Weeds in New Tall Fescue Lawn: Mow High, Fertilize Lightly, Skip Herbicides

Why Mowing Height Beats Weed Killers for Tall Fescue

A Covina, CA homeowner seeded Marathon I Tall Fescue but faces a weed invasion. Research from Purdue, University of Illinois, and Kansas State confirms: mowing at 2.5–3.5 inches drastically reduces crabgrass and broadleaf weeds compared to 1–2 inch heights. Taller grass shades out weed seedlings and promotes deeper roots.

Nitrogen: The Unexpected Weed Fighter

University of Illinois trials showed any nitrogen fertilization reduced broadleaf weeds in Tall Fescue versus unfertilized plots. The rate mattered less than the presence of N. Corn gluten meal (a natural pre-emergent) supplies slow-release N while suppressing weed seed germination.

Action Plan

1. Mow at 3 inches now; raise to 3.5 inches in summer heat.
2. Apply corn gluten meal at 20 lbs/1,000 sq ft in early spring (when forsythia blooms) for pre-emergent control + N.
3. Spot-pull large weeds; never let them seed.
4. Soil test this fall; amend per results.

Herbicides on seedling grass risk injury. Patience + proper cultural practices = a dense, weed-resistant lawn.