Iris ensata (syn. Iris kaempferi) is one of the archetypal plants of the Japanese garden. They are rhizomatous, beardless irises, native to East Asia. They have fans of strap shaped leaves and flowering stems 2-4ft tall. They flower in early summer, with two or three large, exotic-looking flowers on a stem, in shades of purple, blue, white, and pink.
They are extremely hardy, down to -20ºC, zone 4 in the US. They will grow in full sun to partial shade, as long as they get at least 6 hours of sun, and like a loose, slightly acidic damp soil with plenty of organic matter. It is especially important in the spring, in the run up to their early summer flowering, that they have plentiful moisture.
They are sometimes sold as marginal plants, and are happy to be grown in shallow water, as long as the crown of the plant is not covered, during spring and summer. However, they need to be lifted out of the water over winter or they are likely to rot. The old foliage should be cut back to the ground in the winter, after it has been knocked back by the first frost.
Iris ensata are heavy feeders and will benefit from a generous feed of a balanced fertiliser for acid-loving plants in the early spring and again after flowering, and a good mulch to help retain moisture. It’s good to add plenty of organic matter to the soil when they are planted or divided, but don’t add fertiliser at this point. Bone meal should be avoided as it is actively harmful to Japanese irises.
It’s very important to divide and transplant Japanese Irises every three to four years to maintain vigour. New roots form above the old each year, so over time the crown of the plant rises above the soil, potentially causing it to fail. To divide, dig up the plant and divide it into individual sections or fans, or slightly larger sections of 2-4 fans each. The oldest roots and rhizomes – the lowest – should be removed, and the foliage cut back to about 1/3 of its length. The divisions should be planted 2-3 inches deep. in fresh soil where Iris ensata has not been grown for the previous three years, and kept well watered. It is best to divide Japanese Irises in spring or autumn, avoiding the heat of mid-summer, though in cooler areas they can be divided immediately after flowering.
Iris ensata have been cultivated in Japan for at least five hundred years, and possibly much longer. Extensive Iris breeding began in the late Edo period, around the 1800s. Japanese Irises were introduced to the West by Carl Thunberg in 1794, and again by von Siebold in the 1850s (as Iris kaempferi). Many varieties have been bred in the United States as well. Some 2000 varieties are now available.
Japonica Plants Nursery 