Applying iron sulphate in November can kill lawn moss within about two hours and give grass a deeper green colour, according to lawn care specialists. The product is widely used in UK lawn care and is best applied in autumn while grass is still recovering from summer use.
Why iron sulphate works against moss
GreenThumb, a lawn care company, states that iron sulphate (ferrous sulphate) is “the most effective and widely used moss killer in UK lawn care”. It “turns moss black” rapidly, “killing it quickly”. LawnSmith specialists add that it delivers a “wonderful green up” and helps “keep the grass healthy and moss-free” during cooler months.
According to an Iron Sulphate retailer, after application moss should “quickly turn black and die, usually within about two hours”, though it may occasionally take several days. Iron sulphate contains natural minerals needed by plants to produce sugars through photosynthesis, which strengthens grass and gives it a lush, deep green colour, as explained by gardening specialists at Moowy.
How to apply iron sulphate in four steps
To prepare the lawn, mow a few days in advance, clear away leaves and debris, avoid trimming the grass too short, and check the weather forecast. This helps ensure even coverage.
For application, mix 25g per litre of water and use a sprayer or watering can; one litre covers five square metres. Apply uniformly and avoid overlapping, as patchy application can lead to inconsistent results.
After treatment, moss will darken, dry out, and die. A lawn turning black is normal and indicates the iron sulphate is working; the lawn may briefly look worse before improving.
Once the moss has fully blackened, rake it thoroughly, scarify heavily damaged lawns, and collect all loose debris. Removing dead moss improves air circulation, water absorption, seed-to-soil contact, and grass recovery; leaving it in place can hinder the lawn's ability to bounce back.
