Plants to 5 dm, downy. Stems simple or branching. Leaves to 5 dm; petiole to 2 dm; blade pale abaxially, deeply 5-7-lobed; margins irregularly dentate or crenate. Flowers: pedicels 25-80 mm; sepals ca. 15 mm; petals yellow, obovate, 2-3 cm wide; style 3-6 mm. Capsules ellipsoid, 20-35 mm, pubescent. Seeds pale brown, reticulate-pitted. 2 n = 20 (cult.). Flowering spring. Moist deciduous woods, thickets, and cedar barrens, often on slopes, occasionally in fields or on shaded dunes, in loam or sand; 100-600 m; Ont.; Ala., Ark., Ill., Ind., Ky., Mich., Mo., Ohio, Tenn., Va., W.Va. Vegetatively, this native species closely resembles the more frequent and widespread Chelidonium majus , introduced from Eurasia. Various authors have reported Stylophorum diphyllum from western Pennsylvania, but W. E. Buker and S. A. Thompson (1986) could not confirm its past or current native presence there.
From Flora of Indiana (1940) by Charles C. Deam
Infrequent to frequent or local in all parts of the state or possibly absent in some counties. I have not found it in the southwestern counties although I have done much collecting there.
Stem 3-5 dm at anthesis; basal lvs several, long-petioled, thin, broadly oblong to ovate, pinnately divided almost or quite to the midvein into 5-7 oblong or obovate, obtusely lobed or toothed segments; cauline lvs 1 pair, smaller than the basal; buds on erect pedicels 2-5 cm; sep hairy; pet 2-3 cm; style 1 cm; fr ellipsoid or thick-fusiform, bristly, 2-3 cm; 2n=20. Rich, moist woods; w. Pa. to s. Mich. and Wis., s. to Tenn. and Ark. Apr.-May.
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.