Leaflets of Western Botany 5: 116. 1948.
Herbs, annual, cespitose, 0.09–0.6 dm. Culms to 20. Leaves to 2.2 cm. Inflorescences headlike clusters, each with 1–2 flowers; bracts subtending inflorescence 2–4, ovate, inconspicuous, 0.8–1.5 mm, membranous, apex acute. Flowers: tepals 4–6, chestnut brown to black, 1.8–2.8 × 0.8–1.5 mm; inner series usually slightly longer than outer, apex acuminate to attenuate; stamens 2–3, filaments 0.6–1.1 mm, anthers 0.3–0.4 mm; style 0.1–0.3 mm, stigma 0.4–0.6 mm. Capsules tan or apex reddish, 2– or 3-locular, globose to obovoid, 1.2–2 × 1.1–1.5 mm, usually shorter than tepals. Seeds ellipsoid-ovoid, 0.5–0.8 mm. n = 18.
Phenology: Flowering spring–mid summer.
Habitat: Moist, bare flats, short turf, and mossy areas in meadows, stream banks, and seepage areas on outcrops (usually granite)
Elevation: 1200–3200 m
Juncus capillaris occurs in California in the Sierra Nevada and in Oregon in the Steens Mountains.