On almost any old wall you will find small grey buns that look dusted with frost in the middle of summer. Pull one close and the frost turns out to be hair: every leaf of grey-cushioned grimmia (Grimmia pulvinata) ends in a fine white point, and a whole cushion of them reads silver from a few feet off. It is one of the most dependable mosses to name in a town, partly for that hoary look and partly for a capsule that behaves like no other moss on the wall.
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The hoary grey cushion
Colour is the first thing you notice, and it changes with the weather. Dry, the cushion pulls its leaves in tight and the white hair-points stand proud, so the whole dome looks grey, almost silvered, as though someone had breathed frost across it. Wet it and the leaves relax and open, the green beneath comes through, and the grey softens to a sage tone with the hairs lying flatter. Those hairs are awns, glassy clear extensions of the leaf tip with no green in them, and they do real work on a wall that bakes from noon, shading the leaf a little and helping to run the morning dew down into the cushion where the plant can use it. Each hummock sits neat and rounded, rarely more than two or three centimetres across, firm enough that you can press a thumb into it and feel it spring back.
A capsule that hides in the cushion
Most wall mosses hold their capsules up on a straight stalk, waving them clear of the leaves where the wind can carry the spores away. This one does the reverse. Its seta is short and strongly curved, bent like a swan's neck, so the ripening capsule gets drawn back down and tucked in among the hair-points instead of lifted above them. Often you have to part the cushion with a fingernail to find the capsules at all, sitting half-buried and faintly ribbed down their sides as they dry. Nothing else on a typical garden wall does this, which makes it the quickest feature to settle the name by. The wall screw-moss growing a hand's width along the same coping behaves quite differently, shooting its narrow capsules up on tall straight stalks and uncoiling a long orange twist of teeth at the mouth, so once you have met both there is no mistaking them.
Where it grows
This is a plant of hard, lime-rich, sun-struck surfaces, and towns suit it well. Run your eye along the mortar between bricks, the coping stones on top of a wall, concrete posts, asphalt, roof ridges and the flat tops of gravestones, especially where a bird habitually perches and leaves a little nitrogen behind. It wants sun rather than shade, which already sets it apart from most of the mosses people picture, and it handles drought by shutting down altogether: a cushion can sit bone-dry and grey through weeks of summer, then green within minutes of a shower. Grimmia pulvinata was among the plants that crept back into our cities as the coal smoke faded, and it now grows across most of Britain and far beyond, about as cosmopolitan as a moss can be.
Telling it apart on the wall
The hoary hair-points alone will usually do it, but one or two near neighbours can muddle a quick glance. Other small cushion mosses carry whitish hair-points too, so when the colour leaves you unsure, go straight to the capsule, since the curved stalk and the buried, ribbed capsule are the thing that clinches it. The grey, bun-like habit also invites confusion with bun moss proper, though that is a far larger, softer, whitish-green dome of shaded woodland and would never set up on a dry wall top. Up on acidic hill rock you may meet its relatives in the same genus, which can be genuinely awkward to separate; down among brick and mortar in the lowlands, a grey hairy cushion with a hidden curved capsule is almost always this species.
Leave it on its wall
I have never tried to cultivate it and see little reason to. It belongs to brutal little habitats, the top of a sun-struck wall or a concrete gatepost, the sort of place that finishes off most mosses in a fortnight, and it has no interest in the cool damp corners where a gardener actually wants moss to spread. Its value is as a teacher instead. It fruits freely, it turns up everywhere once your eye is tuned to it, and that swan-necked capsule gives you a clear, satisfying scrap of anatomy to practise on before the fiddlier sorts in how to identify moss have you reaching for a microscope. Next time you pass an old churchyard wall in full sun, run your eye along the coping for little grey frosted buns, then part one with a fingernail and look for the ribbed capsules curled back inside.