Linnaeus, Sp. Pl. 1: 326. 1753.
Juncus conglomeratus Linnaeus; J. effusus var. brunneus Engelmann; J. effusus var. caeruleomontanus H. St. John; J. effusus var. conglomeratus (Linnaeus) Engelmann; J. effusus var. costulatus Fernald; J. effusus var. dicipiens Buchenau; J. effusus var. exiguus Fernald & Wiegand; J. effusus var. gracilis Hooker, J. effusus var. pacificus Fernald & Wiegand; J. effusus var. pylaei (Laharpe) Fernald & Wiegand; J. effusus var. solutus Fernald & Wiegand; J. effusus var. subglomeratus Lamarck & de Candolle; J. griscomii Fernald, J. pylaei Laharpe
Herbs, perennial, 4--13 dm. Rhizomes short -branched, forming distinct, often large clumps. Culms erect, terete, 1--2.5 mm diam. at top of sheaths. Cataphylls several. Leaves: blade absent. Inflorescences lateral, compound dichasia, many flowered; primary bract erect, terete, extending well beyond dichasium. Flowers: tepals tan or darker, usually with greenish midstripe, lanceolate, 1.9--3.5 mm; inner slightly shorter; stamens 3, filaments 0.5--0.8 mm, anthers 0.5--0.8 mm; style 0.2 mm. Capsules greenish tan or darker, 3-locular, broadly ellipsoid to oblate, 1.5--3.2 mm. Seeds amber, (0.3--)0.4---0.5 mm. 2n = 40, 42.
Flowering summer, fruiting summer--fall. Swamps and their edges, marshes, moist meadows, and moist or saturated soils, often conspicuous in pasture meadows where it is shunned by grazing animals; Habitat??; 0--2500 m; B.C., Man., N.B., Nfld. and Labr. (Nfld.), N.S., Ont., P.E.I., Que.; 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.Y., N.C., Ohio, Okla., Oreg., Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wash., W.Va., Wis.
The Juncus effusus complex has been variously recognized as containing several species or a single species with numerous infraspecific taxa. Unfortunately, North American treatments have dealt primarily with taxa in either the eastern or western portions of the continent. In considering the continent as a whole, little sense can be made of these treatments. The North American J. effusus complex is one that is in obvious need of modern systematic scrutiny.
Fernald, M. L. and K. M. Wiegand. 1910. The North American variation of Juncus effusus. Rhodora 12: 81--93. Hämet-Ahti, L. 1980. The Juncus effusus aggregate in eastern North America. Ann. Bot. Fenn. 17: 183--191.