Plants medium, rather slender. Stems 3-6(-10) cm. Leaves 6-8(-10) mm, spreading when moist; marginal lamina 3-5(-10) cells wide, plane to erect or somewhat inflexed; lamellae (3-)4-6 cells high, entire or distantly and finely serrulate, the marginal cells narrowly elliptic, not or only slightly thickened. Sexual condition polygamous; males intermingled with the female or male and female inflorescences very rarely observed on the same stem. Capsule rather slender, pale yellowish brown; hypophysis cylindric but not sharply delimited; peristome teeth 50-64, somewhat irregular in size and shape.
Soil or humus in damp to wet, chiefly coniferous forests; moderate to high elevations; B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que.; Alaska, Calif., Colo., Maine, Minn., N.H., N.J., N.Y., Pa., Tenn., Wash., Wis., Vt.; s, e Asia (Japan, Nepal).
Variety densifolium is absent from the Arctic, but widespread across northern North America. Most of the eastern North American plants that have been called Polytrichastrum formosum probably belong to this variety, although many of the older herbarium specimens so named are P. ohioense. Variety densifolium is evidently monoicous (see T. Osada 1966), although admittedly this is difficult to demonstrate.
Plants medium to large and robust. Stems 5-10(-20) cm. Leaves 6-12 mm, subsquarrose and broadly spreading-recurved when moist; marginal lamina 2-5 cells wide, erect; lamellae 5-7 cells high, entire, the marginal cells rounded and ± thick-walled. Sexual condition dioicous. Capsule short rectangular, yellowish to brownish; hypophysis cylindric, delimited by a shallow groove; peristome teeth 64, very regular in form.
Low to moderate elevations (0-500 m); B.C., N.B., Ont.; Alaska, Wash.; Europe; sw Asia (Caucasus Mountains, s Russia, Turkey); n Africa (Algeria); Atlantic Islands (Iceland, Macaronesia).
Variety formosum is rare in North America, and its distribution and ecology poorly documented. It may possibly be an introduction, like Atrichum undulatum, another common European species believed to be introduced here.
Plants: medium and slender to large and robust, green to dark olive green to blackish, in loose tufts. Stems: (2–)3–8(–20) cm, mostly unbranched. Leaves: 6–8(–12) mm, erect to erect-spreading when dry, spreading to subsquarrose and broadly recurved when moist; sheath ovate to elliptic, yellowish, hyaline-margined, gradually tapering or abruptly contracted to the blade, the cells at the shoulders forming a differentiated hinge; blade lanceolate to linear; costa prominent abaxially and toothed near the tip; excurrent as a short, toothed point; marginal lamina erect, (2–)3–5(–10) cells wide, plane or erect, sharply toothed from apex nearly to the sheath; lamellae (3–)4–5(–7) cells high, margins ± entire to finely serrulate in profile, the marginal cells in section rounded to narrowly elliptic and somewhat taller than the cells beneath, the cell walls not or moderately thickened; median cells of sheath 8–12 µm wide, narrowly rectangular, 5–7(–10):1; cells of marginal lamina subquadrate, 10–15 µm. Sexual: condition dioicous or polygamous; perichaetial leaves similar to the foliage leaves, or somewhat longer, with a longer sheath. Seta: 3–6 cm, yellowish to reddish brown. Capsule: 4–7 mm, rather slender or short-rectangular, acutely 4(–6)-angled, inclined to almost horizontal, pale yellowish brown to brownish; hypophysis cylindric, indistinctly delimited or set off by a shallow groove; exothecium smooth or the cells weakly convex, quadrate to hexagonal, without a central thin spot; peristome 600 µm, divided to 0.6, the teeth 64 and highly regular in form or fewer and somewhat irregular, pale to brownish; epiphragm absent marginal teeth. Spores: 12–16 µm. Widespread, temperate to cool temperate latitudes in the Northern Hemisphere.
Varieties 3 (2 in the flora). European treatments often assert a similarity between Polytrichastrum formosum and Polytrichum commune, which cannot be said of the North American expression of the species. The habitat and ecology of the European plants are also distinct: A. J. E. Smith (2004) described P. formosum in Britain as a common and weedy species of heaths, moorland, woods, outcrops, and old walls.