Watercresses (Nasturtium officinale, N. microphyllum; formerly Rorippa nasturtium-aquaticum, R. microphylla) are fast-growing, aquatic or semi-aquatic, perennial plants native from Europe to central Asia, and one of the oldest known leaf vegetables consumed by human beings. These plants are members of the Family Brassicaceae or cabbage family, botanically related to garden cress and mustard — all noteworthy for a peppery, tangy flavour.
The hollow stems of watercress are floating and the leaves are pinnately compound. Watercresses produce small white and green flowers in clusters.
Nasturtium nasturtium-aquaticum (nomenclaturally invalid) and Sisymbrium nasturtium-aquaticum L. are synonyms of N. officinale. Nasturtium officinale var microphyllum (Boenn. ex Reich.) Thellung is a synonym of N. microphyllum (ITIS, 2004). These species are also listed in some sources as belonging to the genus, Rorippa, although molecular evidence shows that the aquatic species with hollow stems are more closely related to Cardamine than Rorippa (Al-Shehbaz & Price, 1998). Watercresses are not related to the flowers in the genus Tropaeolum (Family Tropaeolaceae), popularly known as "nasturtiums".