Rhodora 6: 35. 1904.
Herbs, perennial, cespitose, 1.4–5.5(–7) dm. Culms erect, terete, 1–3 mm diam., smooth. Cataphylls 0–1, straw-colored to pink, apex acute. Leaves: basal 1–3, cauline 1–2; auricles 0.5–3 mm, apex rounded to truncate, scarious; blade terete, 1.5–25 cm × 0.5–2.5 mm. Inflorescences terminal panicles or racemes of 2–35 heads, 1–12 cm, branches erect; primary bract erect; heads 2–8-flowered, ellipsoid to narrowly obconic, 2–9 mm diam. Flowers: tepals green to light brown, lanceolate; outer tepals 2.3–3.1 mm, apex acuminate to rarely obtuse; inner tepals 2.5–3.2 mm, apex acuminate; stamens 3 (or 6), anthers 1/4–1/2 filament length. Capsules exserted, chestnut brown, imperfectly 3-locular, narrowly ellipsoid to prismatic, 3.2–4.8 mm, apex acute proximal to beak, valves separating at dehiscence. Seeds fusiform, 0.7–1.2 mm, tailed; body covered with whitish translucent veil. 2n = 80.
Phenology: Fruiting mid summer–fall.
Habitat: Generally in acidic or peaty moist sites, including emergent shorelines and aroundg hot springs
Elevation: 100–2500 m
Alta., B.C., Man., N.B., Nfld. and Labr., N.S., Ont., P.E.I., Que., Sask., Ariz., Colo., Conn., Maine, Md., Mass., Mich., Minn., Mont., N.H., N.Y., N.C., Oreg., Pa., R.I., Tenn., Utah, Vt., Va., W.Va., Wis., Wyo.
Populations from around hot springs in the western United States have been treated as Juncus tweedyi, with the remaining populations called J. brevicaudatus, but no morphologic distinction appears to exist between the two taxa. This species was called J. brevicaudatus in the print edition of the Flora of North America; however, that name at the rank of species was published in 1904 so the name J. tweedyi has priority.