On almost every garden wall in the temperate world there is a moss you have walked past a thousand times without a glance. Small grey-green cushions, each no bigger than a coin, sit along the coping and in the mortar joints, looking frosted and dead in dry weather and freshening to green after rain. This is wall screw-moss, Tortula muralis, and learning to name it puts a face to one of the most reliable plants in any built-up place.
Recognising it
The cushions are low and neat, a centimetre or two across, packed tight with short upright shoots. Each tongue-shaped leaf finishes in a long, colourless hair-point, a fine glassy thread, and it is the massed fringe of these pale tips that lends a dry cushion its hoary, almost silvered look. Wet the moss and the leaves spread out and flush a clean mid-green; let it dry and they twist inward and the hair-points close over the top like a drawstring. From early spring it lifts slender reddish stalks, each bearing a narrow upright capsule, and on a thriving wall these are produced so freely that the whole surface seems to bristle.
The twist behind the name
The screw is hidden inside the ripe capsule and rewards a hand lens. Around the capsule mouth sits a ring of long, thread-like teeth, the peristome, and in Tortula these are wound into a tight spiral, coiling like the thread of a screw or a stick of barley sugar. They are hygroscopic, flexing as the air's humidity changes, so that with every swing between damp and dry the spiral works loose a few spores and lobs them clear of the parent cushion. Tortula means, plainly enough, "little twist". If you want to see it for yourself, the close-looking techniques in how to identify moss will get you there.
Built for the wall top
A sun-baked wall top offers no soil, little shelter and long droughts broken by sudden downpours, conditions that see off most mosses. Wall screw-moss prospers there on two tricks. That glassy hair-point on every leaf acts as a tiny parasol, reflecting fierce light and slowing evaporation from the green tissue below. And in common with all mosses it can dry to a brittle crisp and simply wait, snapping back into growth within minutes of a shower, though few manage the trick after as thorough a baking as this one shrugs off. It is fond of lime too, which is why mortar, concrete and limestone suit it so well, and it shrugs off urban soot, so it greens the walls of cities where choosier mosses fail. Standing upright in compact domes, it is a textbook acrocarp, the habit set against the creeping carpet mosses in acrocarpous and pleurocarpous mosses.
Where to look
The name does not mislead: walls come first. Glance along the coping of an old garden wall, the joints between bricks, the capping of a churchyard wall or the parapet of a road bridge and the grey-green cushions are very probably this. From there it strays onto pavements, the plinths of statues, gravestones, asbestos sheeting and old roof tiles, settling on any limey, stable surface out in the light. Across Europe and much further afield it ranks among the commonest mosses of pavements and masonry, often growing cheek by jowl with silvery thread-moss in the same gritty cracks, a pairing worth knowing when you start reading the moss in paving. Damp acid woodland and bog are where it will not be; this moss belongs to bare stone and open sky.
Encouraging it, and living with it
You seldom need to plant wall screw-moss, since it finds any suitable masonry by itself given a few seasons. To hurry it onto a new wall or a concrete ornament, the slurry method serves well: blend a pinch of the moss with rainwater or buttermilk, work the paint into the joints and crevices of a shaded face, and keep it damp while it knits down. Skip the rich compost, because what it wants is the lime and grit it would meet in the wild. On sound stonework it does no harm whatever, taking nothing from the wall and only softening its lines; only where the mortar is already failing does it root into the gaps, and there its presence is a nudge to repoint rather than anything to blame. For most of us it stays what it has always been, the quiet green proof that even bare brick is never truly without life.