There is a moss that lives where almost nothing green has any business growing, in the crack of a city pavement, along a clogged gutter, on a flat roof baking in the sun. It gives itself away by a metallic shimmer that no other common moss quite manages. Silvery thread-moss, Bryum argenteum, ranks among the most widespread plants on earth, and once its sheen is in your eye you will catch it on half the pavements you walk.
Where the silver comes from
The shimmer is a trick of the leaves. Each tiny shoot is packed with leaves that overlap closely all the way up, so the shoot stays smooth and rounded like a green catkin or a short length of plaited cord, a shape botanists call julaceous. The upper half of every leaf carries no chlorophyll and stays colourless and translucent, and it is the massed gleam of those pale tips over the green beneath that turns a dry cushion the soft silver-grey behind the name. A shower greens it again as the leaves drink; let it dry and the silver creeps back. The shoots are short, a centimetre at most, mounded into low, neat turf wherever a film of grit has lodged. Upright and tuft-forming, it counts as an acrocarp, the habit explained in acrocarpous and pleurocarpous mosses.
A moss of the city
Other mosses put up with towns; this one seems to relish them. It does best on the hard, hot, disturbed surfaces that see off nearly everything else: the mortar at the foot of a wall, the silt in a paving joint, the gravel of a flat roof, the dust along a kerb. Part of the secret is a hunger for nitrogen, since it grows lushest where bird droppings, dog urine or plain street grime enrich the ground, so a colony will often mark the spot beneath a favourite perch or beside a much-used corner. It shrugs off trampling, drought, road salt and the heavy metals of traffic, and is studied as a quiet gauge of urban pollution for exactly that toughness. Well beyond the city it reaches almost anywhere wind and people can carry it, from Antarctic gravel to high on the world's mountains, which puts it among the small handful of truly cosmopolitan mosses.
How it gets everywhere
Spores barely account for its success. In many districts it seldom bothers to fruit, though where it does it hangs reddish, drooping capsules on short stalks. Its real means of travel is breakage. The brittle shoots part at a touch, and each loose scrap can root and grow afresh, so a fragment caught on a shoe sole or a bicycle tyre is ferried off to found a fresh colony streets away. Many shoots also pinch off minute green pellets, gemmae, that achieve the same thing in miniature. Between the two it spreads without ever waiting on a partner or a ripe capsule, which goes most of the way to explaining how one small moss came to circle the globe.
Living with it
On a path or a drive a haze of silver moss does no harm worth the name, binding a little grit and softening the bare stone underfoot; where it makes a paving joint slippery after rain a stiff brush lifts it easily, the mild end of the methods gathered under removing moss. Should you want to encourage it instead, you need do very little, for a shaded, gritty, faintly grimy surface left undisturbed tends to grow its own crop within a season or two. It keeps regular company with wall screw-moss in the same gritty cracks, the two of them the staple greenery of paved places and well worth learning side by side once you begin reading the moss in paving. To pin a name on it among its neighbours, the close-looking habits in how to identify moss will settle most doubts, because that silver catkin shoot, once properly seen, is hard to take for anything else.