and MSMA, are not for St. Augustine grass. UF/IFAS says no post-emergent herbicide on the market is labeled to pick crabgrass out of St. Augustine selectively. So the plan for Gulf Coast and Southern lawns is a pre-emergent applied before crabgrass germinates in late winter, a second application about 60 days later, and a thick lawn that leaves crabgrass no room. Plants that slip through get pulled or carefully spot-treated.
SELECTIVE POST-EMERGENT
None labeled, per UF/IFAS
FLORIDA PRE-EMERGENT DATE
Feb 1 (south) to Mar 1 (north)
SECOND PRE-EMERGENT
About 60 days after the first
LSU MOWING HEIGHT
2.5 to 3 inches
This guide is written for St. Augustine lawns in Florida, the Gulf Coast and the rest of the Deep South, and the dates come from Florida, Louisiana and South Carolina extension programs. For the full range of weeds that turn up in this grass, see our guide to common weeds in St. Augustine grass lawns. This page is just about crabgrass.
Crabgrass vs St. Augustine: how to tell them apart
Both creep sideways, and a crabgrass plant sprawling into the lawn can look like a stray St. Augustine runner. Check these points before you treat anything:
| Feature | Crabgrass | St. Augustine |
|---|---|---|
| Leaf blade | Narrower, with a sharp point; large crabgrass is hairy on both surfaces | Wide, folded blade with a broad, rounded tip |
| Growth habit | Stems radiate from a center point and may root at the lower nodes | Thick aboveground runners (stolons) that knit into a mat |
| Seedhead | Several thin, finger-like spikes from the top of the stem | One thick spike sitting just above the leaves |
| Life cycle | Mostly a summer annual, though some Florida species persist longer | Perennial lawn grass |
The crabgrass details come from NC State TurfFiles and LSU AgCenter, and the St. Augustine traits from LSU’s St. Augustinegrass home lawn sheet. UF/IFAS Pinellas County notes that Florida has five common crabgrass species, and that blanket and India crabgrass can live past a single season (UF/IFAS Pinellas). If you’re unsure, our guide to what crabgrass looks like has photos by growth stage.
Why the usual crabgrass killers are off the table
St. Augustine is sensitive to most of the products that work on crabgrass in fescue, bluegrass or bermuda. The University of Georgia turf weed control guide spells out the exclusions, and LSU’s crabgrass table agrees on quinclorac and fenoxaprop:
- Quinclorac, sold as Drive XLR8 and Quinclorac 75 DF: UGA says not to apply it to St. Augustine, centipede, bahia or carpetgrass. These are good crabgrass killers, just not for this lawn.
- Quinclorac combinations such as Q4 Plus: not labeled for St. Augustine or centipede, per UGA.
- Fenoxaprop (Acclaim Extra): UGA lists it for bluegrass, fescues, ryegrass and zoysia only, and LSU marks it “No” for St. Augustine.
- MSMA: UGA notes the label says not to use it on St. Augustine or centipede, and the EPA cancelled all residential lawn use in 2009 anyway.
- Typical 2,4-D weed and feed: Scotts Turf Builder Weed & Feed (2,4-D and dicamba) says not to use it on St. Augustine, including Floratam, and crabgrass isn’t on its weed list.
Those products are the backbone of our crabgrass killer comparison, which is written for cool-season and bermuda lawns. In St. Augustine, read every label for the turf list before you buy.
What Southern extension programs actually list
UF/IFAS’s crabgrass publication ENH1134 is blunt about it: no post-emergent product is currently labeled to remove crabgrass from St. Augustine without harming the turf. The few products in its St. Augustine post-emergent list come with limits. Dithiopyr works only on crabgrass at the three-leaf stage or smaller, and amicarbazone gives suppression only. For mature crabgrass, ENH1134 points to hand pulling or spot treatment, and warns that non-selective products such as glyphosate or glufosinate will leave a hole in the turf.
County-level UF advice says the same thing more bluntly. UF/IFAS Clay County tells homeowners they won’t find an herbicide that controls actively growing crabgrass in St. Augustine. Its options are pulling, or careful spot treatment with glyphosate or glufosinate, used sparingly because both kill St. Augustine too.
What about atrazine, the classic St. Augustine weed killer? UGA lists it for St. Augustine, but for annual bluegrass and broadleaf weeds, and calls its control of summer annual grasses weak. It isn’t a crabgrass answer.
Pre-emergent timing for Gulf Coast and Southern lawns
Because killing crabgrass in St. Augustine is so limited, timing the pre-emergent is the most important step. It has to be in the soil before seeds sprout. LSU AgCenter puts germination at soil temperatures of 55 to 60°F in late winter.
- Florida: ENH1134 gives February 1 for south Florida, February 15 for central Florida and March 1 for north Florida, with a follow-up application about 60 days later that it calls very important.
- Louisiana: LSU says as early as late January around New Orleans and by late February around Shreveport, depending on the winter. Its pre-emergent guide puts Baton Rouge at mid to late February and north Louisiana in early March.
- South Carolina: Clemson HGIC recommends early to mid-February, repeated in 8 to 10 weeks. Water granular products in with half an inch of irrigation within seven days, or sunlight breaks them down.
