Push aside the leaf litter on a chalky woodland floor and you may turn up a moss the size of a small fern frond, its stems distinctly reddish, its leaves flung out on every side so that a single shoot looks faintly electrified. This is big shaggy-moss, among the largest and most conspicuous mosses in Britain, and once its look is fixed in your eye you will spot it wherever the ground runs sweet.
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A moss you notice from standing height
Most mosses ask you to kneel before you can make out anything of them. This one you can pick out on your feet. The stout shoots reach ten to fifteen centimetres and sprawl loosely through grass and dead leaves rather than knitting into a cushion or a close mat. Its main stem is wiry and clearly red, a good first clue, and it branches without much order, sending side shoots off at all angles. The result is generous and untidy, a scramble of pale to mid green across the ground that in good light carries a soft sheen. For a woodland moss, being so plainly unkempt turns out to be a help rather than a nuisance when you are trying to put a name to it.
Leaves that point everywhere at once
The English name does most of the describing. Each leaf is broadly triangular, drawn out to a fine tip and pleated lengthwise so it holds the light in narrow folds. What marks the moss is the way those leaves sit: they spread widely from the stem on every side, upward, downward and outward, giving the shoot its bristling, shaggy silhouette. A hand lens shows the leaf margins toothed towards the point and the nerve short and forked rather than a single line reaching the tip. The Latin triquetrus means three-cornered, a nod to the way the leaves are ranked so that a branch can look faintly three-sided when you roll it between finger and thumb.
It likes its ground sweet
A healthy carpet of big shaggy-moss is a fair sign of base-rich conditions. It leans towards neutral to lime-rich soils and shows up most on the floor of chalk and limestone woodland, in old unimproved grassland, on shaded banks, and in the damp hollows of coastal dunes where blown shell sand keeps the ground alkaline. On sour, peaty soil it dwindles and gives way to mosses that relish acid, which is partly why it seldom bothers a garden lawn unless that lawn lies over chalk. Where it does grow among turf it arrives as scattered shoots rather than a smothering sheet, so it rarely calls for any discouragement; and if you would sooner encourage it, a shaded, limey bank is the place to look.
Sorting it from the other big carpet mosses
Three other large mosses share the woodland floor and account for most of the muddles. Its close cousin springy turf-moss (Rhytidiadelphus squarrosus) is smaller and spongier, its leaves kinked abruptly back at almost a right angle, and it is the one that runs into lawns. Glittering wood-moss (Hylocomium splendens) builds in neat stepped storeys, each year's growth arching over the last, a habit big shaggy-moss never adopts. Red-stemmed feather-moss (Pleurozium schreberi) shares the red stem but keeps its leaves combed close, so its shoots read as smooth and rounded instead of shaggy. All four are creeping, branching pleurocarps, members of the mat-forming tribe rather than the erect one.
A new name, and a moss that seldom seeds
Two footnotes are worth carrying. Botanists have lately moved this species into a new genus, so recent keys may print it as Hylocomiadelphus triquetrus; the plant has not changed, only its label. And it very rarely raises capsules in Britain, so you will almost never meet it in fruit. It persists instead by patient spread and by broken fragments taking root where they fall, a strategy that suits a moss whose business is holding stable woodland floors and long-settled turf. A thick, thriving colony therefore hints at ground left undisturbed for many years, precisely the sort of quiet, unimproved place that shelters a great deal of other life besides the moss itself.