Plants 2--3-pinnate with fronds 20--35(--50) mm broad in typical forms (1-pinnate with fronds as narrow as 8 mm in reduced Arctic-alpine forms), 1--3 mm wide measured across leafy stem, to 20 cm long. Stem leaves oblong-ovate to ovate, 1.6--3.2(--4) × 1--1.7 mm; apex abruptly narrowed to a long crinkled acumen in typical forms (in reduced forms, ovate, 1--1.8 × 0.7-1 mm; apex rounded to obtuse or often with an abruptly acute tip); median cells 30--70 × 4--6 µm. Primary branch leaves ovate to elliptic, concave, 0.8--1.3(--2) × 0.3--0.6(--0.9) mm. Branchlet leaves ovate to elliptic-lanceolate, concave, 0.3--0.8 × 0.2--0.4 mm. Seta 5--25 mm. Capsule ovoid to ellipsoid from distinct neck, 1.7--2.8 mm.

Carpeting soil, humus, rotten logs, and rocks over extensive areas of forest, especially boreal conifer forest, also common in arctic and alpine tundra; 0--3700 m; Greenland; St. Pierre and Miquelon; Alta., B.C., Man., N.B., Nfld., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Alaska, Calif., Colo., Conn., Ga., Idaho, Iowa, Maine, Md., Mass., Mich., Minn., Mont., N.H., N.J., N.Y., N.C., Oreg., Pa., R.I., S.Dak., Tenn., Vt., Va., Wash., W.Va., Wis., Wyo.; Eurasia; n Africa; Australia; Pacific Islands (New Zealand).

Hylocomium splendens is one of the most common and widespread mosses of the circumboreal forest and Arctic tundra, which covers huge areas of Alaska, Canada, northern Europe, and Siberia. Variation in nutrients and climate produces variation in size, growth pattern, and leaf morphology over this broad range. Plants from coniferous rainforests of the Pacific Northwest are robust, 3-pinnate, and form deep wefts of stair-step innovations. The stem leaves have long, crimped acumens. Plants from the Arctic tundra, however, are much smaller, only once- or twice-pinnate, and usually lack the arching innovations. The stem leaves have obtuse or abruptly acute apices. Although these two forms are quite distinct and could easily be named as separate species, between the temperate rainforest and the Arctic tundra there exist plants of every intermediate form. It is best, therefore, to treat this continuum as a single variable species. It does not seem worthwhile to apply names to the infraspecific variants