Nuphar lutea (L.) Sm.
Flower width 4-6 cm.
An aquatic plant which grows from thick, woody rhizomes anchored in the mud underwater. It has very large floating green leaves which can cover a fairly large body of water. The small, solitary flowers are bright yellow and fairly small. The fruit is green and shaped like a flask.
It flowers June to August, and is found in freshwater lakes, ponds, canals, and slow-moving streams and rivers. Common and widespread throughout Britain except the far north.