Infrequent throughout the state in marshes, wet borders of lakes, wet woods, swamps and wet interdunal flats. In some large marshes it is common and in these a study of its variation may be made. I have done so and found plants of varying size with all of the leaflets toothed, usually above the middle, some with nearly all of the leaflets entire or some with all of the leaflets entire. Usually the larger plants have larger leaves and the leaflets are more toothed, and the leaves of the smaller plants have fewer teeth or are entire. The form with entire leaflets has been given a varietal name but I believe this is only a form of the species without taxonomic significance. The plants with entire leaflets conform to the type in other characters. The width of the widest leaflets of the plants with entire leaflets is as follows: 2.5 mm; 5.5 mm; 6 mm; 7 mm; 8 mm; and 15 mm. Wilson reports this species as "common" in Hamilton and Marion Counties. No doubt this report should be referred to Cicuta maculata which is a common plant in those counties and which he does not report
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.