Sium suave is a perennial herb to 2 m tall; stems stout, angular, solitary; from a fibrous-rooted crown.
Leaves alternate, pinnate, leaflets opposite, 7-17, to 15 cm long, reduced upwards, linear to lance-shaped, sharply toothed along entire length, pointed at both ends, stalkless, leaf stalk sheathing at base, submersed leaves, when present (as in young plants of first year rosette), twice pinnately dissected into filament-like segments.
Flowers white, very small. radially symmetrical, in umbrella-shaped clusters 3-12 cm wide; blooming July-Sept.
Fruit dry, to 0.3 cm long, almost as wide.
Wetland status: OBL.
Frequency in NYC: Very infrequent.
Habitat: Open swamps, muddy soil, wet meadows.
Notes: Apiaceae of open sites are eaten by larvae of the black swallowtail butterfly, Papilio polyxenes (Papilionidae); (Tallamy 2003; Pyle 1981).