Rhizomes pink, freely branching, producing extensive clumps, 2-3 cm diam., with fibrous remains of old leaves; roots fleshy. Stems usually 1-branched, solid, 7-15 dm. Leaves: basal deciduous, at first erect, then recurved, blade dark green, with prominent median thickening, 4-10 dm × 2-3 cm, slightly glaucous basally; cauline equaling inflorescence unit. Inflorescence units 4-12-flowered; spathes green with brown margins, outer spathe strongly keeled, inner without keel, 6-9 cm, subequal, margins not scarious. Flowers: perianth bright yellow; floral tube 0.6-0.8 cm, with no constriction into ovary; sepals bright yellow or cream colored, lanceolate to ovate or suborbiculate, 5-7.5 × 3-4 cm, base abruptly attenuate, claw ca. 1/2 length of limb, signal a darker yellow basal patch limited by short, brown lines; petals without veining, lanceolate to spatulate, 2-3 cm; ovary triangular in cross section with concave sides and narrow groove at each angle, 1.5 cm; style keeled, 3-4 cm, crests spreading, 1-1.2 cm, laciniate at apex; stigmas rounded with prominent tongue; pedicel 2.5-7 cm. Capsules prismatic to oblong-ovoid, obscurely 3-angled with obvious groove at each angle, 3.5-6 cm, beak 5 mm. Seeds D-shaped, flattened, 6-7 mm, corky, lustrous. 2n = 34.
Lvs stiff and erect, broadly ensiform from densely crowded rhizomes; stem 0.5-1 m, shorter than or equaling the lvs; fls 7-9 cm wide, bright yellow or cream-color; sep spreading, the crest area outlined by an irregular series of brown markings; pet erect, ligulate, constricted at the middle, 1-2.5 cm, unmarked; capsule 6-angled, cylindric-prismatic to ellipsoid, 5-8.5 cm, the valves widely spreading at maturity; 2n=24-34. Swamps and shallow water along streams and ponds; native of Europe, and widely established in our range. Apr.-June.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.