Plants robust, stiff; green, pale green, yellow-green; large terminal bud; typically as loose carpets in coniferous forests. Stem green to red-brown; 2-3 superficial cortical layers. Stem leaves shorter than branch leaves, ovate-lingulate to oblong-lingulate, 1.6-1.8 × 1-1.2 mm; hyaline cells mostly nonseptate. Branches long and tapering with distinct squarrose spreading leaves, often terete in tundra forms. Branch fascicles with 2 spreading and 2-3 pendent branches. Branch stems with 1-2 layers of cortical cells. Branch leaves larger than stem leaves, 1.9-2.8 mm, conspicuously squarrose from ovate-hastate base and abruptly narrowed 1/2-1/3 distance from apex into involute-concave acumen, often terete in tundra forms; hyaline cells convex on both surfaces, non-ringed pores at ends and corners of cells, ringed pores on concave surface (4-8/cell) and nonringed pores (2-4/cell) on convex surface, internal commissural walls smooth or indistinctly papillose, chlorophyllous cells ovate triangular with widest part at or close to the convex surface. Sexual condition monoicous. Spores 17-30 µm; proximal surface finely papillose, distal surface smooth with raised bifurcated Y-mark sculpture; proximal laesura more than 0.5 spore radius.
Sporophytes abundant, capsules mature early to mid summer. Forming loose carpets in rich habitats such as wet coniferous forests, Thuja swamps, karrs, medium fens, and stream margins; low to high elevations; Greenland; Alta., B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon; Alaska, Ariz., Calif., Colo., Conn., Idaho, Ill., Ind., Iowa, Maine, Mass., Mich., Minn., Mont., N.H., N.J., N.Y., N.C., Oreg., Pa., S.D., Tenn., Vt., Wash., W.Va., Wis., Wyo.; Eurasia; Pacific Islands (New Zealand).
In its typical robust form with strongly squarrose branch leaves, Sphagnum squarrosum is unmistakeable. Smaller forms such as occur in the higher mountains may be difficult to identify accurately without careful examination of microscopic details. In the tundra there sometimes occur large, terete forms of S. squarrosum but these are usually considerably more robust than S. teres. See also discussion under 14. S. strictum.