Helophyte. Glabrous perennial, to 110 cm; rhizome short, ± vertical, 11–22 mm thick, chambered. Stem widely hollow; basal part 5–12(–18) mm thick, terete, usually purplish and glaucous; upper internodes terete or angled to slightly sulcate. Leaves 2–3 at the base and 4–7 on the stem (the innermost basal one or the lowest stem leaf is usually the largest); sheath rather broad, sometimes purplish; petiole 10–31(–39) cm; blade 1–2-pinnate, 8–25(–33) × 4–13(–22) cm, with a length/width ratio of (1.2–)1.6–3.3(–3.8), thin, lower side ± glaucous. Primary leaflets 3–5(–6) pairs; angle leaflet/rachis 25–45°. Apical leaflet 1–2-pinnatifid, with 1–2 pairs of lobes; petiolule 10–32 mm; blade 26–85(–122) × (10–)15–80(–102) mm, with a length/width ratio of (0.7–)1–2.1(–3); margin serrate to doubly serrate, with acute to acuminate teeth; base usually broadly cuneate; apices usually acute. Apical lobe 17–72(–88) × (2–)5–21(–27) mm, with a length/width ratio of 3.1–8.3(–13).

Distribution. Nem–SBor (MBor–NBor) (in the west), Nem–NBor (in the east). – D common to rather common in most areas, but scattered to rare in southern Sjæ, LFM and Brn (5 localities, extant at least at Snogebæk). N scattered in the southeast north to He Engerdal and Op Gudbrandsdalen, and scattered to rare along the coast to SF Solund (but unevenly distributed and hardly in the fjords); around Trondheimsfjorden (ST Ørland and NT 5 localities), NT Nærøy, and VFi Kautokeino (2 localities). S fairly common to scattered (but unevenly distributed) on the mainland north to Mpd and southeastern Jmt, and near the coast in the northern provinces; rare in the northern inland below the mountains; not known from Öl or Gtl (a living specimen, however, found in shore drift in northeastern Gtl). F scattered in A and in the southwestern archipelago, common or rather common although unevenly distributed north to KiL and SoL, more scattered in Kn, Ks, northern KiL and SoL, and rare in southwestern EnL and InL Inari.

Habitat. In full sun or moderate shade in shallow water or at the water edge; indifferent to lime. Preferably on clay or mud at nutrient-rich lakes and rivers, ponds and pools, but also in quagmire bordering ± oligotrophic forest tarns, between stones in rapids of rivers, and (in the north) in eutrophic and herb-rich fens. Also in slightly brackish water: on shores of the Gulf of Bothnia e.g. at the outlet of streams and in lagoons and marshes with freshwater supply. Favoured by overgrowth, eutrophication and lowering of lakes (e.g., one of the few species which can penetrate into dense Glyceria maxima belts); increasing at least in some areas.

Biology. The rhizome contains the deadly poisonous polyacetylene cicutoxine. The fruits are water-spread; the large, air-filled cavities in the ridges of the mericarps enable them to float. The chambered rhizomes may also spread short distances.

Variation. Northern plants are sometimes smaller, have narrower leaf-lobes with fewer teeth, umbels with fewer umbellules and purplish flowers. They often grow in oligotrophic places and might be interpreted as modifications (starved plants). On the other hand, at its northernmost localities the northern morphotype mainly occurs in eutrophic mires, and plants from oligotrophic habitats in the south are not particularly small. The northern plants thus may represent genetically fixed ecotypes adapted to low nutrient availability. They are provisionally treated here as var. angustifolia, but cultivation experiments are required to establish whether they really deserve taxonomic recognition.

Similar taxa see Sium latifolium.

Distribution and habitat. In most habitats of the species. In the southern and central parts corresponding to the distribution of the species, northwards declining, but occurring all along the Bothnian coast both in S and F, in the inland reaching S Jmt and F Ks, SoL and KiL.

Plant 40–60 cm; stem and leaves usually partly purple. Leaf blades 9.5–12 × 4–6 cm, with 3–4 pairs of primary leaflets. Apical leaflet 26–51 × 10–24 mm (length/width ratio 1.4–3). Apical lobe 17–44 × 2–5 mm (length/width ratio 6.9–13), with 3–7 teeth on each side. Umbel with 8–17 umbellules. Bractlets 7–11, 2.5–5 × 0.4–0.7 mm. Flowers 20–35 per umbellule; petals and anthers usually purplish.

Distribution and habitat. Mainly in eutrophic birch and flark fens, mesotrophic flark fens, more rarely and further south on shores of peat-forming pools, lakes and rivers. – N He Engerdal, NT Frosta and Stjørdal, VFi Kautokeino (Guovdageaidnu). S the northern inlands below the mountains, from western Dlr to TL Karesuando and Nb Pajala. F in the northern parts, fairly common in KiL and SoL, infrequent south to KP Lappajärvi, PS Kiuruvesi and PK Polvijärvi.– An old specimen from S Ög Kättilstad is probably mislabelled.