Watercress likes to grow with its feet in the edge of a lazy stream. This colony grew in a little tributary of the Pine Creek in Wexford, where it was blooming profusely in late October. The main blooming season is in the spring, but the cool weather of fall seems to give the plants their second wind.
Gray lists the plant as Radicula nasturtium-aquaticum:
RADICULA [Dill.] Hill. WATER CRESS
Pod a short silique or a silicle, varying from slender to globular, terete or nearly so; valves strongly convex, nerveless. Seeds usually numerous, small, turgid, marginless, in 2 irregular rows in each cell (except in R. sylvestris). Cotyledons accumbent. Aquatic or marsh plants, with yellow or white flowers, and commonly pinnate or pinnatifid leaves, usually glabrous. (Name meaning a little radish.) RORIPA Scop. NASTURTIUM R. Br.
1. Petals white, twice the length of the calyx, pods linear; leaves pinnate.
R. NASTURTIUM-AQUATICUM (L.) Britten & Rendle. (TRUE W.) Perennial; stems spreading and rooting; leaflets 3-11, roundish or oblong, nearly entire; pods (1.2-1.6 cm. long) ascending on slender widely spreading pedicels. (Sisymbrium L.; Nasturtium officinale R. Br.; Roripa Nasturtium Rusby.)—Brooks, ditches, etc., originally cultivated. (Nat. from Eu.)