High on a windswept plateau, where the last grasses give out and the ground is little more than bare stone and gravel, a springy grey carpet often takes over, dull silver in dry weather and greening within minutes of a shower. Underfoot it feels like a thick, faintly crisp mattress. That is woolly fringe-moss, Racomitrium lanuginosum, the great coloniser of Britain's coldest and most exposed uplands, and one of very few plants content to make a living where the wind never rests.
The wool in its name
Leaf after leaf, each shoot ends in a long colourless hair drawn out from the leaf tip, and it is these hairs, packed by the million across a cushion, that lend the plant its felted, grey-white look when dry. Under a lens the hairs prove jagged rather than smooth, minutely toothed along their length like the edge of a tiny saw, and the green leaf cells below them are roughened with small bumps. Wet the moss and the whole picture shifts: the shoots swell, the green floods through, and the carpet darkens to olive in seconds. Let it dry and the silver creeps back. Read from a hillside away, the colour of a woolly fringe-moss heath is a fair guide to how long it has been since the last rain.
Where it takes charge
Acid rock is its element, and it turns its back on anything chalky. It runs over exposed boulders and block scree, sheets the gravelly plateaux of the higher hills, threads through blanket bog, and, most striking of all, blankets the cooled lava of the north. Cross the great flows of southern Iceland and mile upon mile of black basalt lies under an unbroken quilt of this single species, grey and soft and deep enough to swallow a boot; it is often the first plant to gain a foothold on raw rock, trapping the grit and dust that in time become a thin soil for others. In Britain and Ireland its kingdom is the mountain top, from the Cairngorms and the Highlands down through Snowdonia to the Dartmoor tors, wherever exposure and poor acid rock together keep taller plants at bay.
Built for the worst weather
What lets it hold such ground is an extraordinary tolerance of hardship. Like most mosses it can dry to a brittle crisp and simply wait, sometimes for weeks, reviving within minutes of the next shower, a knack looked at in reviving dried moss. The pale hair-points earn their keep here too, breaking the wind across the cushion and combing moisture from drifting cloud, so the living tissue below sits a shade cooler and damper than the bare air around it. Frost and gales scarcely register, and it will sit out months under snow without complaint. What it cannot abide is competition on gentler ground, which is exactly why it withdraws to the places where its rivals give up.
A carpet in retreat
For all its toughness against the weather, woolly fringe-moss heath has been thinning across Britain for decades, and the cause is largely of our making. The moss is tuned to ground almost bare of nutrients, so the nitrogen now falling in rain and mist from farming and traffic works on it like a slow poison, letting coarse grasses muscle in where the moss once ruled unchallenged. Heavy sheep grazing and trampling complete the damage, tearing the carpet and opening it to those same grasses. Where the silver heath has given way to a dull green sward of mat-grass, the shift usually signals enrichment rather than any natural succession, the airborne-nitrogen story told from the other direction in moss and air quality. Once these heaths are gone, coaxing them back is slow and far from certain.
Knowing it on the hill
Few upland mosses are so easy to name where they grow. A grey, hair-fringed, springy carpet on exposed acid rock or high plateau is close to unmistakable, for no other moss forms such vast silvery sheets at altitude. Up close, the likeliest confusion is with the pincushion grimmias of walls and boulders, which also carry white hair-points but sit in small tight domes rather than sprawling wefts, and their dry-country habits are set out in common pincushion. Its close cousins among the fringe-mosses share the toothed hair yet lean to neater or shaggier forms on different ground. Once you have knelt on a summit and felt that deep hoary mattress give under your palm, you will know Racomitrium lanuginosum for good.