Plants moderate-sized to robust, open, very stiff and slender, less frequently compact, capitulum large, flat, and stellate; typically deep green in shaded sites to yellowish brown in more open sites; without metallic lustre when dry. Stems pale green to yellow-brown; superficial cortical cells with a single large round pore in distal portion of cell usually free from cell wall. Stem leaves lingulate, broadly lingulate to lingulate-spatulate; 0.8-1.3 mm, apex broad, truncate and lacerate, border broad at base (more than 0.25 of base); hyaline cells rhomboid, efibrillose, and rarely septate. Branches typically long and tapering, not 5-ranked. Branch fascicles with 2 spreading and 1-2 pendent branches. Branch stem with solitary retort cells or in groups of 2-3, necks moderately distinct. Branch leaves ovate to ovate-lanceolate, 1-1.4(-1.8) mm, concave, straight, apex strongly involute, margins entire; hyaline cells on convex surface with numerous elliptic pores along the commissures, grading from small pores near the apex to large pores near the base, concave surface with large round pores along the margins and base. Sexual condition dioicous. Spores 21-27 µm, moderately to coarsely papillose on both surfaces; proximal laesura less than 0.5 spore radius.
Capsules mature late summer. Shade tolerant, forming carpets on moist forest floors, along small streams, up through subalpine zone; 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, Calif., Colo., Conn., Idaho., Ill., Ind., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., N. Dak., Ohio, Oreg., Pa., R.I., Tenn., Vt., Va., Wash., W.Va., Wis.; Eurasia.
Sporophytes are uncommon in Sphagnum girgensohnii. This species is most frequently associated with S. russowii, but also found growing with S. centrale, S. fallax, S. fimbriatum, S. warnstorfii, and S. magellanicum when growing in shaded sites of mires. It is very similar to S. rubiginosum, but S. girgensohnii lacks any reddish pigments, has only 2 spreading branches per fascicle, infrequently produces sporophytes, and differs in spore morphology. Throughout much of its range, S. girgensohnii is readily recognized by its green color and its large, slender, strongly stellate capitulum. In the more northern portion of its range, it frequently forms compact stands with a golden brown color and then the stem leaf must often be examined for accurate identification. In Alaska it overlaps morphogically with S. fimbriatum subsp. concinnum, which can look very similar but will have a more spatulate stem leaf that is lacerate completely across the broad flat apex and slightly down the sides. Sphagnum girgensohnii, on the other hand, has stem leaves only lacerate for about 3/4 of the apex width and less conspicuously broadened at the apex.