Controlling Dollarweed in St. Augustine Grass: Water Management and Organic Strategies

Dollarweed: The Moisture-Loving Invader

Dollarweed (pennywort) thrives in wet, poorly drained St. Augustine lawns. Chemical herbicides often fail and damage soil biology. The most effective control is cultural: change the conditions dollarweed loves.

The Water Connection

Research (Turfscience.com) shows dollarweed declines under reduced irrigation. Test three regimes:

• High: daily watering
• Moderate: weekly deep soaking at first wilt
• Low: brief watering at wilt

Preliminary results favor moderate to low schedules. In Tarpon Springs' humid climate, drying the soil is challenging but possible.

Practical Steps

1. Water deeply but infrequently: Soak to 6 inches, then wait until the top 2-3 inches are dry and grass shows slight wilt (blades fold lengthwise). This encourages deep St. Augustine roots and stresses shallow-rooted dollarweed.
2. Improve drainage: Aerate, top-dress with compost, or install French drains in low spots.
3. Hand-pull survivors: Remove dollarweed plants before they set seed.
4. Consider clear plastic: During rainy periods, temporarily cover worst areas with clear plastic to shed water and heat the soil (solarization effect).

Organic Soil Building

A strong St. Augustine lawn is the best defense. Follow an organic program:

• Sugar (4-5 lbs/1,000 sq ft) spring and fall to feed microbes.
• Alfalfa meal and lava sand for minerals.
• Corn gluten meal (10 lbs/1,000 sq ft) in spring as pre-emergent and nitrogen source.
• Mow high (3-4 inches), leave clippings.

What About Geese or Sugar as Herbicide?

Geese eat dollarweed but are impractical for most. Table sugar is not a herbicide—it feeds microbes that improve soil, indirectly suppressing weeds over time. It won't kill existing dollarweed quickly.

Patience and consistent cultural practices will shift the balance. A dense, deep-rooted St. Augustine lawn on living soil will eventually crowd out dollarweed.