Iris robusta Dark Aura

In stark contrast to our garden irises, Iris x robusta 'Dark Aura' is a stately plant that thrives in wet soil and prefers heavy, moisture-retaining soil in summer. It is ornamental for two reasons: firstly, its spring foliage is a deep purple, gradually turning green-grey with a purple base, and secondly, its summer flowering consists of elegantly shaped flowers, with bright blue-violet petals and a white and yellow throat, carried on tall, dark purple stems. This extremely hardy rhizomatous plant eventually forms striking clumps that are simply stunning in borders or near water features, where they reflect a perfect elegance on calm water. 

The Iris x robusta is a result of crossing I. versicolor and I. virginica, both hardy and native to wet and marshy areas in the eastern United States. They have little in common with our classical garden irises, except for their belonging to the Iridaceae family. 

The 'Dark Aura' cultivar has received the Royal Horticultural Society's Award of Garden Merit, which is given to particularly ornamental and easy-to-grow plants. The plant forms an erect, bushy clump, reaching about 90cm (35in) in height when flowering and spreading 30 to 40cm (12 to 16in) wide. It slowly spreads on the ground through its rhizomes, becoming more beautiful year after year. The deciduous foliage emerges in spring with a strong red-purple hue, gradually turning green, while the base of the leaves retains this decorative purple tint. The foliage consists of long, sheathing, ribbon-like leaves with a prominent central vein. The flowering occurs between June and July, depending on the climate. Tall dark purple flower stems emerge from the leaf clusters. Each stem carries 4 to 5 flowers, 12cm (5in) in diameter, which open one after the other, from top to bottom. They have 3 large, slightly drooping, spatulate sepals in a vibrant blue-violet colour veined with dark blue, and are white at the base with a bright yellow signal. These 3 sepals are topped with 3 smaller, upright petals in the same blue-violet colour, but thinner and more delicate. 

Often grown on the banks of ponds and basins, Iris x robusta, like many marsh plants, can live with their "feet" in water throughout the year. In our gardens, these irises will also thrive in well-drained loamy soil mixed with compost, preventing them from drying out in summer. For example, plant them in heavy soil forming a slight depression, at the bottom of a bank, or on an embankment where the soil becomes moist through capillary action. For a beautiful combination, plant 'Dark Aura' with Anemone rivularis, Ligularia dentata 'Desdemona', Darmera peltata, Lobelia 'Queen Victoria', astilbes, Physostegia virginiana, lysimachias, and pulmonarias.