Where common haircap wants a bog, this one takes the dry bank above it. Juniper haircap makes its living on sun-baked acid ground that would leave its larger cousin gasping: heathy banks, sandy tracks, forest rides, the raw shoulders of a quarry. Its shoots stand shorter and bluer than common haircap's, tipped with a reddish glint once the weather dries, and the way each leaf rolls its edges inward is the surest mark of the species.
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A haircap built for dry ground
Juniper haircap (Polytrichum juniperinum) is a medium-sized member of the haircap tribe, its stiff unbranched shoots standing a few centimetres tall, now and then a little more, in loose turfs or scattered through the heather. In good light the colour is a distinctive glaucous blue-green, often flushed reddish-brown on the older growth and over a whole colony in a dry spell. It keeps the family's upright, spiky bearing, so a patch reads as a low bristly sward rather than a soft carpet. What it will not abide is a soaking. Common haircap holds the wet peat and the pool margins; juniper haircap settles the free-draining ground alongside, the crest of the bank, the gravelly path, the thin acid soil over rock where the sun reaches and the water runs away fast.
The inrolled leaf and the red tip
Lift a shoot, look along a leaf with a hand lens, and you meet the feature that names it. Like all haircaps it carries rows of green photosynthetic plates, the lamellae, along the upper face of the leaf. Common haircap leaves its margins flat and finely toothed, standing out to the sides; juniper haircap folds its margins right over the top of those plates, so the two edges meet down the middle and roof the green tissue in. That inrolled margin is smooth, without teeth, and it does genuine work: by covering the lamellae it shields them from drying wind and hard light, part of how the plant holds on where its bog-dwelling relative could not. Each leaf finishes in a short, stiff hair-point, and in this species the point is reddish-brown rather than clear or white. Let the ground dry and the leaves shut in hard against the stem, so the shoots look thin and almost twiggy; wet them and they lift out again into the familiar haircap star.
Little cups on the male plants
In spring the male shoots do something that stops you looking at anything else. Juniper haircap keeps its sexes on separate plants, and each male stem ends in a flat rosette of reddish or orange leaves shaped like a shallow cup, a centimetre or so across and easily taken at a glance for a small flower. These are splash-cups. A raindrop landing squarely in one throws out a spray of droplets carrying the sperm cells, flinging them far enough to reach the female plants nearby. Where fertilisation takes, the females raise the spore capsule on a long reddish stalk through the summer. The capsule itself is sharply four-angled, a little box tipped on its stalk, and while it ripens it wears the shaggy golden hood of hairs that lends the whole group its English name.
A coloniser of bare and burnt ground
This is among the first mosses to move onto ground that has been stripped or scorched. Bare acid soil, road cuttings, the spoil around old workings, sandy heath opened up by feet or fire: juniper haircap arrives early, and its shoots knit a thin turf that starts to bind the loose surface. After a heath or moorland fire it is a familiar sight among the pioneers, keeping company with bonfire moss and redshank moss on the blackened ground before the heather returns. It is common and widespread across Britain and well beyond, one of the more cosmopolitan mosses, at home from lowland heath to mountain slope wherever the ground is acid, open and quick to drain.
Telling the haircaps apart
Three haircaps turn up often enough to be worth holding apart. Common haircap (Polytrichum commune) is the giant of wet acid bog, tall and deep green, its flat toothed leaf margins spreading out to the side. Juniper haircap sits on the drier ground between, bluer, its margins folded over the leaf and its hair-tip red. Smaller and greyer again is bristly haircap (Polytrichum piliferum), which takes the very driest and most open places of all, bare sand and gravel and the tops of walls, and carries a long silvery-white hair-point that catches the light where juniper haircap's stays reddish. Draw a fingertip and a lens over the leaf edge and note the colour of the hair, and the three usually sort themselves out. Reading a moss from its overall form first and its fine points second is the habit set out under how to identify moss; the wet-loving cousin has its own account at common haircap.