Plants medium-sized, 4--8 cm, generally regularly pinnate, dark green to golden green or nearly black, procumbent to suberect, often firmly attached by rhizoids, but when suberect generally without rhizoids; branches 0.2--1.5 cm, leafy stems 0.05--0.15 cm wide Stems red-brown to nearly black, hyalodermous, central strand poorly differentiated; pseudoparaphylla foliose, broad. Leaves of stem falcate, leaves curved downward towards substratum, ovate-lanceolate, tapering to a narrow acumen, 1--1.5 × 0.4--0.5 mm, curving slightly to insertion, margins plane, entire to sinuate in basal half, but often sharply serrate toward apex, costa usually indistinct; median cells 40--60 × 3--4 µm, basal cells often pigmented yellowish to brownish, especially adjacent to hyaline alar cells, porose, rectangular, shorter and wider than median cells; alar cells in a well-defined excavate group of thin walled, mainly rectangular cells, area 2--3 cells high and 3--4 cells deep, the outer ones thin-walled and somewhat collapsed inward, with a few triangular cells distally, often bulging 40--65 × 20--30 µm. Sexual condition dioicous; inner perichaetial leaves erect, lanceolate and finely pointed, plicate, toothed, costa obscure. Seta 2--4.5 cm, red-brown when mature. Capsule red-brown,usually nodding when mature, 3--4.5 mm excluding conic operculum, annulus 1--2-seriate, cilia of endostome rudimentary.

Sporophytes produced summer; capsules mature June-July. Terrestrial, rocks, logs, near water courses, seepage areas, open to somewhat shaded areas, but usually where persistent moisture is available; 0--1500 m; B.C.; Alaska, Calif., Oreg., Wash; Asia (Japan south to Kyushu).

Hypnum dieckii is distributed around the north Pacific, and can be frequent when growing on logs or outcrops near streams. This is a distinctive species, especially when sporophytes are present, as it is the only North American species that has nodding capsules. The dark stems, and pinnate branching associated with the excavate region of thin-walled alar cells set off by inner thick-walled and pigmented basal cells, are usually enough to separate it. It can be distinguished from Hypnum lindbergii by the decurrent thin-walled alar cells and the longer and weakly toothed leaves of that species.