Yellow Iris

ris pseudacorus is a species of Iris, native to Europe, western Asia and northwest Africa. Common names include yellow iris and yellow flag. Its specific epithet, meaning "false acorus," refers to the similarity of its leaves to those of Acorus calamus, as they have a prominently veined mid-rib and sword-like shape.
It is a herbaceous perennial plant growing to 1-1.5 m (or a rare 2 m) tall, with erect leaves up to 90 cm long and 3 cm broad. The flowers are bright yellow, 7-10 cm across, with the typical iris form. The fruit is a dry capsule 4-7 cm long, containing numerous pale brown seeds.

Iris pseudacorus grows best in very wet conditions, and is often common in wetlands, where it tolerates submersion, low pH, and anoxic soils. The plant spreads quickly, by both rhizome and water-dispersed seed. It fills a similar niche to that of Typha and often grows with it, though usually in less deep water. While it is primarily an aquatic plant, the rhizomes can survive prolonged dry conditions. Yellow iris has been used as a form of water treatment since it has the ability to take up heavy metals through its roots. (Wiki)