Most mosses keep their heads down; the common haircap stands up. On a wet heath or the floor of a sodden wood it raises what looks like a miniature pine forest, ankle-deep and sometimes deeper, every shoot a stiff stem set with narrow dark leaves. It ranks among the tallest mosses on earth, and it is one of the more satisfying to learn by sight.
A forest in miniature
Polytrichum commune grows as unbranched upright stems, often a hand-span high and, in a really wet bog, taller still, each one crowned in its prime by a spreading star of leaves. Run a finger along a leaf and it feels faintly rough; the upper surface carries rows of tiny green plates called lamellae, which deepen the colour and do much of the plant's photosynthesis. Dry weather folds the leaves in tight against the stem, and a shower throws them open again into that characteristic star. The English name points to the spore capsules, lifted high on reddish stalks through summer, each wearing a shaggy golden cap of hairs.
Plumbing inside the stem
What really sets the haircaps apart is hidden in the stem. A typical moss drinks over its whole surface and has next to no internal transport, but Polytrichum runs a central core of stretched, specialised cells, hydroids and leptoids, that carry water up and sugars down in much the way the veins of a larger plant do. That inner plumbing is a good part of why it can afford to grow so tall and stand so stiffly where flatter mosses would flop. It offers a glimpse of the very problem, lifting water against gravity, that the flowering plants would later answer on a far grander scale, which is why this humble moss turns up so often in botany classes.
Where it grows
Seek common haircap on wet, acid, peaty ground: bogs, the margins of moorland pools, damp heath and the boggy floors of acid woodland. It is a plant of the sour and the saturated, intolerant of lime, and where the ground suits it can sheet over wide stretches. On drier banks you are more likely to meet its shorter cousins, the juniper and bristly haircaps, which take more sun; true commune wants its feet in the wet. As an upright, dome-forming moss it is a textbook acrocarp, the contrasting habit described in acrocarpous and pleurocarpous mosses.
A long record of use
Haircap has served people for centuries, which wins it a place in any account of moss through human history. The wiry, durable stems were bundled into besoms, the old twig brooms used to sweep yards and hearths. In parts of Scotland and Scandinavia the same toughness saw it twisted into ropes and woven into baskets and mats, and gathered by the sackful to stuff mattresses and pillows, where its springiness and slight water-resistance made a serviceable filling. Households on the northern moors are recorded sleeping the winter through on beds of it.
Growing it
In the garden, common haircap is less obliging than the creeping carpet mosses, since it insists on the acid, perpetually damp conditions of its native bog. Meet those, in a boggy corner, at a pond margin or in a lime-free terrarium kept good and humid, and it lends a height and forest-floor character the flat mosses cannot. Move it as intact clumps carrying their own pad of peaty substrate rather than as loose fragments, water it only with rainwater, and never lime it. Whether it will settle for you comes down almost entirely to acidity and wetness, the sort of judgement worth making before you start, as weighed up in will moss grow where I want it?.