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Moss control with iron

Iron kills moss and deepens the green, but it only treats the symptom. Moss grows where grass is weak.

Plate 60 — Moss control (iron)Loops · silent
When
Apr 1 – May 31 (northern hemisphere) · Oct 1 – Nov 30 (southern)
Material
iron fertilizer (ferrous sulfate)
For
Cool-season lawns (ryegrass, fescue, bluegrass)
Rule
sp-moss-iron · Hessayon; RHS; Turgeon
Iron suppresses moss and deepens the green — but it only treats the SYMPTOM. Moss thrives where grass is weak: acidic soil (test pH, lime if low), shade, and compacted/poorly-drained ground. Fix those and reseed the thin grass, or the moss returns every year.

Why moss appears

Moss does not push grass out; it moves into space the grass has already left. The usual causes are shade, acidic soil, compacted or poorly drained ground that stays wet, and mowing too short. Unless at least one of those changes, moss returns every year.

Treating it with iron

  1. In April or May, on a still day, apply iron sulfate (ferrous sulfate) or a lawn feed containing it, at the rate on the label.
  2. Within days the moss turns black. Leave it about a week.
  3. Rake or scarify out the dead moss. Expect a lot of material and a thin-looking lawn.
  4. Overseed the bare patches straight away, before weeds or new moss fill them.

Iron sulfate stains paving, stone and clothing rust-brown. Keep it off hard surfaces, and wash it off immediately if it lands there.

Fixing the cause

  • Acidic soil: run a pH test and lime if it is below about 6.0.
  • Compaction and wet ground: aerate and top-dress with sandy material.
  • Shade: thin overhanging branches and use a shade-tolerant seed mix with a high share of fine fescue.
  • Short mowing: raise the cut, especially in shaded areas.

Sources

  • D.G. Hessayon — The Lawn Experthessayon
  • Royal Horticultural Society — lawn guidancerhs
  • A.J. Turgeon — Turfgrass Managementturgeon