The short answer
Moss is a symptom of shade, compaction, poor drainage, low fertility or cutting too short. Killing it without changing one of those means it returns within a season.
- Work out which cause you have. Shade under trees, waterlogging after rain, hard compacted soil, or grass that is thin and pale.
- Compaction: aerate with a garden fork or a hollow tine aerator, pushing in every 10cm or so.
- Drainage: aerate and top dress with sharp sand and compost.
- Shade: thin overhanging branches, or accept it and plant something that likes shade.
- Fertility: feed in spring and autumn with the appropriate seasonal feed.
- Then scarify to pull the dead moss out, and overseed the bare patches.
What will ruin it
Moss killer and lawn feed products containing ferrous sulphate stain paving, concrete and clothing permanently. Sweep any that lands on a path immediately, before it gets wet.
Why it works
Moss does not out-compete healthy grass; it colonises where grass is already struggling. That is why treating the moss alone is always temporary.
Tipking is general household information, not professional advice. Test anything on a hidden patch first. Full disclaimer.