Stems (2-)4-6(-10) cm, simple to fasciculately branched. Leaves(4-)5-8 mm, coarsely toothed. Capsule 3-5 × 0.8-1 mm, short-cylindric to long-cylindric and subarcuate.
Soil or humus, shaded non-calcareous rock outcrops, banks, and other shady situations; moderate to high elevations; Greenland; B.C., N.B., N.W.T., N.S., Nunavut, Ont., Que., Yukon; Alaska, Calif., Colo., Idaho, Maine, Mass., Mich., Minn., Mont., N.H., N.Mex., N.Y., N.C., Oreg., Pa., Vt., Wash., W.Va., Wis.; Mexico; temperate s South America; Europe (Turkey); n, c Asia (Japan, New Guinea); s Africa; Pacific Islands (New Zealand); Australia; Antarctica.
Variety alpinum is widely distributed across northern North America, growing in thick masses in crevices and ledges on moist, shaded rock outcrops, also common at all elevations in the Arctic, on tussocks in open tundra, stony banks, and outcrop ledges. In Nunavut, it is known from Bathurst Island and Ellesmere Island. Variety arcticum has traditionally been the repository for plants with cylindric capsules (as opposed to the smaller, ovoid capsules of var. septentrionale) and probably comes closest to being 'typical' Polytrichastrum alpinum. The common expression of P. alpinum in eastern North America has a distinctive aspect, tall and gracile, with slender, subtubulose leaves, and elongate, slender, distinctly curved and inclined capsules (G. E. Nichols 1937), and has no exact counterpart among the traditionally recognized varieties of the species. Polytrichum alpinum var. brevifolium has a more northerly distribution and is smaller in all its parts, but has the toothed leaves and cylindric capsule of the typical form.