Iris pseudacorus


 * This article is about the yellow flag iris. For the penalty flags in American football, see American football.  For the yellow flag used in auto racing, see Racing flags.

Iris 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.

Large iris stands in western Scotland form a very important feeding and breeding habitat for the endangered Corn Crake.

I. pseudacorus is one of two Iris species native to Britain, the other being Stinking Iris (Iris foetidissima).

Cultivation and uses
The rhizome has historically been used as a herbal remedy, most often as an emetic. When applied to the skin or inhaled, the tannin-rich juices can be acrid and irritating.

It has been planted nearly worldwide as an ornamental plant, with several cultivars selected for bog garden planting.

In some regions it has escaped from cultivation to establish itself as an invasive aquatic plant which can create dense, monotypic stands that outcompete other plants in the ecosystem. Where it is invasive, it is tough to remove on a large scale. Even ploughing the rhizomes is often ineffective, and its eradication is highly unlikely. It has been banned in some areas but is still widely sold in others for use in gardens, and it will continue to be planted by gardeners unaware of or unconcerned with its invasive potential.