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 x  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 x 1.1--1.5 mm, usually shorter than tepals. Seeds ellipsoid-ovoid, 0.5--0.8 mm. n = 18.

Flowering spring--mid summer. Moist, bare flats, short turf, and mossy areas in meadows, stream banks, and seepage areas on outcrops (usually granite); 1200--3200 m; Calif., Oreg.

Juncus capillaris occurs in California in the Sierra Nevada and in Oregon in the Steens Mountains.