Plants: medium to robust, dark green to brownish with age, in loose or rather dense, tall tufts, often in extensive patches. Stems: (2–)5–10(–70) cm, stiffly erect to decumbent, mostly simple or rarely forked, bracteate proximally, densely to rather distantly leafy. Leaves: 6–8(–12) mm, erect or erect-spreading when dry, flexuose, with distinctly recurved tips, spreading to broadly recurved when moist, or the blade sharply squarrose-reflexed from the base; sheath oblong to elliptic, involute-tubular and clasping the stem, often golden yellow and highly nitid, abruptly contracted to the narrowly lanceolate blade; marginal lamina plane or erect, narrow, 2–3(–7) cells wide, toothed from the base of blade to the apex, the teeth unicellular, embedded in the margin; costa toothed abaxially near the apex, excurrent, ending in short, rough awn; lamellae crenulate in profile, 5–9 cells high, the margin distinctly grooved as seen from above, with 2 rows of paired, projecting knobs, the marginal cells in section narrow or more typically enlarged and wider than those beneath, retuse to deeply notched (rarely divided by a vertical partition), ± thick-walled and brownish, smooth; sheath cells 60–90 × 10–13 µm, elongate-rectangular to linear (as much as 20:1), narrower toward the margins; cells of the marginal lamina 10–15 µm wide, subquadrate. Sexual: condition dioicous; perichaetial leaves with a long, scarious-margined sheath and blade greatly reduced, gradually narrowed to a finely acuminate tip, the margins toothed, denticulate to subentire, the costa short- to long-excurrent, roughened to almost smooth. Seta: 5–9 cm, stout, yellowish to reddish brown. Capsule: 3–6 mm, short-rectangular to cubic, brown to dark reddish brown, glaucous when fresh, sharply 4-alate, inclined to horizontal; peristome 250 µm, divided to 0.6, the teeth 64, pale. Calyptra: golden yellow to brownish, completely enveloping the capsule. Spores: 5–8(–12) µm. Widely distributed in the North America, throughout temperate and boreal latitudes in the Northern Hemisphere, Mexico, Pacific Islands (New Zealand), Australia.

Widely distributed in the North America, throughout temperate and boreal latitudes in the Northern Hemisphere, Mexico, Pacific Islands (New Zealand), Australia. The only North American species likely to be confused with Polytrichum commune is Polytrichastrum pallidisetum. When sterile, Polytrichum commune is easily distinguished by cross-sections of the leaves, particularly by the greatly broadened costa, and the notched marginal cells of the lamellae. In Polytrichastrum pallidisetum the marginal cells of the lamellae are broadened and shallowly retuse, but not distinctly notched, and in Polytrichum commune the arc of guide cells (seen in section below mid-point of blade) is broad, 18–24 cells wide, compared with 8–12 cells wide in Polytrichastrum pallidisetum. When capsules are present, the two species can be distinguished at a glance. A striking form of Polytrichum commune growing in wetlands, var. uliginosum, has greatly elongate stems and tightly sheathing, glossy leaf bases, the blades are flexuose and squarrose-recurved. Recent European genetic studies (R. J. Bijlsma et al. 2000) suggest that it should be recognized as a separate species.

Plants: highly variable in size and habit but usually rather tall and robust. Stems: (2–)5–10(–70) cm. Leaves: loosely imbricate to rather distant; sheath clasping the stem and usually plainly visible wet or dry; blade typically divergent and sharply toothed to the base; marginal cells of lamellae in section broader than those beneath, retuse to distinctly notched, with prominent knobs; perichaetial leaves not markedly longer than the foliage leaves, ending in a short, roughened spinulose tip. Capsule: short-rectangular.

Moist organic soil in wet habitats, pastures, old fields, and meadows, peatlands, margins of bogs or swamps, often covering extensive areas, low to moderate elevations, widespread in the boreal forest, rare north of the tree line and absent from the high Arctic (D. G. Long 1985). Greenland, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Ont., P.E.I., Que., Sask., Yukon, Alaska, Ark., Calif., Colo., Conn., Del., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., N.H., N.J., N.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., n, c Europe, n, e Asia, n Africa, Atlantic Islands (Macaronesia), Pacific Islands (Hawaii, New Zealand), Australia. Discussion.

Plants: in compact, rather dense tufts. Stems: to 6 cm. Leaves: densely imbricate and crowded, somewhat less sharply toothed than in var. commune; marginal cells of lamellae in section narrow, more shallowly grooved; perichaetial leaves conspicuous and ribbon-like, exceeding the foliage leaves, mostly hyaline, subentire, ending in a long, nearly smooth awn. Capsule: short-rectangular to cubic.

Humus and moist sandy soil, open woods, and roadside banks. mainly low elevations. B.C., N.B., N.S., Ont., Yukon, Alaska, Ark., Conn., Fla., Ill., Ky., La., Maine, Md., Mass., Mich., Mo., N.J., N.Y., N.C., Okla., Pa., R.I., S.C., Tenn., Va., Wash., Wyo., Europe, n Africa.

Variety perigoniale has a distinctly †œbushy† aspect, the leaves thick set and densely imbricate. The typical expression is common on the Atlantic coastal plain, as in the Pine Barrens of New Jersey (the type was from Carolina). Elsewhere, its distribution and ecology are poorly documented, but it is probably not simply a dry land phase (H. A. Crum and L. E. Anderson 1981), occurring sporadically throughout the range of the species. The perichaetial leaves are not only longer than the foliage leaves, but distinctly ribbon-like, irregularly twisted and curled when dry.