Accidental Preen Application on St. Augustine Grass: Risks and Recovery Steps

Applying Preen Weed Preventer (containing dithiopyr) to St. Augustine grass is concerning but not necessarily catastrophic. Dithiopyr is a pre-emergent herbicide that inhibits root development in germinating seeds—and unfortunately, it can also affect existing turf roots.

How Dithiopyr Affects St. Augustine

Dithiopyr prevents root growth, which stops weed seeds from establishing. However, St. Augustine spreads via stolons (above-ground runners) that root at nodes. The herbicide can inhibit this rooting process, potentially thinning the turf. Since dithiopyr binds tightly to soil and doesn't leach readily, its effects may persist for several months.

Immediate Actions to Support Recovery

• Do not water excessively: Overwatering won't "wash away" the chemical and may worsen root stress.
• Apply foliar fertilizers: Liquid fertilizers absorbed through leaves bypass root uptake limitations. Use balanced formulations at reduced rates.
• Maintain proper irrigation: Water deeply but infrequently to encourage deep rooting where herbicide concentration is lower.
• Avoid additional stress: Postpone aeration, dethatching, or heavy foot traffic.

Regulatory Compliance

Always follow label directions—it's the law. Check local watering and fertilizer restrictions (common in Texas) to ensure your recovery practices comply with municipal regulations.

Monitoring and Patience

Healthy St. Augustine has vigorous stolon growth and abundant root initials. With proper care, the turf can outgrow the herbicide's effects as it degrades over time. Watch for new rooting at stolon nodes—a sign of recovery. If large areas fail to recover after 8–12 weeks, consult a lawn care professional for renovation options.