Perennial grasslike forb 30 - 90 cm tall Leaves: two to four, basal, 15 - 30 cm long, 0.5 - 2.3 mm wide, slender, grasslike, flattened in cross section, and lacking crosswise partitions inside (non-septate). The leaves also have a fragile, easily broken, transparent, membranous, basal, 2 - 3.5 mm long, pointed-tipped, ear-like appendage (auricle) at the top of the clear-edged leaf sheath. Inflorescence: a terminal, widely spreading, often flat-topped, forked, multi-branched structure with ten to one hundred, single, short-stalked (0.1 - 0.5 mm), radially symmetric, small flowers at the ends of at least 0.6 cm long secondary branches. The entire branched inflorescence is subtended by a large (usually longer than full inflorescence), erect or ascending bract, and the individual stalked flowers have two small bracts (bracteoles) positioned directly below the tepals. Stamens: six, with 0.8 - 1.1 mm long filaments, and 0.3 - 0.7 mm long anthers. Pistil: with one superior ovary, a 0.2 mm long style, and three stigmas. Fruit: single-chambered, tan, 2 - 3.2 mm tall (obviously shorter than tepals), 1.1 - 1.6 mm wide, widely ellipsoid to inversely egg-shaped, distinctly stalked capsules borne widely spaced along spreading branches. Each capsule is surrounded by the erect, remnant tepals. Stems: tufted, erect, smooth, round in cross section. Seeds: many, tan, 0.3 - 0.6 mm long, ellipsoid with short-pointed or abruptly narrowed tips, but without distinct narrowed tails at the ends. Tepals: six in two whorls of three, green, 3.2 - 4.5 mm long (all about same length), 0.7 - 1 mm wide, lance-shaped with pointed tips.
Similar species: Juncus anthelatus is most similar to J. tenuis, but that species is typically smaller (rarely to 70 cm tall), and it has taller capsules (3.3 - 4.4 mm) borne on shorter and more congested inflorescence branches. Also very similar are J. dudleyi and J. interior, except neither of those species have long, pointed, fragile ear-like appendages (auricles) at the summit of the leaf sheaths, rather the auricles are well under 1 mm long and hard or even leathery. Other species such as J. vaseyi and J. greenei differ by having leaves that are almost round in cross section. The non-native J. compressus should be obviously differentiated by its much shorter auricles (only up to 0.5 mm), blunt-tipped tepals, and darker colored capsules.
Habitat and ecology: Not common, typically in moist or seasonally wet and usually sandy soil.
Occurence in the Chicago region: native
Notes: Older treatments often consider this species as a variety of J. tenuis, to which it is very closely related and often confused.
Etymology: Juncus is the classical name for Rush. Anthelatus is Latin for "bring in advance" or "place before".