Weed Control in Shady Floratam St. Augustine: Mowing High and Atrazine Limits

Broadleaf weeds in shady Floratam under oaks require cultural management (high mowing) because atrazine (in weed & feed) has strict annual limits and shade reduces turf competitiveness.

Atrazine Constraints

• Low rate in weed & feed: Requires perfect calibration for efficacy.
• Annual limit: 2 applications/year max (FL label).
• Seasonal window: Typically fall/winter only.
• Already used? Cannot reapply legally this season.
• Oak roots: Atrazine uptake by trees possible; label restrictions near driplines.

Cultural Weed Suppression: The Primary Tool

1. Mow 3.5–4 inches WEEKLY: Removes weed flowers before seed set; maximizes St. Augustine leaf area.
2. Shade management: Arborist thinning of oak canopy (not removal) increases dappled light.
3. Irrigation: Deep, infrequent (¾–1 inch/week); shade stays wet longer.
4. Fertilization: Minimal N (0.5 lb/1,000 sq ft spring); excess feeds weeds.

Why This Works

• St. Augustine is aggressive laterally—high mowing encourages stolon density.
• Weeds prevented from seeding = population declines annually.
• Dense turf shades soil = prevents weed germination.
• No chemical resistance issues.

If Chemical Needed (After Atrazine Limit)

• Metsulfuron (Manor/MSM): Safe on St. Augustine; broad spectrum; very low rate.
• Celsius WG: Professional; excellent on tough weeds; St. Augustine tolerant.
• Spot-treat only: Backpack sprayer; surfactant required.

Weekly high mowing is the sustainable solution. It's labor, but it builds turf density that permanently suppresses weeds. Atrazine is a crutch with expiration date.