Sp. Pl. 2(1): 214. 1799.
Herbs, perennial, tufted, 1.5–5 dm. Rhizomes densely branching. Culms few–20. Leaves basal, (1–)2–3; auricles 2–5 mm, apex acute, membranous; blade flat, 3–12 cm × 0.5–1 mm, margins entire. Inflorescences 5–40-flowered, borne congested or branch internodes ca. as long as tepals, ssomewhat loose, 1–5 cm; primary bract usually longer than inflorescence. Flowers: bracteoles 2; tepals greenish, lanceolate, 3.3–4.4 mm; outer and inner series nearly equal; stamens 6, filaments 0.5–0.9 mm, anthers 0.1–0.2 mm; style 0.1–0.2 mm. Capsules tan or light brown, 1-locular to pseudo-3-locular, ellipsoid, (3.3–)3.8–4.7 × (1.1–)1.3–1.7 mm, nearly equal to tepals. Seeds tan, ellipsoid to lunate, (0.52–)5.5–0.65(–0.7) mm, not tailed. 2n = 80.
Phenology: Flowering spring–early summer.
Habitat: Exposed or shaded sites in soils ranging from sandy to clayey under moist or drier conditions, oftentimes these sites naturally or otherwise disturbed (e.g., game or human trails)
Alta., B.C., Man., N.B., N.S., Ont., P.E.I., Que., Sask., Ala., Alaska, Ariz., Ark., Calif., Colo., Conn., Del., D.C., Fla., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Utah, Vt., Va., Wash., W.Va., Wis., Wyo., introduced worldwide.
Juncus tenuis occurs throughout North America. It is particularly abundant in northeastern United States and eastern Canada, although infrequent in the south and west.
Through the use of isozyme electrophoresis, hybridization can be demonstrated between various members of the Juncus tenuis complex, including Juncus tenuis, J. anthelatus, J. interior, J. secundus, and J. dichotomus (R. E. Brooks, unpubl.). Juncus ××oronensis is thought to be a hybrid between J. tenuis and J. vaseyi in the northeast.