Broadleaf Weed Control in St. Augustine: Selective Herbicides and Cultural Recovery

Broadleaf weeds overtaking St. Augustine after weed & feed failure require targeted selective herbicide application once grass breaks dormancy, combined with cultural practices to restore density.

Why Weed & Feed Failed

• Atrazine rate too low: Weed & feed contains minimal atrazine; inaccurate spreader calibration = sublethal dose.
• Timing wrong: Applied November—weeds not actively growing; poor translocation.
• Fertilizer stimulated weeds: Nitrogen fed weeds more than dormant St. Augustine.
• Oak canopy: Shade reduces St. Augustine competitiveness; favors shade-tolerant weeds.

Selective Herbicide Options for St. Augustine

Application Protocol

1. Wait for full green-up: St. Augustine actively growing (typically March–April in N. FL).
2. Spot-treat: Backpack sprayer; target weeds only. Broadcast only if >50% weed cover.
3. Add surfactant: 0.25% non-ionic—critical for waxy leaf penetration.
4. Repeat in 3–4 weeks: Perennial weeds need sequential applications.
5. No mowing 3 days before/after: Maximizes leaf area for absorption/translocation.

Cultural Recovery (Parallel Track)

• Mow 3.5–4 inches: Shades weed seedlings; maximizes St. Augustine photosynthesis.
• Weekly mowing: Removes weed flowers before seed set.
• Fertilize moderately: 0.5 lb N/1,000 sq ft at green-up; slow-release.
• Irrigate deeply/infrequently: 1 inch/week; discourages shallow-rooted weeds.

The Amdro link references a specific atrazine product. For severe infestations under oaks, metsulfuron (Manor) often outperforms atrazine. Patience—two seasons of combined chemical + cultural management typically restores dominance.