Iris ensata Royal Banner - Japanese Water Iris

Iris ensata 'Royal Banner' wonderfully lives up to its name of "royal banner". The large velvety flowers of this Japanese water iris, with their deep purple colour, feature magnificent standards in the centre, shaped like golden tongues bordered with white.  In stark contrast to our garden irises, this rhizomatous perennial is at home in non-limestone and waterlogged soils in summer. Stunning when planted en masse in wet areas and along the banks of water bodies, Iris ensata were once intensively cultivated in Japan, particularly around Edo (the ancestor of Tokyo), for cut flowers. 

Iris ensata (synonym kaempferi), hanashōbu in Japanese, known more commonly as Japanese water iris, bear little resemblance to our classic garden irises, apart from their membership in the family Iridaceae. The original Iris ensata is a fibrous rhizome plant native to Asia, specifically to the wetlands of Japan, the Himalayas, and Siberia. These irises, known for being difficult to cultivate, thrive in acidic and peaty soils (they cannot tolerate excess limestone), and are thirsty for water during their growth and flowering period, but appreciate slightly less moist soils in winter: in nature, they are found above the water level, which often decreases in winter and rises again in spring with the melting of snow. 

The 'Royal Banner' cultivar is a horticultural achievement. The plant forms an upright, rather erect clump, reaching approximately 90cm (35in) in height when flowering and 50cm (20in) in width. This iris slowly spreads on the ground through its rhizomes, becoming more beautiful year after year. The deciduous foliage, of medium green colour, consists of long and slender ribbon-like leaves, sheathing at the base, marked by a prominent central vein. Flowering takes place in summer, usually in July-August, depending on the climate. Flower stalks emerge from the clusters of leaves. Each stalk bears 2 to 3 large enormous flowers. Each flower consists of 3 large, flat, slightly pendulous sepals, topped by 3 slightly smaller, also pendulous petals. In the corolla's centre, there are also 3 decorative petal-like styles.

Often cultivated along pond and basin banks, Iris ensata, like many marsh plants, can tolerate having their "feet" in water during summer, but their rhizomes do not tolerate being submerged during winter, especially if it is freezing. In our gardens, these Japanese irises will thrive in a loamy soil, enriched with compost, but not drying out in summer. For example, plant them in a heavy soil forming a slight depression, at the bottom of a slope, or on a bank where the soil becomes moist through capillarity. Combine Iris ensata 'Royal Banner' with Anemone rivularis, Darmera peltata, Lobelia 'Queen Victoria', Astilbes, Lysimachias and Ligularias , for example.