Most mosses shrink from a baking, sun-struck surface, yet here is one that seeks it out. Great hairy screw-moss, Syntrichia ruralis, colours the tops of walls, the sunny slope of an old tiled roof and the fixed sand of a coastal dune, forming loose olive turfs that shrivel to a dull khaki in drought and open into small green stars within a minute of rain. It ranks among the toughest mosses you can meet, and once you have learned its trick with water you will notice it almost everywhere you look.
Free PDF
Get the free Moss Care Cheat Sheet
One printable page on light, water and patience for terrariums, walls and gardens. Leave your email and pick it up, plus occasional unhurried notes on growing moss.
Free, one click to unsubscribe. The download starts as soon as you have signed up.
Where it grows
This is a plant of open, sunny, lime-rich ground, the sort of place that punishes softer species. It favours mortar and the coping stones of walls, weathered concrete and asbestos tiles, the gritty edges of tarmac, chalk and limestone turf, and above all the stabilised grey dunes of the coast, where the sand-hill form (subspecies ruraliformis) can bind whole acres into a springy crust. Full sun and sharp drainage are what it asks; deep shade and standing damp send it packing, which is why you seldom find it beneath trees. On many an old roof it is the moss doing most of the greening, and it arrives early on the thin substrates of engineered green roofs before the sedums have knitted over. Give it grit, lime and light and it wants for nothing else.
The star in the leaf
Wet a dry shoot and watch closely. Within seconds the leaves peel back and outward from the stem, spreading into a flat rosette that genuinely reads as a small green star, which is where the English name comes from. Each leaf is tongue-shaped with its margins rolled under, and the midrib runs on past the blade as a long, toothed, glass-clear hair, so a damp turf looks dusted with fine white points. That silvery hair-tip is not decoration: it gathers dew and reflects away some of the fierce midday heat, both of which help the plant endure a surface that can cook at noon. Shoots stand a centimetre or two tall and weave into open wefts rather than firm domes, so the turf feels soft and yielding underfoot.
Its trick with drought
Star moss is one of the classic resurrection plants, and it has drawn a good deal of laboratory attention for it. As the surface dries the leaves twist tightly round the stem, curling and pressing inward until the whole turf turns a lifeless grey-brown and looks quite dead. It is nothing of the sort. The cells park their machinery, shed almost all their water and simply wait, sometimes for months, then reassemble within minutes of wetting and photosynthesise as though no time had passed. I once kept a dried scrap on a windowsill through a whole summer and greened it under the tap in front of doubtful visitors; the effect is quietly astonishing, and that same knack is precisely what lets the moss live where it does. Anyone who wants to watch it happen will find the method under reviving dried moss.
Telling it from wall screw-moss
Its nearest source of confusion is its smaller cousin wall screw-moss (Tortula muralis), which shares the hair-point and grows on some of the very same walls. Size decides most cases: Tortula makes neat little cushions a few millimetres across on shaded mortar, while Syntrichia sprawls into looser, taller turfs across open sunny ground. The wet rosette is the other tell, for star moss flings its leaves right back into a flat star, whereas wall screw-moss holds them more forward and its cushions never open so widely. On the coast the sand-dune form muddies the naming again, and specialists now separate several near-identical Syntrichia on microscopic characters. Naming the group is plenty for most of us, and the broad habit reading in how to identify moss is the sensible level to work at.
Living with it
On a roof or a wall top this moss does no structural harm, though a thick sunny mat can shed grit into gutters and, on a shallow pitch, slow the run-off, so the measured approach under moss on roofs applies if it starts to bother you. Left in peace it is a benign and rather beautiful colonist, and on a living roof it is a positive asset, holding moisture and sheltering seedlings through the first dry summers. I would not try to cultivate it in the garden, since it wants more sun and sharper drainage than a shaded border tends to offer, but if you have a sun-baked wall or a gritty flat roof, it will very likely find its own way to you.