Buy a fern, stand the pot on a shaded step for a season, and a soft green moss will very likely settle on the compost before you have lifted a finger to invite it. As often as not that volunteer is Bryum capillare, capillary thread-moss, among the most abundant and adaptable mosses in the world and quite possibly one you already own.
Recognising it
The shoots stand upright in low cushions seldom more than a centimetre or two tall, each crowded with leaves broadest above the middle and drawn to a fine, clear tip. That tip is a hair, the leaf nerve running out past the blade as a colourless point, and it hands the plant both its Latin name (capillare, from capillus, a hair) and half its character. The surest field mark shows only as the cushion dries. A damp shoot could pass for any small green moss, yet a drying one twists its leaves into a tight spiral round the stem, so the whole shoot appears to corkscrew; wet it again and the leaves unwind within seconds. No other common wall or pot moss performs that spiral so readily, and a single dry shoot rolled between finger and thumb is often enough to name it.
The hanging capsules
Through much of spring the moss carries spore capsules, and they are as good as a signature. Each rides atop a long stalk, or seta, that begins fresh yellow and ripens to a deep translucent red, while the capsule hangs downward from the tip like a small green pear rather than standing straight up. Swollen and green at first, it dulls to nodding brown as the spores mature, its mouth ringed with fine teeth that open in dry weather to shed the spores. A wall studded with these drooping red-stalked lanterns in April is almost certainly capillary thread-moss or one of its close kin, and that pendent capsule marks the whole Bryum tribe apart from the upright-fruited mosses growing alongside.
A moss you may already own
Few mosses are so willing to arrive uninvited. It colonises plant pots and greenhouse staging, wall mortar, roof tiles, gravel, weathered tarmac and the bare soil of a shaded bank, asking only a little grit to lodge on and shelter from the harshest sun. Most gardeners meet it as the green film creeping over the compost of a long-standing pot, where it does no harm at all and, if anything, says the watering has been steady. Drought it survives by shrinking to a dull olive crust and greening again at the next shower, the same knack that keeps most of its neighbours going, discussed under watering a moss. That toughness, with a readiness to grow from the smallest broken scrap, has carried it to every continent, Antarctica included.
Telling it apart
Its closest look-alike is its own cousin, silvery thread-moss (Bryum argenteum), though the two seldom cause lasting trouble: argenteum is the metallic silver moss of hot pavements, its blunt leaves pressed into smooth catkin-like shoots, while capillare stays plainly green, its pointed leaves loose enough to spiral when dry. On a wall it often grows shoulder to shoulder with wall screw-moss (Tortula muralis), which also carries a fine hair-point; there the drying test decides it, for Tortula curls each leaf inward like a scroll rather than twisting the shoot bodily, and holds its capsules bolt upright on straight stalks. Strictly, capillare is an aggregate, a knot of near-identical segregates that specialists separate under the microscope by leaf shape and by minute tubers on the rhizoids; for the rest of us, naming the group is plenty, and the broad-brush habit reading in how to identify moss is the right level to work at.
Growing it, or leaving it
There is rarely any call to plant capillary thread-moss, since a shaded, gritty, undisturbed surface tends to recruit its own within a year or two, but it takes kindly to deliberate use. It holds a neat low cushion and fruits so freely that it makes a quiet feature on the compost of a shaded houseplant or in an open dish garden, the spring capsules a small reward for no effort. It transplants without fuss: press a lifted patch onto damp gritty compost, keep it out of direct sun and mist it while it knits down, much as you would when starting a moss lawn from fragments. Where it strays onto a path or pot rim, a stiff brush clears it; otherwise it is a moss to notice rather than to manage, arriving unbidden and asking nothing in return.