How to Kill Smooth & Large Crabgrass in St. Augustinegrass Without Hurting Your Lawn
Direct Agronomic Answer
To selectively kill Smooth & Large Crabgrass in St. Augustinegrass without damaging your turf, apply Hi-Yield Atrazine Weed Killer (Atrazine 40.8%), Barricade 4FL (Prodiamine 65 WDG) (Prodiamine 40.7% (or 65% WDG)) when air temperatures are between 50°F and 85°F.
For early 1–3 leaf stages and mature 2–4 tiller plants, Quinclorac paired with a methylated seed oil (MSO) surfactant provides rapid systemic control within 7 to 14 days. In St. Augustine and Centipede lawns where Quinclorac causes severe chlorosis, Atrazine or Mesotrione (for Centipede only) must be used instead.
1. Labeled Safe Herbicides That Kill Smooth & Large Crabgrass in St. Augustinegrass
| Recommended Product | Active Ingredient & Group | Application Temp (°F) | Surfactant & Mixing Protocol | Where to Buy |
|---|---|---|---|---|
| Hi-Yield Atrazine Weed Killer EPA Reg. 34911-15-7401 |
Atrazine 40.8% Group 5 (Photosystem II Inhibitor / Triazine) |
50°F – 85°F Rainfast: 6h |
Do NOT add surfactant when spraying actively growing St. Augustine or Centipede above 80°F to avoid foliar yellowing. Irrigate lightly within 48 hours for root uptake. | Check Price at DoMyOwn View on Amazon |
| Barricade 4FL (Prodiamine 65 WDG) EPA Reg. 100-1139 |
Prodiamine 40.7% (or 65% WDG) Group 3 (Microtubule Assembly Inhibitor / Dinitroaniline) |
45°F – 90°F Rainfast: 0h |
Pre-emergent soil barrier: do NOT use foliar surfactant. Must be watered in with 0.5 inches of irrigation or rainfall within 14 days of application to activate. | Check Price at DoMyOwn View on Amazon |
2. What NOT to Spray on St. Augustinegrass When Treating Smooth & Large Crabgrass
Warning: Common Smooth & Large Crabgrass Killers That Damage St. Augustinegrass
Many retail herbicides advertised for Smooth & Large Crabgrass are formulated for different turf zones and will severely injure St. Augustinegrass:
- AVOID Tenacity (Mesotrione 40%): Rated 'sod_production_only' on St. Augustinegrass — high risk of turf phytotoxicity or stunting.
- AVOID Drive XLR8 (Quinclorac 18.92%): Rated 'avoid' on St. Augustinegrass — high risk of turf phytotoxicity or stunting.
- AVOID Acclaim Extra (Fenoxaprop-p-ethyl 6.59%): Rated 'avoid' on St. Augustinegrass — high risk of turf phytotoxicity or stunting.
- AVOID Pylex Herbicide (Topramezone 29.7%): Rated 'avoid' on St. Augustinegrass — high risk of turf phytotoxicity or stunting.
3. Optimal Seasonal Timing & Temperature Thresholds
Apply pre-emergent Prodiamine or Dithiopyr when 4-inch soil temperatures reach 50°F–55°F for three consecutive days, and spray post-emergent herbicides in late spring when seedlings have 2 to 3 leaves.
Post-emergent applications are most effective when Crabgrass is actively growing in moist soil and air temperatures remain below 85°F. Spraying mature multi-tiller plants in late August during drought stress significantly reduces herbicide translocation.
4. Diagnostic Checklist: Confirm It Is Smooth & Large Crabgrass Before Spraying
- Light apple-green coarse blades growing in a prostrate star-like rosette
- Membranous, tall jagged ligule with sparse hairs at the collar region
- Finger-like seedheads (3 to 5 spikes) emerging in mid-to-late summer
Not 100% sure? Use our AI Photo Weed Scanner or review the full Smooth & Large Crabgrass Botanical Profile.