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.
Densely cespitose; stems to 1+ m, smooth or with 30-60 very low, inconspicuous ridges, the epidermal cells over the ridges not enlarged; basal sheaths to 2 dm, bladeless, mostly reddish-brown; invol lf (10-)15-25(-35) cm, appearing like a continuation of the stem; infl apparently lateral, many-fld, usually open, the longest branches (3-)4-10 cm; fls prophyllate; tep 2-2.5(-3) mm, stramineous, broadly lanceolate, acute, the outer commonly a bit longer than the inner; anthers 3; fr trilocular, slightly shorter to slightly longer than the tep, mostly obtuse to truncate, seldom J. griscomi) mucronate; 2n=40, 42. Open marshes and wet meadows; nearly cosmop., and throughout our range. Our common native plant, throughout e. U.S. and se. Can., is var. solutus Fernald & Wiegand, with rigid tep closely investing the fr. The European var. effusus, with soft tep ±spreading from the base, is locally intr. from Nf. to Me. and inland to Mich. and Minn. (Var. compactus, a form with compact infl)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.