Herbs, perennial, rhizomatous, 0.3--7 dm. Rhizomes 1--3 mm diam., nodes not swollen. Culms erect, terete, 1--2 mm diam., smooth. Cataphylls 0 (rarely 1, straw-colored, apex obtuse). Leaves: basal 0--2, cauline 1--4, small fascicles of short capillary leaves often on rhizomes and stems; auricles 0.3--1 mm, apex rounded, membranaceous; blade terete, 1.5--11 cm x 0.8--1.1 mm. Inflorescences terminal cymes, flowers single or paired at nodes, (rarely in 3s), 2--25 cm, branches spreading to erect; primary bract erect. Flowers: tepals straw-colored, oblong; outer tepals 1.6--2.3 mm, apex obtuse; inner tepals 1.8--2.8 mm, apex obtuse; stamens 6, anthers 2--3 times filament length. Capsules included to exserted, chestnut brown, 1-locular, narrowly ovoid, 1.5--3.5 mm, apex acute proximal to beak, valves separating at dehiscence, fertile only proximal to middle. Seeds ovoid, 0.3--0.5 mm, not tailed; body clear yellow-brown. 2n = 40. Fruiting late summer--fall. Shores, peat bogs, sandy soils, pools, occasionally submersed in lakes, rarely in salt water; 0--600 m; B.C., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; Ala., Conn., Del., D.C., Fla., Ga., Ind., Maine, Md., Mass., Mich., Minn., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Vt., Va., Wis. Populations from Virginia and south have been separated as Juncus pelocarpus var. crassicaudex (J. abortivus) based on their taller culms and thicker rhizomes. The evidence used to support the varietal status for the southeastern plants (N. A. Murray and D. M. Johnson 1987) clearly demonstrates that these plants are the southern end of a cline. The flowers are often replaced by bulbils.
Rhizomatous, colonial, ±erect, mostly 1-5 dm, with very slender, obscurely septate lvs; infl much branched, 5-15 cm, broadly ovoid or obpyramidal with ascending branches to flattened with divaricate branches, bearing numerous solitary or paired, distinctly secund fls, some or all of the fls usually replaced by subulate bulbils; fls eprophyllate; tep oblong, scarious on the margins and at the obtuse tip, the sep 1.6-2.3 mm, the pet 1.8-2.8 mm; anthers 6, longer than the filaments; fr narrowly ovoid-ellipsoid, 2.4-3.1 mm, gradually acuminate into a slender beak, unilocular, the placentae fertile only below the middle; seeds few, plump, 0.4 mm; 2n=40. Moist boggy or sandy soil and shores, in soft-water habitats with a seasonably variable water-level. Var. pelocarpus, ranging from Lab., Nf. and Que. to Minn., s. to Del., Md., and n. Ind., is dwarf, mostly 1-3 dm, erect or prostrate (but not repent), the rhizome 1 mm thick or less. The ill-defined clinal var. crassicaudex Engelm. (J. abortivus), occurring irregularly from se. Va. to Fla., is stouter, 3-8 dm, erect, the rhizome 2-5 mm thick.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.