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.)

A popular bedding plant that liberally seeds itself. When seeds get washed downhill, they lodge in sidewalk cracks, where they’re quite happy to grow and bloom all summer and well into fall, producing more seeds to lodge in sidewalk cracks, and making the garden Alyssum one of our more common urban weeds. This plant grew where the sidewalk met a stone retaining wall in Beechview.

From Gray’s Manual:

LOBULARIA Desv. SWEET ALYSSUM

Pod small, orbicular, with only one or two wingless seeds in a cell; valves nerveless, somewhat convex, the margin flattened. Petals white, entire. Cotyledons accumbent. Hairs of the stem and leaves 2-pointed, appressed, attached in the middle. (Latin lobulus, a little lobe, probably referring to the 2-lobed hairs. )

1. L. MARITIMA (L.) Desv. Slightly hoary; leaves linear; flowers small, honey-scented. (Alyssum Lam.; Koniga R. Br.) Often cultivated, and occasionally spontaneous. (Introd. from Eu.)