Canad. Rec. Sci. 6: 72. 1894.
Plants medium-sized to large, brownish, pale pink, or sometimes green or yellowish. Stems regularly radially branched; axillary hair apical cell elongate-rectangular to short-linear. Stem leaves broadly ovate-cordate; costa single, strong, (88–)95–282 µm wide near base, ending shortly before apex; alar regions sharply delimited, triangular, from margins to costa or nearly so; medial laminal cells 40–164 × 7–11(–12) µm. Sexual condition dioicous. Spores 15–21 µm.
Habitat: Wet, ± mineral-rich fens, ditches, shores, floating or submerged in lakes
Elevation: low to high elevations
Greenland, Alta., B.C., Man., N.B., Nfld. and Labr., N.W.T., N.S., Nunavut, Ont., P.E.I., Que., Sask., Yukon, Alaska, Colo., Conn., Idaho, Maine, Mass., Mich., Minn., Mont., N.Y., N.Dak., Pa., Vt., Wash., Wis., Wyo., s South America, Eurasia, Pacific Islands (New Zealand).
Plants of Calliergon giganteum frequently resemble miniature spruce trees because of dense and successively longer branches from the shoot apex downwards, regular radial branching, and somewhat spreading branch leaves. The branch leaves are often markedly narrower and more ovate than the stem leaves and may resemble those of C. cordifolium in shape and costa width, but their alar regions are more sharply delimited. The stem leaves are usually broadly ovate-cordate with a broad costa and large and sharply delimited alar regions. Plants with a characteristic pink hue are most frequent in exposed habitats of the North. Differences between this species, 1. C. cordifolium, and 4. C. richardsonii are discussed under the latter two.