Herbs erect, 2–7 dm, sometimes with ascending branches distally. Stems: internodes 4-lined or 4-angled at first, then terete. Leaves sessile, sometimes amplexicaul; blade usually ovate or triangular-ovate to elliptic or oblong, rarely oblanceolate, 20–65 × 10–22(–30) mm, base shallowly cordate, apex usually rounded, rarely obtuse to retuse, gland dots laminar (relatively dense, large) and intramarginal (relatively small). Inflorescences laxly cylindric to pyramidal, 3–15-flowered from terminal node, sometimes with subsidiary inflorescences from to 4 proximal nodes and flowering branches from to 6 further nodes. Flowers 10–15 mm diam.; sepals oblong to elliptic-oblong or oblong-lanceolate, 4–7(–8) × 1–2 mm, apex acute to acuminate; petals oblong-elliptic to oblanceolate, 6–9(–10) mm; stamen fascicles 4–6.5 mm; filaments 1/5 connate; styles 2–3.5 mm. Capsules cylindric to ellipsoid or ovoid-ellipsoid, (8–)9–10(–12) × 4–5 mm, apex acute. Seeds 0.5–1.2 mm. 2n = 38.
Phenology: Flowering summer–fall (Jul–Oct).
Habitat: Swamps, marshy shores, poor fens
Elevation: 0–500 m
N.S., Ont., Ala., Ark., Conn., Del., D.C., Fla., Ga., Ill., Ind., Ky., La., Maine, Md., Mass., Mich., Miss., N.H., N.J., N.Y., N.C., Ohio, Pa., R.I., S.C., Tenn., Tex., Vt., Va., W.Va., Wis.
Triadenum virginicum is the most widely distributed North American species of Triadenum. In the southwestern part of its range, the leaves are longer and narrower, approaching those of T. tubulosum and T. walteri; it is nearly always distinguishable from the northern T. fraseri by the sepals and styles (see key).