Sium suave Walter

Water Parsley

© SRTurner

Family - Apiaceae

Habit - Fibrous-rooted perennial forb, glabrous. Often emergent aquatic.

Stem - Ascending to erect, to 2 m, often ridged relatively stout, glabrous.

	Stem and node.
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Leaves - Alternate and sometimes basal, dimorphic, petiolate or upper leaves sessile, sheathing bases not or slightly inflated. Submerged leaf blades multiply pinnately or dichotomously dissected. Leaf blades 2-30 cm long, narrowly oblong to broadly ovate in outline, 1 time pinnately compound with 7-17 leaflets, those of the uppermost leaves sometimes simple, the leaflets 10-100 mm long, lanceolate to 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 usually deeply and finely dissected into linear segments.

	Submerged leaf.
© SRTurner

	Leaf.
© SRTurner

	Leaflet adaxial.
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	Leaflet abaxial.
© SRTurner

	Leaf sheath.
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Inflorescences - 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.

	Inflorescence.
© SRTurner

	Involucre.
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© SRTurner

	Involucel.
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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.

	Flowers.
© SRTurner

Fruits - Schizocarps 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.

	Fruits.
© SRTurner

Flowering - July - September.

Habitat - Marshes, swamps, bottomland prairies, pond margins, ditches.

Origin - Native to the U.S.

Lookalikes - Oxypolis rigidior, more broadly, many other members of the Apiaceae, including species of Angelica, Levisticum, Ligusticum, Cicuta, Perideridia, and others.

Other info. - This characteristic species of good-quality wetlands is found in scattered locations in Missouri, predominantly in the eastern half of the state. Its broader range includes much of the northern half of the continental U.S., and also Canada and Alaska.

As noted above, the plant has numerous lookalikes, with the closest being Oxypolis rigidior. These can be differentiated by their leaflets, which in Oxypolis are entire or with only a few teeth toward the tips. Leaflets of Sium suave have numerous, regular teeth along the entire margin. The plant is quite variable and this should be kept in mind when determining IDs within this family.

There are literature reports that the root of this plant, raw or cooked, is edible with a nutty flavor. However, other reports state that the plant, or at least parts of it, are toxic due to content of polyacetylenes. In any case, the plant strongly resembles others which are unequivocally lethally toxic, and it seems wise to avoid consumption on this basis alone.

Photographs taken at Shaw Nature Reserve, Franklin County, MO, 7-30-2007, and at Marais Temps Clair Conservation Area, St. Charles County, MO, 8-2-2018 and 7-28-2020 (SRTurner).