Cardamine pratensis (Cuckoo Flower or Lady's Smock), is a flowering plant in the family Brassicaceae, native throughout most of Europe and western Asia.
It is a herbaceous perennial plant growing to 40-60 cm tall, with pinnate leaves 5-12 cm long with 3-15 leaflets, each leaflet about 1 cm long. The flowers are produced on a spike 10-30 cm long, each flower 1-2 cm diameter with four pale pink (rarely white) petals. It grows best close to water.
Cardamine pratensis is grown as an ornamental plant in gardens, and has as a result of cultivation become naturalised in North America. In some European countries the plant is now under threat ( e.g. parts of Germany). It is a food plant for the orange tip butterfly (Anthocharis cardamines) and makes a valuable addition to any garden which aims at attracting wildlife. It was once used as a substitute for Watercress.
In folklore it was said to be sacred to the fairies, and so was unlucky if brought indoors. It was not included in May day garlands for the same reason.
Description from Wikipedia: http://en.wikipedia.org/wiki/Cardamine_pratensis