Biennial or perennial herb 20 - 50 cm tall Leaves: alternate, stalkless or short-stalked, 5 - 15 cm long, 1.5 - 4 cm wide, narrow lance-shaped to oblong, wavy to toothed, covered with stinging barbed hairs. Flowers: borne near branch tips, pale yellow to whitish, with 100 to 200 stamens, opening in late afternoon and closing at midnight. The ten petals are 4 - 7 cm long, narrow and inversely egg-shaped, and usually overlap when open. Fruit: a cylindric capsule, 3 - 5 cm long, 1.5 - 2 cm wide, opening at the tip. The seeds are numerous, flat, and have well developed wings. Stems: one to several, erect, stout, branching on upper half, whitish and exfoliating near base.
Similar species: Mentzelia nuda is less coarse, has shorter petals (2 - 5 cm long) that do not tend to overlap and flowers that close near sunset.
Flowering: August to October
Habitat and ecology: Introduced from farther west, this species grows near railroads and in sandy disturbed soils.
Occurence in the Chicago region: non-native
Etymology: Mentzelia is named after German botanist Christian Mentzel (1622 - 1701). Decapetala means ten-petaled.
Stout, sparingly branched biennial 2-5 dm; lvs sessile or short-petioled, narrowly lanceolate or oblong, sharply cleft or pinnatifid, the larger 6-15 cm; pet 10, pale yellow to nearly white, 5-7 cm; stamens 100-200; fr 3-5 cm; 2n=22. Dry prairies and plains; N.D. to Alta. and Ida., s. to w. Io., Tex., and n. Mex.; adventive in Ill. July-Sep. (Nuttallia d.)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.