In last week’s “Gardening Corner,” I wrote about tall, bearded iris, which some gardeners plant as late spring to early summer focal points. There is no denying the beauty and grace of these ruffled popstars, but intermediate bearded cultivars and dwarf varieties can also offer color and charm to small garden beds and containers.
If you have plenty of bearded irises, consider Siberian (Iris siberica) or Japanese (Iris ensata) irises. Both are beardless species that will add variety in size, color, and bloom time to your beds and borders. Another member of the Siberian iris group, Iris sanguinea, native to Russia, Korea, and Japan, is sometimes called Siberian iris. Iris japonica, native to woodlands in Japan and China, is also known as Japanese iris.
Siberian iris (I. siberica) is native to Central Europe, Ukraine, and Central Asia. In North America, Siberian iris grows in USDA Hardiness Zones 3-9. Unlike sun-loving bearded iris, Siberian iris requires light to moderate shade during hot summers. This species needs moist, organically rich, well-drained soil with an acidic to neutral pH of 6.0 to 7.0, and one inch of water each week during the growing season. A light application of organic mulch will conserve soil moisture and keep the roots cool during the hottest days of summer.
Plant Siberian iris rhizome crowns about one inch deep on a small mound of soil; spread the roots; and cover with two inches of soil. Water weekly for 6 to 8 weeks. The slender, grass-like blue-green leaves grow to 3 to 4 feet in height, forming a large clump. In May and June, two to five flowers appear on each stalk, mostly in shades of lavender, blue, purple, and white with erect standards and drooping falls. Some cultivars are pink, wine, or yellow.
After the flowers have faded, cut back the spent stalks. The leaves will remain upright and attractive into the fall. In the following spring, cut the leaves back to one inch above the ground. Lift and separate clumps every few years, or when flowering decreases.
Siberian iris tolerates deer, rabbits, drought, and clay or wet soils. It is less susceptible to the iris borer and bacterial rot than bearded iris.
Japanese iris (I. ensata), also called Japanese water iris, is a spectacular addition to any perennial garden. This iris blooms from mid-June through early August, later than other iris species. The standards, tall and upright in bearded and Siberian irises, are small, and the falls are wide, draping down to give the large flower an open appearance. Flowers can be single or double in shades of white, pink, lavender, red-violet, or blue with a variety of speckling and marbling effects on the petals. The green leaves are straight and sword-shaped, reaching 2 feet in height. The long flower stems can increase the plant height to 3 to 4 feet.
The Japanese native thrives in USDA Zones 4 -9. It requires heavy, moist soil that is rich in organic material, especially peat or manure, with a mildly acidic pH of 5.5 to 6.5. Sufficient moisture is a key ingredient for success, and placement by a pond or stream edge is ideal, although you must take care not to let the roots stand in water, especially during the winter months.
Plant the finger-shaped rhizomes upright in 2 to 4 inches of soil, after soaking them in water overnight. Water the rhizomes daily for several weeks, then offer 2 inches of water weekly during the growing season. Mulch with pine or oat straw.
Japanese iris tolerates deer and wet soil. Thrips and iris borers can infest the plants.
For information on all species of irises, go to the American Iris Society website. From there, you can access detailed information on Siberian and Japanese irises. The Missouri Botanical Garden Plant Finder offers entries on numerous species of irises. The Penn State Extension article “Iris Diseases” provides information on symptoms of iris diseases and how to manage them.