Roundup for St Augustine grass remains a popular option for broadleaf weed control in warm-season lawns. This guide covers how to apply it safely, the best timing, and how it compares to alternatives for dense turf.
Before treating, it helps to understand product options, grass response, and local regulations. The summary below highlights key application windows, surfactant needs, and tradeoffs between convenience and lawn stress.
| Product Name | Key Active Ingredient | Best Used On | Notes |
|---|---|---|---|
| Roundup Max Control | Glyphosate Isopropylamine Salt | Bermuda, Zoysia, Centipede, St Augustine | Higher glyphosate concentration for faster kill of weeds without surfactant in some versions |
| Roundup for Lawns Ready-To-Spray | Glyphosate Isopropylamine Salt | Small areas with labeled broadleaf weeds | Convenient bottle, but less concentrated; requires careful rate adjustment for St Augustine |
| Southern Ag 41% Glyphosate | Glyphosate Isopropylamine Salt | Tough perennial weeds in dormant or actively growing St Augustine | Agricultural label; allows surfactant choice for better coverage and turf recovery |
| Tenacity Mesotrione | Mesotrione | St Augustine with Quinclorac-free weed control | Slower but less grass injury; often preferred when Bermudagrass or centipede are mixed in |
Ideal Application Window for Roundup on St Augustine
Seasonal Timing and Turf Conditions
Apply Roundup for St Augustine grass when the lawn is actively growing, typically late spring through summer. Avoid applications when daytime temperatures exceed 90°F, because heat stress increases the risk of turf damage. Early morning or late evening spraying reduces rapid evaporation and improves coverage on broadleaf weeds.
Weather and Rain Considerations
Do not mow or irrigate before application, and avoid rain or heavy dew within six hours of spraying. Rain shortly after treatment can wash herbicide off leaves, reducing effectiveness on weeds and prolonging recovery for the grass. Wind under 10 mph helps prevent drift onto desirable plants and nearby garden beds.
Weed Spectrum and Surfactant Requirements
Broadleaf Control and Grass Safety
Roundup controls many broadleaf weeds such as dandelion, clover, and chickweed in St Augustine when used at labeled rates. Grasses and sedges are not controlled, so it works best as a targeted spot treatment. Adding a non-ionic surfactant approved for St Augustine improves leaf coverage and adhesion, especially on waxy broadleaf species.
Resistance, Mixtures, and Alternatives
Glyphosate-resistant weeds are increasingly common, so rotation with other chemistry helps maintain control. For sensitive St Augustine varieties, Tenacity or products with Quinclorac can reduce turf injury while still managing difficult broadleaf weeds. Always confirm herbicide compatibility with your specific cultivar before treating large areas.
Herbicide Injury Symptoms and Recovery Steps
Visual Signs and Temporary Setbacks
Herbicide injury on St Augustine often appears as yellowing, cupped leaves, or tip burn within a few days of treatment. Severe exposure can cause patchy yellow or white tissue, followed by slow dieback along edges. Most recovery is possible if the crown and stolons remain alive and moisture is adequate. p>
Post-Treatment Care and Monitoring
After applying Roundup, limit foot traffic and avoid mowing injured areas until new growth resumes. Apply one inch of water weekly if rainfall is insufficient, and consider a light fertilizer only when temperatures moderate and weeds are no longer active. Reassess the lawn after four to six weeks before deciding on additional treatments.
Responsible Use and Long-Term Lawn Strategy
- Read and follow the specific rate and safety instructions on the Roundup label for St Augustine grass.
- Spot-treat weeds instead of broadcasting to reduce herbicide load on the lawn and surrounding areas.
- Use a surfactant approved for warm-season turf to improve coverage and minimize repeat applications.
- Rotate herbicide sites of action to help manage glyphosate-resistant weed populations.
- Monitor lawn health after treatment and address bare areas promptly with sod or plugs to prevent erosion and new weeds.
FAQ
Reader questions
Can I use Roundup on newly established St Augustine plugs?
Avoid spraying herbicide on plugs until they have filled in and been mowed at least three times. Young plants are more sensitive, and uneven coverage can create bare spots that invite weeds.
Is it safe to use Roundup near trees, shrubs, or vegetable gardens?
Use extreme caution, because drift and root uptake can damage nearby desirable plants. Apply only with proper shielding, low-wind conditions, and a shielded sprayer to protect non-target vegetation.
How long does it take for weeds to die after spraying?
Broadleaf weeds may show yellowing within two to seven days, with complete dieback in one to three weeks under ideal conditions. Resist retreating too soon, as turf recovery may lag behind weed kill.
Can Roundup be mixed with fertilizer or other lawn chemicals?
Do not tank-mix unless the label explicitly permits it, as physical or phytotoxic reactions can occur. Apply herbicide and fertilizer separately, and follow all compatibility and reentry instructions to protect grass health.