Yellow Iris

This herbaceous flowering perennial plant grows to 100–150 cm (39–59 in), or a rare 2 m (6 ft 7 in) tall, with erect leaves up to 90 cm (35 in) long and 3 cm (1.2 in) broad. The flowers are bright yellow, 7–10 cm (2.8–3.9 in) across, with the typical iris form. The fruit is a dry capsule 4–7 cm (1.6–2.8 in) long, containing numerous pale brown seeds.

I. pseudacorus grows best in very wet conditions, and is 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 shallower water. While it is primarily an aquatic or marginal plant, the rhizomes can survive prolonged dry conditions. Large I. pseudacorus stands in western Scotland form a very important feeding and breeding habitat for the endangered corncrake. I. pseudacorus is one of two iris species native to the United Kingdom, the other being Iris foetidissima (stinking iris).

The plant's roots and leaves are toxic. This plant has been used as a form of water treatment since it can take up macronutrients (such as nitrogen and phosphorus) through its roots, and is featured in many AS Level Biology practicals as its ability to grow in low pH levels makes it a useful indicator. It can also withstand high salinity levels in the water.

It is widely planted in temperate regions as an ornamental plant, with several cultivars selected for bog garden planting. The following cultivars have gained the Royal Horticultural Society's Award of Garden Merit:

'Roy Davidson' 'Variegata' (it has leaves that are edged with deep white stripes) Other cultivars known include Alba (with pale cream flowers) and Golden Fleece (with dark yellow flowers).