Plants medium sized, generally regularly and closely pinnate to plumose, soft, pale yellow green to golden green, procumbent, 2--10 cm; branches 0.2--1.5 cm, leafy main stems 0.5--1 mm broad. Stems yellowish to pale green, hyalodermous, central strand weak or absent. Leaves of stem circinate-secund, 1.5--2 × 0.5--0.8 mm, not markedly curved to insertion, margins plane sinuate proximally to serrate toward apex; median leaf cells 50--65 × 4--4.5 µm, those at insertion shorter, broader and pitted, alar regions excavate, of 3--4 cells at extreme leaf angle, slightly bulging, hyaline and thin walled. Sexual condition dioicous; inner perichaetial leaves lanceolate, weakly or not plicate, slightly toothed or entire near apex. Seta 2.5--3 cm, red-brown. Capsule cylindric, erect to suberect, 3--5 mm excluding conic to conic or rostellate operculum; annulus 1--3-seriate.

Sporophytes produced spring, capsules mature May--June. Predominantly epiphytic on living trees or on cliffs and perpendicular rock faces but persisting on decaying logs, sometimes terrestrial; 0--1500 m; B.C., N.W.T., Nunavut, Yukon; Alaska, Calif., Idaho, Mont., Oreg., Wash; e Asia.

Specimens of Hypnum subimponens from Arctic regions are problematic; in fact any specimens from horizontal surfaces can be difficult to place accurately. Sporophytes are frequent only on vertical surfaces and then often are abundant. Unlike similar species in Western North America, this species produces cylindric, erect sporangia (slightly curved below mouth when mature). Hypnum lindbergii has falcate-secund leaves while those in H. subimponens are circinate-secund the circinate to strongly hamate Key traits of the stem leaves separate H. subimponens from H. hamulosum, while the former is usually more than twice the size of the latter. See also discussions of H. callichroum, H. holmenii and H. plicatulum.