UGA’s 2024 turf weed control guide includes St. Augustine on the labels of dithiopyr, prodiamine, pendimethalin, indaziflam and the dimethenamid plus pendimethalin combination, and UF/IFAS ENH1134 puts all five on its St. Augustine pre-emergent list. UGA notes one St. Augustine limit: indaziflam is capped at 6 fl oz per acre per application. Products we carry include Dimension 2EW (dithiopyr, which also catches very young seedlings) and Prodiamine 65 WDG. Check each label for St. Augustine and for any limits on newly laid sod or plugs, and apply at the label rate. We compare more options in our crabgrass preventer roundup.
One Florida caution: UF/IFAS Clay County advises against getting your crabgrass pre-emergent from a weed and feed or other combination product, and says to hold turf fertilizer until April 15 in Northeast Florida. Pre-emergent timing and fertilizer timing don’t line up in St. Augustine, so apply them separately.
A dense lawn is the long-term fix
Crabgrass seed needs light and open soil, and a thick St. Augustine canopy denies it both. LSU’s St. Augustine sheet recommends mowing at 2.5 to 3 inches, never removing more than a third of the blade at once, and up to 3 lb of nitrogen per 1,000 sq ft per year applied from midspring to midsummer, no more than 1 lb per application (LSU AgCenter). Florida guidance runs taller: UF/IFAS Pinellas suggests 3.5 to 4 inches for standard St. Augustine and 2 to 2.5 inches for dwarf varieties such as Captiva, Delmar, Sapphire and Seville. It also advises bagging clippings while grassy weeds are in seed and watering only when about 30% of the lawn shows morning wilt.
Florida nitrogen limits and local ordinances vary, which we cover in our St. Augustine fertilizer guide. For mowing, watering and thin spots, see our St. Augustine grass care guide and how to make St. Augustine grass thicker.
If crabgrass is already up
- Pull it early. Young plants come out easily, and every plant removed before it seeds means fewer next year.
- Catch seedlings with dithiopyr. Only while plants are at the three-leaf stage or smaller, per ENH1134. Past that, it won’t do much.
- Spot-treat clumps you can’t pull. Glyphosate or glufosinate on the crabgrass only, knowing the St. Augustine underneath dies too. Let the runners fill back in, or plug the gap.
- Stop seed production. Bag clippings while seedheads are showing.
- Mark your calendar. Clemson notes crabgrass dies with the first frost. The real fix is a pre-emergent on time next winter.
Don’t spray Drive XLR8, Quinclorac 75 DF or Q4 Plus on St. Augustine to rescue it from crabgrass.
Those labels exclude St. Augustine, and the damage lands on the grass you are trying to save. The same goes for a 2,4-D weed and feed and for an old jug of MSMA. If crabgrass has filled large areas, you are past spot treatment: pull or spot-treat what you can this season, then rely on a well-timed pre-emergent and a thicker lawn next year.
If you would rather have someone handle the timing, our lawn weed control page connects you with a lawn care company serving your area.
St. Augustine crabgrass FAQs
What is the best crabgrass killer for St. Augustine grass?
There isn’t a selective one. UF/IFAS says no post-emergent herbicide is currently labeled for selective crabgrass control in St. Augustine. The best product is a pre-emergent labeled for St. Augustine and applied on time, plus pulling or spot treatment for escapes.
Will Drive XLR8 kill crabgrass in St. Augustine?
It would kill the crabgrass, but its active ingredient, quinclorac, is not for St. Augustine. UGA’s guide says not to apply quinclorac to St. Augustine or centipede.
Can I use Tenacity on St. Augustine?
Not as a crabgrass killer. UF/IFAS lists mesotrione only as a pre-emergent at reduced rates on well-established St. Augustine, and UGA lists it for St. Augustine sod production. Follow the current label.
When should I put down crabgrass preventer on St. Augustine?
Before crabgrass starts sprouting in late winter. UF/IFAS ties Florida timing to soil temperatures holding above 50°F, and LSU puts germination at 55 to 60°F. In Florida that means February 1 in the south to March 1 in the north, and in Louisiana late January to early March depending on location, with a second application about 60 days later in Florida.
Does crabgrass die in winter in Southern lawns?
Most crabgrass is a summer annual that dies with the first frost, per Clemson. In Florida, blanket and India crabgrass can persist beyond one season.
Related guides
Sources: UF/IFAS ENH1134: Crabgrass Biology and Management in Turf (rev. 2016) · UF/IFAS Clay County: Crabgrass in St. Augustinegrass Lawns · UF/IFAS Pinellas County: Controlling Crabgrass in Lawns · LSU AgCenter: Crabgrass · LSU AgCenter: Preemergence Options for Crabgrass Control in Louisiana Lawns · LSU AgCenter Pub. 3624-SS: St. Augustinegrass · Clemson HGIC: Weed of the Month, Crabgrass · University of Georgia: Turfgrass Weed Control for Professional Managers (2024) · NC State TurfFiles: Large Crabgrass · EPA: MSMA (organic arsenical) status · Scotts: Turf Builder Weed & Feed product page
