- Name also: Great Water Dock, Giant Water Dock
- Family: Dock Family – Polygonaceae
- Growing form: Perennial herb.
- Height: 100–200(–250) cm (40–80(–100) in.). Stem green when young, later reddish, grooved, branching from inflorescence, inflorescence branches ascending, branching angle wide.
- Flower: Regular (actinomorphic), Approx. 5 mm (0.2 in.) across, bisexual. Perianth-segments in two whorls of three. Segments of outer whorl small and spreading–obliquely forward-pointing. Inner whorl forms fruit valves, which are widely ovately triangular. Stamens 6. Pistil of 3 fused carpels, styles 3. Joint on flower-stalk (5–10 mm (0.2–0.4 in.)) near base. Flowers whorled. Inflorescence a compound raceme.
- Leaves: Spirally. Lower leaves long-stalked, glabrous. Leaf blade lanceolate–ovate, sharp-tipped, 30–80 cm (12–32 in.), almost waveless. Upper leaves several.
- Fruit: Achene 3–4 mm (0.12–0.16 in.), brownish. Perianth segment lobes roundly triangular–ovate, 5–7 mm (0.2–0.3 in.) long, often toothed from base, long reddish tubercle. Joint on flower-stalk (5–10 mm (0.2–0.4 in.) near base, inconspicuous.
- Habitat: Rich shores by lakes, rivers and the sea. Usually by water line or in shallow water.
- Flowering time: July–September.
Genus Rumex is comprised of some 200 species wordwide, of which approx. 20 grow in Finland. They are quite a troublesome group because members easily cross-breed – water dock especially with Scottish dock (R. aquaticus) and broad-leaved dock (R. obtusifolius). The calyx of the ripe fruit is helpful (and often necessary) to identify the plant.
Water dock is easiest to find in the south of Finland and is the largest and longest-leaved of the genus Rumex plants that grow in the country. Its basal leaves are usually over half a metre (20 in.) long. Water dock looks quite similar to Scottish dock (R. aquaticus), which grows across almost all of Finland. The species can be differentiated from each other according to their different sizes and also the fruit calyx. All the leaves on water dock’s fruit calyx have a taper-tipped tubercle. Scottish dock can occasionally reach two metres in height, and its fruit has no tubercles. Additionally, the base of Scottish dock’s leaves are wider, and sometimes cordate-based. Water dock’s leaves always have a tapering base (if there has been no cross-breeding).