Plants medium-sized, with red or dark red secondary coloration, sometimes green; branch and shoot apices not pencil-like; cells of stem epidermis not widened; pseudoparaphyllia broadly ovate to broader than long; axillary hairs with 1--4(--5)-celled distal portion, hyaline when young. Stem leaves straight, loosely imbricate or erect-spreading, oblong, ovate or narrowly ovate, in distal portion ± suddenly narrowed to rounded- or acute-apiculate apex, short apiculus (rarely absent) often bent inwards over leaf, concave; margins entire or almost so; costa ending 80--95% distally in leaf; alar cells in upwards distinctly delimited, transversely triangular group that ± gradually passes into strongly incrassate cells near costa, not or hardly decurrent, supra-alar cells weakly differentiated. Sexual condition dioicous.

Warnstorfia sarmentosa is easily separated from the other North American species of the genus by its oblong or ovate stem leaves, suddenly narrowed to a rounded-apiculate or acute-apiculate apex. The apiculus is most distinct in young leaves and is rarely lacking completely in these, but may be worn off in older leaves. In late snow beds or in cold springs in higher mountains and in the Arctic, modifications with less well delimited alar cells occur. The species could perhaps be confused with Calliergon species, which, however, are larger, have broader leaves and never get a clear red color, or with Straminergon stramineum [treated in the FNA as Calliergon stramineum], which is usually pale or whitish green to yellow-green (never red) and lacks the apiculus near the leaf apex. The latter is normally more sparsely branched (sometimes unbranched) than W. sarmentosa, and in addition the alar groups are ovate or broadly ovate in Straminergon, transversely triangular in W. sarmentosa. The type material of Calliergon subsarmentosum Kindberg. (Vancouver Island) belongs to Calliergon giganteum (Schimper) Kindberg.

Intermediately mineral-rich fens, often around springs or in late snow-beds, sometimes submerged in lakes; 0--3960 m; Greenland; Alta., B.C., Man., Nfld., N.W.T., Nun., Ont., Que., Yukon; Alaska, Colo., Mont., N.H., N.Y., Wash., Wyo.; South America, Eurasia (including Papua New Guinea), e Africa, Australia, Pacific Islands (New Zealand), Antarctica.

Warnstorfia sarmentosa was earlier placed in the genus Calliergon because of general similarities in leaf shape and orientation. However, in the frequent presence of red pigments, appearance of the alar groups, nondecurrent leaves, general size of the plants, and in its habitat preferences it is more similar to certain Warnstorfia species, such as W. exannulata. In addition, the differences between W. sarmentosa and W. exannulata are bridged by the South American Warnstorfia luipichensis (R. S. Williams) Hedenäs, which is closely related to W. samentosa but has leaves slightly more gradually narrowed to the apex and more or less denticulate leaf margins, and W. pseudosarmentosa, which, despite a leaf shape that is similar to that in straight-leaved specimens of W. exannulata is reminiscent of W. sarmentosa in its habit and entire leaf margins. Entire or subentire stem leaf margins are also found in the Eurasian W. procera (Renauld & Arnell) Tuomikoski, of which the leaf shape is similar to that in falcate-leafed specimens of W. exannulata. Considering all characters, there is no justification for excluding W. sarmentosa from the genus Warnstorfia, unless a few easily visible characters are given much higher weight than other ones.