Leaflets: blade apex acute to obtuse. Inflorescence bracts lanceolate to ovate. Petals white (sometimes pinkish), (3-)5-7 (-8) mm. Stamens (6-)7-10 (-14) mm, ± equaling or slightly longer than petals. Seeds roughened. 2n = 20. Flowering summer-early fall. Roadsides, riverbanks, gravel bars, flood-scoured shorelines, grasslands, disturbed sites; 0-1100 m; Ont., Que.; Ala., Ariz., Ark., Calif., Colo., Conn., Ga., Idaho, Ill., Ind., Iowa, Kans., Ky., La., Maine, Md., Mass., Mich., Minn., Mo., Mont., Nebr., Nev., N.H., N.J., N.Mex., N.Y., N.Dak., Ohio, Okla., Oreg., Pa., S.Dak., Tex., Vt., Va., W.Va., Wis., Wyo. Subspecies dodecandra is smaller-flowered than the other subspecies and is most often found in northeastern and midwestern states. `White Spider´ is a cultivar sometimes offered in seed catalogues. Occurrences in New England (except Vermont) probably represent adventives from farther west; this plant is common along weedy roadsides.
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, 3.5 - 6.5 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, 4 - 10 mm long, not 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. trachysperma has longer petals (8 - 16 mm) and longer stamens (12 - 30 mm), with the stamens usually much longer than the petals.
Flowering: mid June to early October
Habitat and ecology: This species grows in gravelly soil, often along railroads or along river banks.
Occurence in the Chicago region: native
Etymology: Polanisia comes from the Greek words polys, meaning many, and anisos, meaning unequal, referring to the stamens. Dodecandra means "with twelve stamens."