More people meet moss as a lawn complaint than in any other way. Before reaching for the iron sulphate, it pays to read what its arrival is telling you about the ground.
Why the moss is there
Moss colonises turf wherever grass is already losing its grip. Shade, compacted soil, poor drainage, scalping the lawn with a low mower and thin, acidic, hungry ground all weaken grass and happen to be the conditions moss prefers. So the green carpet creeping through the lawn is really a reading of the site. Clear it off and leave those conditions untouched, and it will be back inside a season.
Reading the signs
Each cause leaves a clue. Worst under trees or along a fence line, and shade is your answer. Worst where the lawn squelches underfoot or sits on heavy clay, and the trouble is drainage and compaction. A thin general invasion across an old, hungry, close-mown lawn points instead to low fertility and cutting too hard. Working out which one you have matters, because some of these yield to effort and others, deep shade above all, simply will not.
Getting the grass back
Where the cause can be addressed, go for the cause rather than the moss itself. Thin overhead growth to let in light, spike or hollow-tine to relieve compaction, sort out drainage, raise the cutting height, and feed so the grass can muscle back in. Scarifying drags out the loose moss and opens room for grass to thicken, and on a truly acidic lawn a measured dose of lime shifts the balance. Iron-based mosskillers blacken it within days, which feels like progress but lasts only until the same conditions grow it back.
Living with a moss lawn
The alternative is to stop fighting. In a shaded, damp garden a moss lawn is a real pleasure underfoot, green right through winter and drought, and free of mowing, feeding and watering once it has settled in. Clearing the leaves in autumn is about all it asks. Where grass has never properly taken, this tends to be both the better-looking surface and far the less demanding, and plenty of gardeners now choose it on purpose rather than inheriting it by defeat. There is a full how-to in how to make a moss lawn.
Which moss makes a lawn
If you are encouraging it rather than evicting it, lean towards the creeping, carpet-forming kinds over tight cushions, because they spread into continuous, walkable cover. The springy turf-moss already colonising many lawns is one of them; the feather mosses are another. Cushion-formers look marvellous in a moss garden but spread too slowly and lumpily to make a lawn.