This midwestern species occurs on base-rich soils in varied types of woods, including moderately disturbed areas. In the central Bluegrass, Short (1828-9) noted: "Grows in rich woods or grass grounds." Further west, his account of the "barrens" also indicates that Trilliums such as sessile did occur in relatively open land. T. sessile is sometimes confused with cuneatum or (in forms with paler flowers) luteum. It differs as follows (FNA 26, J, W, Ch): more elongated stigmas (nearly 2 x ovary versus about as long); stamens ca. 1/2 as long as petals (versus 1/3), the anthers with mostly latrorse dehiscence (versus strictly introse) and more elongated connectives (2-5 mm beyond sacs and beaked versus 0-0.5 mm and blunt); petals less elongated (1-3 cm long and elliptic versus 3-6 cm and cuneate); leaves generally smaller (ca. 4-10 x 2-8 cm versus 7-18 x 7-13 cm); stems generally shorter (ca. 10-20 cm versus 20-40 cm). In Ky. some colls. are referable to forma viridiflorum Beyer (GRAV, JEFF, JESS, MEAD) or forma luteum (CAMP). A coll. from HENR (KY) has four sets of petals and sepals.