Sp. Pl. 1: 340. 1753.
Rhizomes horizontal, brownish, thick, praemorse, fleshy. Scapes 1–3, round in cross section, 0.8–2.5 dm, slender to stout, glabrous. Bracts held well above ground, sessile; blade green to bluish green, strongly to sparsely mottled, mottling becoming obscure with age, oval to suborbicular, 4–10 × 2–8 cm, base broadly attached, apex rounded-acuminate to bluntly parallel sided-acuminate (rounded basally to its broad attachment). Flower erect, odor pungent, spicy; sepals displayed above bracts, spreading, green, variously streaked with maroon, lanceolate-oblanceolate, 9–35 × 4–8 mm, margins entire, apex rounded-acuminate; petals long-lasting, erect, ± connivent, ± concealing stamens and ovary, maroon, brownish maroon, green, or yellowish green, not spirally twisted, oblanceolate to elliptic, occasionally almost orbicular, 1.7–3.5 × 0.7–2 cm, thick-textured, narrowed near basal attachment (but not truly clawed), margins entire, apex gradually rounded-tapered to acute; stamens straight, 10–23 mm; filaments red-purple, 2–5 mm, dilated basally; anthers erect, straight, gray-purple, 9–16 mm, thick, dehiscence introrse; connectives purplish brown, straight, projecting 2–5+ mm beyond anther sacs; ovary greenish white basally, purple distally, ovoid to globose, 6-angled, pyramidally narrowed to stigmas, 4–8.5 mm; stigmas erect, divergent-recurved, distinct, purple, subulate, 1–5 mm, ± fleshy. Fruits baccate, dark greenish purple, odorless, subglobose, 6-angled, angles somewhat winglike, pulpy, not juicy. 2n = 10.
Phenology: Flowering spring (Mar–early May).
Habitat: Rich woodlands, limestone districts, calcareous soils, floodplains, riverbanks, clayey alluvium, less fertile soils, high, dry limestone woods, persists under light pasturing, in fencerows and brushy areas after lumbering
Elevation: 100–300 m
Ala., Ark., Ill., Ind., Kans., Ky., Md., Mich., Mo., N.Y., N.C., Ohio, Okla., Pa., Tenn., Va., W.Va.
Trillium sessile is rather uniform throughout its range, with few color forms.