1. Sium suave Walter (water parsley, water parsnip)

Pl. 212 h–j; Map 882

Plants perennial, glabrous, with fibrous roots. Stems 50–200 cm long, erect or ascending, often relatively stout and strongly ridged. Leaves alternate and sometimes also basal (1 or a few basal leaves sometimes present at flowering), long- to mostly short-petiolate, the uppermost leaves often sessile or nearly so, the sheathing bases not or only slightly inflated. Leaf blades 2–30 cm long, narrowly oblong to broadly ovate in outline, 1 time pinnately compound with (3–)7–17 leaflets (the leaflets several times pinnately or dichotomously dissected in submerged leaves), those of the uppermost leaves sometimes simple, the leaflets 10–100 mm long, those of emergent leaves linear to lanceolate or narrowly ovate, narrowed or rounded at the base, sharply and often finely many-toothed along the margins, narrowed or tapered to a sharp point at the tip, those of submerged leaves in aquatic individuals usually deeply and finely dissected into linear segments. Inflorescences terminal and axillary, compound umbels, short- to long-stalked. Involucre of 6–10 bracts, these 3–15 mm long, linear to lanceolate, the broader ones usually with irregular, white, papery margins. Rays 10–20, 1.0–4.5 cm long. Involucel of 4–8 bractlets, these mostly shorter than the flower stalks, entire, linear to narrowly lanceolate, sometimes with narrow, white margins. Flowers mostly numerous in each umbellet, the stalks 3–5 mm long. Sepals absent or minute triangular scales. Petals elliptic-obovate, shallowly notched or narrowed or tapered abruptly to a short, slender tip, white. Ovaries glabrous. Fruits 2–3 mm long, broadly elliptic to nearly circular in outline, slightly flattened laterally, glabrous, tan to straw-colored, with prominent reddish brown oil tubes filling the spaces between the ribs, the mericarps slightly narrowed along the commissures, with 5 prominent ribs, these with short, thick, corky wings. 2n=12. July–September.

Scattered nearly throughout the state but absent from many of the western counties (U.S., Canada). Bottomland prairies, moist depressions of upland prairies, marshes, swamps, bottomland forests, margins of ponds and lakes, and banks of rivers, streams, and spring branches; also roadsides and ditches; often emergent aquatics.

Species of Sium reportedly contain polyacetylenes similar to those found in Cicuta and should be considered poisonous.