Leaflets: blade apex acute to obtuse. Inflorescence bracts lanceolate to ovate. Petals white, (7-)8-13(-16) mm. Stamens (6-)12-28(-40) mm, longest ones ca. 2 or more times as long as petals. Seeds roughened. 2n = 20. Flowering summer-early fall. Gravelly or sandy, sunny places along streams, sandy places, open woodlands, grasslands, roadsides; 100-1500(-2100) m; Alta., Man., Ont., Sask.; Ala., Ariz., Ark., Calif., Colo., Conn., Idaho, Ill., Iowa, Kans., Ky., La., Md., Mass., Mich., Minn., Mo., Nebr., Nev., N.Mex., N.Dak., Ohio, Okla., Pa., S.Dak., Tex., Utah, Va., Wash., W.Va., Wyo.; Mexico. Subspecies trachysperma is found mainly in the Great Plains and western states. Populations in the eastern United States are presumably adventive, as are those in eastern Ontario, where it has been found spreading in disturbed areas, such as quarries and roadsides (B. McBride 2006).
Annual herb 20 cm - 0.8 m tall Stem: erect, arching with age, branched, covered with sticky hairs. Leaves: compound with three leaflets (trifoliate), stalked, having an unpleasant odor. Flowers: borne in a many-flowered (fifteen to 30) terminal inflorescence (raceme), each flower subtended by a leaf-like bract and having a purple style. The four petals are white or pinkish, 8 - 16 mm long, with the upper pair being longer than the lower pair, narrowing to a long claw at the base, and deeply notched to squared at the tip. Stamens ten to 27, pink to purplish, 12 - 30 mm long, often much longer than petals. Fruit: an erect capsule, 2 - 7 cm long, 5 - 10 mm wide, oblong, flattened but slightly inflated, with interconnecting veins and stalked glands, opening from the tip to two-thirds the length, releasing fifteen to 40 or more dull reddish to dark brown seeds (1.7 - 3 mm). Leaflets: 1.5 - 6 cm long, 0.5 - 2 cm wide, lance-shaped to widest above middle or lance-elliptic.
Similar species: The similar Polanisia dodecandra ssp. dodecandra has shorter petals (3.5 - 6.5 mm) and shorter stamens (4 - 10 mm), with the stamens usually not much longer than the petals.
Flowering: early July to mid October
Habitat and ecology: Introduced from the western United States, this species grows in sandy soils but is rare in the Chicago Region.
Occurence in the Chicago region: non-native
Etymology: Polanisia comes from the Greek words polys, meaning many, and anisos, meaning unequal, referring to the stamens. Dodecandra means "with twelve stamens." Trachysperma comes from the Greek words trachys, meaning rough, and sperma, meaning seed.