Plants in loose tufts, yellow to yellowish green, glossy. Stems 2-8 cm, scarcely tomentose with whitish to reddish brown rhizoids. Leaves erect-spreading, sometimes nearly appressed, flexuose, little changed when dry, undulate or rugose, (3.5-)4-5.5(-6) × 1-1.5 mm, flat to ± concave proximally, subtubulose above, from a lanceolate base to a short, broadly acute apex, distal part of stem often with ovate, short-subulate, blunt leaves; margins serrate in the distal half; laminae 1-stratose; costa ending just before the apex, sometimes with two poorly developed toothed ridges above on abaxial surface, 1/13-1/8 the width of the leaves at base, row of guide cells, two thin stereid bands, adaxial epidermal layer of cells not differentiated, the abaxial layer with a few (usually 2) cells differentiated in distal part of the leaves; cell walls between lamina cells not bulging; leaf cells smooth; alar cells 2-stratose, well-differentiated, sometimes extending to costa; proximal laminal cells long, sinuose, pitted, (28-)47-71(-113) × (5-)9-11(-14) µm; distal laminal cells short-linear, sinuose, pitted, (25-)36-51(-73) × (5-)8-14(-20) µm. Sexual condition pseudomonoicous; dwarf males on rhizoids of female plants; interior perichaetial leaves abruptly long-acuminate, convolute-sheathing. Seta 2.5-3.5 cm, solitary, rarely two per perichaetium, yellowish brown to reddish brown. Capsule 2.5-3 mm, arcuate, inclined to horizontal, striate when dry, yellow-brown; operculum 1.7-3 mm. Spores 14-28 µm.
Capsules mature spring. Mainly in eutrophic fens, sometimes on calcareous soil or rock; 60-1300 m; Greenland; Alta., B.C., Man., Nfld. and Labr. (Nfld.), N.W.T., N.S., Ont., P.E.I., Que., Sask.; Alaska, Idaho, Maine, Mass., Mich., Ohio, Oreg., Pa., Vt., Wash., Wis., Wyo.; Europe; Asia.
Dicranum bonjeanii was recorded from Maine by B. H. Allen (1998b). It is difficult to distinguish from the myriad forms of the polymorphic D. scoparium. Indeed, few of the many herbarium collections from North America named D. bonjeanii are actually that species or at least what is known as that species. It has been noted before (R. R. Ireland 1982) that it may be merely an enviromental form growing in a calcareous, often hydric habitat. Most Europeans (e.g., A. J. E. Smith 1978; E. Nyholm 1986+, fasc. 1) recognize the species as it occurs in Europe, and some that come to North America to collect (e.g., R. K. Tuomikoski et al. 1973) find the species to be distinct on this continent. However, H. A. Crum and L. E. Anderson (1981) and other North American bryologists have synonymized the species with D. scoparium. D. Briggs (1965), who cultivated and studied British plants of both D. bonjeanii and D. scoparium under controlled environmental conditions, found that while they show wide intraspecific variation, especially in regard to the leaf habit and undulation, and thought that they should be kept as separate taxa because each maintains a distinctive array of gametophytic characters. Also, both species are distinctive ecologically: D. bonjeanii prefers eutrophic fens, whereas D. scoparium usually grows in decidedly dry to mesic woodlands, on soil, humus, humus over rock, stumps and logs, tree bases, etc.
Dicranum bonjeanii is best known by its glossy, mostly erect, nearly straight, undulate leaves with broadly acute apices, weakly developed marginal teeth and, what is most important, two poorly developed ridges present only near the leaf apex on the abaxial surface of the costa. The two ridges on the costa, best seen in cross section, will distinguish the species most of the time from D. scoparium which usually has four ridges on its costae. Its preference for eutrophic fens and other calcareous habitats, while avoiding acid substrates, is important from an ecological standpoint and helps give a clue to the identity of the species.