It is the tallest species in the genus, with flowering stems attaining a height of up to 2 m (6 ft 7 in).

Great water dock is a tall perennial plant reaching a height of 2 m (6 ft 7 in).

During its first year it has a rosette of long-stalked, hairless leaves with lanceolate leaf blades up to 80 cm (31 in) in length.

These leaves are large and taper at both ends, and their lateral veins are at right angles to the midrib.

In subsequent years it produces an upright flowering stem, unbranched until the base of the inflorescence.