Few plants are as widely travelled as this one. Ceratodon purpureus grows on every continent, from Arctic gravel to Antarctic rock and across deserts, roadsides and city roofs, and it is very likely the small dark moss furring the ridge tiles above your head or the cinder track under your feet. In spring it gives itself away, when a plain green cushion flushes suddenly to a deep reddish-purple, the colour behind its old country name of redshank.
The purple in the name
For most of the year redshank is easy to walk past, a low turf of short shoots, dark green above and browner below, seldom taller than a couple of centimetres and packed tight over bare ground. The change comes with fruiting. From late winter the colony sends up a dense forest of slender stalks, the setae, coloured a rich wine-red to purple, each carrying a capsule at its tip, and from a little distance the massed stalks wash the whole patch crimson. The Latin purpureus records exactly this, as does the older English redshank. No other common moss of open ground turns colour so boldly, or so conveniently for anyone trying to put a name to it.
Recognising it up close
Out of fruit you have to look harder. The leaves are narrow and taper to a fine point, and their most telling feature shows under a hand lens: each margin is rolled under, curved back on itself down both sides of the leaf. When the shoot dries, the leaves twist loosely around the stem and the whole plant takes on a slightly dishevelled, corkscrewed look; wet it and they spring straight again within seconds. The ripe capsule repays a glance too, held at a slant rather than upright, reddish-brown, and grooved along its length like a furrowed little barrel once it has shed its spores and dried. Take the recurved leaf edge together with that inclined, furrowed capsule on its purple stalk and the species is pinned down.
The great coloniser of waste ground
What marks redshank out is less any single character than its flat indifference to hardship. It is a pioneer, first onto the ground that other plants refuse: cinders and mine spoil, gravel car parks, the trodden edges of paths, thin acid soil over rock, and the grit that gathers in roof valleys and gutters, where it turns up in the company of the mosses described under moss on roofs. It survives drought by shutting down when the surface bakes and quickening again after rain, tolerates a level of air pollution that would see off fussier plants, and ranks among the very few mosses able to grow on earth poisoned by lead, copper and zinc near old workings. On town pavements it shares the cracks with the weedy survivor of silvery thread-moss, and between them the two account for a fair share of all the moss in any city.
A follower of fire
Redshank is also one of the classic mosses of burnt ground, springing up on the blackened soil after a heath or moorland fire alongside its better-known rival for scorched earth. That rival is bonfire moss, Funaria hygrometrica, and once both are in fruit they are simple to separate: Funaria dangles a fat, pear-shaped capsule from a stalk crooked like a swan's neck, while redshank holds a narrower capsule at a mere slant on a straight purple seta. Fire suits redshank for the usual pioneer reasons, offering bare ground swept clear of rivals, and it can hold a burnt patch for a year or two before grasses and taller plants shoulder in over the top. The fire-following habit gets a fuller airing under bonfire moss.
A moss worth knowing
Because it is so common and so forgiving, redshank tends to be among the first mosses a beginner learns to name with any confidence, helped enormously by that spring flush of purple stalks. It carries a weight in science out of all proportion to its size as well: its toughness and its knack of growing almost anywhere have made it a favourite laboratory subject for studying how mosses withstand pollution and drought, and it has even been flown into orbit to test how it copes with the conditions of space. Next time a roof ridge or a gravel path shows a haze of reddish-purple in early spring, kneel down for a moment. You are looking at one of the most successful land plants on Earth, quietly getting on with things where almost nothing else will stand.