Of the haircaps you are likely to meet, this is the smallest and the hardiest. Bristly haircap settles the rawest ground going: bare acid sand, the gritty cap of an old wall, a heath path scuffed down to mineral grit. Its shoots stand short and grey-green, each one finished with a long silvery hair, so a colony caught in a dry spell looks frosted from a few paces and is as easily taken for a lichen as for one of the lush haircaps of bog and woodland.
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A small haircap of the barest ground
Bristly haircap (Polytrichum piliferum) keeps the upright, spiky bearing of its tribe but on a reduced scale, the unbranched shoots usually a centimetre or two tall and seldom more, growing in low open turfs or scattered singly through the grit. Where common haircap wants wet peat and juniper haircap the dry acid bank, this one goes further out still, onto ground that holds almost nothing: sun-struck sand, gravel pits, thin soil over acid rock, the flat tops of walls, the compacted edges of heathland tracks. It is a plant of the open and the exposed, and it is happiest where little else will even start. The colour is a dull blue-green in growth, hoary over the whole patch once the sun is on it, a grey cast that comes entirely from the hairs the leaves carry.
The long glass hair that names it
The species name, piliferum, means hair-bearing, and the hair is the first thing to look for. Each leaf tapers to a clear, glassy point that runs on well past the green blade, longer and more conspicuous here than on any of its relatives. On juniper haircap the equivalent tip stays short and reddish; on bristly haircap it is drawn out and silvery-white, and on a dry colony those countless points merge into the frosted grey sheen that gives the moss away at a glance. Like the rest of the haircaps it rolls each leaf margin inward over the ranked green plates beneath, roofing in the photosynthetic tissue against sun and wind. The pale hairs do related work, throwing back some of the fierce light and, as on the other mosses that wear white tips, catching dew and trailing it down to the shoot. For a plant living on bare mineral ground with no reserve to fall back on, that is part of how it rides out drought.
Red cups and four-sided capsules
The sexes sit on separate plants. In spring the male shoots flatten their topmost leaves into a shallow rosette, flushed orange or red, which holds rainwater like a tiny saucer; a drop falling into one scatters the sperm cells outward to reach the females growing near. Fertilised plants then raise a spore capsule through early summer on a slender reddish stalk. The capsule is a small four-angled box, carried more or less level or nodding, and while it ripens it wears the shaggy pale hood of hairs the whole group is named for. Once the hood drops and the lid lifts, the spores sift out of a drum closed by a fine membrane, the way all the haircaps release them.
First onto raw sand and burnt ground
This is a true pioneer, among the earliest green things to colonise mineral surfaces that have been laid bare. Its shoots and the threads beneath them begin to knit loose sand and grit into a thin crust, steadying ground that would otherwise shift underfoot or blow away, which makes it quietly useful on eroding heaths and sandy banks. After a moor or heath fire it moves onto the cleared ground early, keeping company with the other quick settlers of scorched acid soil. It is common and widely spread across Britain and one of the more cosmopolitan mosses worldwide, turning up from lowland heath and dune to high mountain gravel wherever the surface is acid, open and sharply drained. I most often notice it on the sandy shoulders of heathland paths, where a season of boots has worn the vegetation back to bare grit and this grey little moss has crept in to hold the edge.
Telling it from the other haircaps
Three haircaps come up often enough that it pays to separate them, and habitat does most of the sorting before you reach for a lens. Find a tall, deep-green, soft-looking haircap in wet acid bog and it is common haircap (Polytrichum commune), whose flat toothed leaf edges spread out to the sides; its account sits under common haircap. On a drier acid bank, bluer and stiffer with a short red leaf-tip, you have juniper haircap. Smallest and greyest, out on the most exposed sand, gravel and wall tops, with that long silvery hair catching the light, is bristly haircap. Running a fingertip and a lens along the leaf edge and noting the colour and length of the hair usually settles it, the patient looking that identifying moss always comes down to.