Rorippa Scop. (including Nasturtium R. Br.)
yellow cress, water cress
cosmopolitan, mostly through introductions
Species commonly cultivated
Rorippa nasturtium-aquaticum (L.) Hayek (Nasturtium officinale R. Br.) (Europe and northern Asia)
Rorippa aquatica (Eaton) E.J. Palmer & Steyerm. (Neobeckia aquatica (Eaton) Greene) (North America)
Rorippa are noxious weeds around the world and in some states in the United States. Most of these are not exclusively aquatic, but tolerate periodic inundation. Rorippa nasturtium-aquaticum is a common weed of irrigation canals and natural waterways but is rarely problematic.
Annual or perennial. Stems compact to erect and ascending, sometimes floating or creeping. Leaves in a basal rosette or cauline, alternate, simple or pinnate, often conspicuously emarginate and undulate. Inflorescence a simple or branched raceme. Flowers zygomorphic; sepals 4; petals 4, white, yellow or blue. Dispersal by seeds or vegetative fragments.
Some taxonomists consider the genus Rorippa to exclude Nasturtium. Here Rorippa includes Nasturtium, Neobeckia (a monotypic genus), and Armoracia aquatica (Eat.) Wieg. (=Rorippa aquatica). R. nasturtium-aquaticum (water cress) is commonly used as a vegetable in many countries. This has led to its wide-ranging introduction in many countries through deliberate plantings in natural waterways. Some authorities prefer the name Nasturtium officinale; both names are in equal usage in the literature.