Rumex hydrolapathum Huds. (great water dock; water dock; giant water dock) is a species of perennial herbaceous plants in the genus Rumex native to fens and freshwater banks of Europe and Western Asia. It is the tallest species in the genus, with flowering stems attaining a height of up to 2 m (6 ft 7 in). It is one of the small number of decaploid organisms, containing two hundred individual chromosomes.