Plants: small, bright green. Stems: 0.5–1.5 cm, distinctly singly rosulate, innovations short, rosulate. Leaves: of main rosette and innovations similar, regularly spirally twisted around stem, erect-spreading when moist, obovate, weakly concave, 0.5–2.5 mm; base not decurrent; margins recurved to mid leaf, distinctly serrulate distally, limbidium present, of 1–3 rows of cells; apex acute; costa long-excurrent, awn sometimes hyaline, slender, irregularly twisted when dry; proximal laminal cells long-rectangular in rosette leaves; medial and distal cells short-rhomboidal, 12–25 µm wide, 3–4:1, walls thin, not porose. : Specialized asexual reproduction by rhizoidal tubers, usually on long rhizoids away from stem base, red-brown, same color as rhizoids, 200–300 µm, smooth or almost so. Sexual: condition dioicous. Capsule: inclined, red-brown, cylindric to elongate-pyriform, 3–5 mm. Phenology: Capsules mature May–Jul.
Traits diagnostic for the protean Rosulabryum capillare are leaves spirally twisted around the stem, long awn, distinct border, and serrulate margins, small size, rosulate innovations, and rhizoidal tubers the same color as the rhizoids. Rosulabryum gemmascens has irregularly contorted leaves with a much weaker border and nearly entire margins, and elongate somewhat imbricate innovations of small red-brown ovate leaves. Rosulabryum laevifilum has a much more variable costa that sometimes does not reach the apex, green leaves, and filiform gemmae. Rosulabryum torquescens has irregularly contorted leaves with strongly serrate margins and red to crimson tubers, brighter than the rhizoids, and is usually synoicous.