Stems frequently pink or reddish, especially with age, 1-6(-8) dm, stipitate glands hyaline or reddish brown and often black- or purple-tipped. Leaves: petiole absent or 1-10 mm; blade 2-18(-24) × 0.5-4(-5.5) cm, glabrous or sparsely stipitate-glandular abaxially and ad-axially, hairs black- or purple-tipped. Inflorescences secund, 2-7-branched, each branch (6-)10-25(-30)-flowered. Pedicels 0.5-3 mm. Flowers: hypanthium 0.5-1.5 × 1.5-3.5 mm; sepals persistent, erect or spreading, unequal, 0.8-2 × 0.4-1 mm, margins entire or serrulate with 1-4 gland-tipped teeth per side; filaments 1-2 mm; anthers 0.7-1 mm; pistil 3-4 mm; stigmas often purple with age. Seeds 0.5-0.7 × 0.2-0.3 mm, tubercles reddish or pinkish. 2n = 18.

Flowering Jul-Oct. Wet soils, stream banks, fresh-water marshes, margins of beaver ponds, pools in floodplain forests, shores, ditches; 0-700 m; B.C., Man., N.B., Ont., Que.; Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Miss., Mo., Nebr., N.H., N.J., N.Y., N.C., N.Dak., Ohio, Okla., Oreg., Pa., R.I., S.C., S.Dak., Tenn., Tex., Vt., Va., Wash., W.Va., Wis.

The seeds of Penthorum sedoides were used by the Meskwaki to make cough medicine, and the leaves were used by the Cherokee as a potherb (D. E. Moerman 1998). The species is introduced in southern British Columbia, Oregon, and Washington, where it grows in cranberry bogs.