Part the low growth on an acid heath, or under a stand of pines, and look at the stems rather than the leaves: if a clear wine-red thread runs through the plump green shoots, you are almost certainly holding red-stemmed feather-moss (Pleurozium schreberi). It carpets the boreal forests of the north by the square mile and ranks among the commonest mosses anywhere, yet in Britain it keeps mostly to poorer, more acid ground, so gardeners used to richer soils seldom meet it by name.
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A red thread through plump shoots
Trust the stem first. Where most sprawling mosses hide a pale or brownish one, this species flushes a distinct wine to orange-red, and it shows plainly the moment you tease the leaves back or hold a shoot up to a window. Above it the plant is fairly stout and faintly glossy, a yellowish mid-green that pales towards gold out in the open. Its branches come off in a regular, once-pinnate, feathery arrangement, but the leaves sit so concave and so closely overlapped that each branch reads as a smooth, rounded bolster rather than a spray of separate leaves. Draw a fingertip along one and it feels full and blunt-tipped; there is no strong single nerve down the leaf, and none of the leaves sweep to one side, which between them see off most of the mosses it grows among.
Where it grows
Lean, acid, undisturbed ground is the pattern. It thrives across heaths and moorland, through pine and birch woods, over peaty banks, in dune slacks and on the drier hummocks of bogs, wherever lime is scarce and the soil poor. Head far enough north and it becomes one of the ruling plants of the forest floor, laying a continuous springy layer with glittering wood-moss and other feather mosses, deep enough to lose a hand in. Gardens on chalk or on fed, cultivated soil almost never carry it, which is part of why it stays a stranger here while blanketing much of the northern hemisphere. On naturally acid ground left alone, though, it tends to turn up among the heather and bilberry uninvited.
A naturalist's name and a nitrogen habit
The species remembers Johann Christian Daniel von Schreber, an eighteenth-century German naturalist, which is why older books file it as Schreber's moss. What earns it real attention, though, is not the moss itself but what lodges on it. In the northern forests the surface of these feather-moss carpets hosts cyanobacteria, chiefly Nostoc, which draw nitrogen straight out of the air and fix it into a form plants can take up. Spread that across a whole boreal landscape and it counts for a great deal: the moss layer, working with its bacterial tenants, supplies a large share of the nitrogen entering these naturally impoverished forests, feeding the trees at one remove. The same deep carpet also insulates the ground below, holding it cool and damp and helping decide where tree seedlings can root. For a plant most people tread on without a second look, it does a fair amount of the forest's quiet book-keeping.
Telling it from glittering wood-moss
The usual muddle is with glittering wood-moss (Hylocomium splendens), which shares the red stem and often the same carpet. The build settles it. Glittering wood-moss grows in stepped storeys, each year's frond arching over the last like a little staircase, and its shoots divide two or three times into a lacy, almost fern-like spread. Red-stemmed feather-moss keeps to a plainer, once-branched plan and those smooth plump shoots, with no tiered steps to it. There is a fuller account of the other species in glittering wood-moss, and the general knack of sorting the sprawling carpet mosses is laid out in how to identify moss. Both belong to the creeping, side-fruiting group described in acrocarpous and pleurocarpous mosses.
In a garden, or left in the wild
You will not be turning this one into a lawn. It belongs to particular, unhurried places and takes badly to rich, disturbed or limy ground, so it repays being left where it already grows rather than lifted and moved. Should you garden on acid soil and covet its deep, forgiving carpet, the reliable route is to build the conditions it wants, poor sandy or peaty ground, shade, steady damp and no feeding, then wait for it to find you. Out on a heath or in a pine wood it is worth kneeling to, both for the clean red stem and for the thought that a mat of moss can quietly govern a whole forest floor. Its place in living systems sits alongside the accounts in moss and wildlife and the mossy Atlantic woods of the temperate rainforest.