Stout or slender, to 1.5 m, with few lvs and branches; some of the roots often distally tuberous-thickened; lvs once pinnate, sessile or nearly so, oblong to deltoid, the 5-9 lfls narrowly linear to oblanceolate or elliptic, 6-14 cm נ5-40 mm, entire to strongly serrate; rays mostly 20+, spreading, forming a loose umbel to 15 cm wide; pedicels 5-20 mm; fr rounded at both ends, 4.5-6 mm, more than half as wide; 2n=32. Swamps, marshes, ditch-banks, and wet prairies; L.I. to s. Ont. and Minn., s. to Fla. and Tex. July-Sept. (O. turgida)
Gleason, Henry A. & Cronquist, Arthur J. 1991. Manual of vascular plants of northeastern United States and adjacent Canada. lxxv + 910 pp.