Sp. Pl. 1: 432. 1753,.
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 adaxially, 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.
Phenology: Flowering Jul–Oct.
Habitat: Wet soils, stream banks, fresh-water marshes, margins of beaver ponds, pools in floodplain forests, shores, ditches
Elevation: 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.