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.