Engelm., Trans. Acad. Sci. St. Louis 2: 483 (1868)
T: California, in uliginosis vallis Sacramento [Sacramento Valley], K.T.Hartweg 2017; syn: W [a duplicate of this collection was mentioned as Juncus acutiflorus Benth., Pl. Hartweg. 341 (1857), a false later interpretation of the note given by Bentham]; California, Mariposa, Big Tree Grove, Jul 1866, H.N.Bolander [G.Engelmann, Herb. Junc. Bor.-Amer. Norm.] 95; syn: MO, PR, W.
Perennials, usually 30–70 cm tall, loosely caespitose, robust; rhizome creeping, branched, to c. 1.5 mm in diam., with internodes long. Stem flattened, two-edged, c. 2.5–6 (–8) mm wide. Cataphyll 0–1; apex short, acuminate. Leaves mostly cauline, 2–5, double-edged-compressed, to c. 40 cm long, shorter than stem, (2.0–) 3.5–6.0 (–9.0) mm wide, pluritubular, imperfectly septate; sheath loose; auricles absent. Lower bract leaf-like, c. 3–8 cm long, shorter than inflorescence. Inflorescence to 20 × 10 cm, anthelate, supradecompound with branches erect to suberect, of many cup-shaped to semiglobose, (2–) 3–6 (–10)-flowered pale brown or stramineous heads. Capitulum bracts scarious to stramineous-membranous, ovate-lanceolate, acuminate, c. 2 mm long. Tepals ±equal, linear-lanceolate, acuminate-subulate, (2.8–) 3.3–4.5 mm long, pale brownish to stramineous; margins narrow, membranous. Stamens 6; anthers c. 0.9–1.1 mm long; filaments 0.5–0.8 mm long; style c. 0.5–0.8 mm long, difficult to distinguish from capsule top; stigmas c. 1.0 mm long, exserted. Capsule elongated, narrowly prismatic-trigonous, very gradually narrowing to a rostrum, unilocular, c. 4.0–4.7 mm long, usually conspicuously exceeding perianth, pale brown to stramineous-brown, shiny. Seeds ±ellipsoid, c. 0.4 × 0.2 mm, pale brown, distinctly longitudinally reticulate, minutely apiculate; appendages absent.