Irises are perennial flowers that offer a huge range of colors and patterns, heights, and bloom times, with variations on a common theme of flower shape and plant form. The basic flower shape consists of three inner (often erect) petals, called standards, surrounded by three other petals (usually arching out and down), called falls. Leaves are almost always flat and long, resembling swords; they grow in rather open to quite dense upright or arching clumps from bulbs or creeping rhizomes. Some have fibrous roots.
By far the most popular group is the large collection of hybrids termed the “bearded” irises, named for the hairy caterpillar-like tuft creeping out of the center of each fall. Flowers range from barely 2 inches wide to 7-inch giants in what is probably the widest color range of any plant group, lacking only pure red. They bloom in early summer, from 2 inches to nearly 5 feet above stiff, swordlike leaves. A number of cultivars rebloom from late summer into fall, to double the show; these reblooming cultivars are worth seeking out. Zones 3–8.
In place of a beard, “beardless” irises flaunt a colorful spot, called a signal, or an intricate pattern of lines. Blooms on Iris sibirica, Siberian iris, rarely exceed 3 inches wide and are more open and graceful then most bearded irises; they occur in shades of white, red-violet, blue, and purple (plus a few rare pinks and yellows, such as the yellow-and-white ‘Butter and Sugar’) on upright, grassy clumps averaging 3 feet tall. They bloom toward the end of the bearded’s season. Zones 3–9. I. ensata (formerly I. kaempferi), Japanese irises, bear 4- to 10-inch, six-petalled flattish or more double flowers in shades of white, pinkish lavender, red-violet, blue, and violet, often exotically edged, lined, and speckled. Most grow to about 2½ to 3 feet and are broader-leaved and less dense than Siberians, blooming a few weeks later. Zones 4–9. Very early-spring-blooming, bulbous I. reticulata, reticulated irises, produce narrow-petalled, sweetly scented 3-inch blooms, mostly in blue and purple shades with orange or yellow signals, among sparse, four-sided leaves that may grow to 1½ feet after bloom. Zones 5–9.
How to grow
Most bearded irises are easy to grow, but they do have specialized needs. Plant and divide every 3 to 4 years in summer or early fall, splitting them into individual “fans” with the rhizome attached, or into divisions with a few fans. Trim leaves back before planting to make up for root loss. They grow best in full sun or very light shade and average to rich, well-drained soil. Barely cover the rhizome and point the leafy end in the direction you want it to grow, ideally out from the center of a group of three to five of a kind.
Bearded irises tolerate drought very well when dormant (usually beginning about six weeks after bloom), but water them well up to the time dormancy sets in and after division. Fertilize routinely in spring and early fall, keep weeds and other plants away from the rhizomes, mulch loosely the first winter after division, and be ready to stake the tall cultivars when they bloom.
Soft rot attacks during wet seasons in poorly drained soil, entering though wounds in the rhizome made from premature leaf removal or too-close cultivation; it can also be carried on the body of the iris borer. The eggs of this pest hatch in spring, producing 1- to 1½-inch-long, fat, pinkish larvae. The larvae enter a fan at the top and tunnel down toward the rhizome, where they may eventually eat the whole interior without being noticed.
In fall, remove dead, dry leaves, which often carry borer eggs, and destroy badly infested fans in spring. You can also crush borers in the leaves by pinching toward the base of the telltale ragged-edged leaves or by running your thumb between the leaves and squashing any borers you find. They are also vulnerable when you divide the clumps; check every rhizome for this pest. If you find a few borers, try cutting them out, but destroy badly infested rhizomes.
Siberian irises enjoy similar conditions to bearded irises, tolerating wetter soil and requiring less frequent division in spring or fall. Be certain to replant as soon as possible after dividing them. Rot and borers seldom plague them.
Grow Japanese irises in much the same way, providing shade from the hottest sun. Water well before and during bloom. They need acid soil and benefit from a few inches of mulch in summer.
Plant reticulated irises in fall about 3 inches deep and a few inches apart in average to more fertile, very well-drained soil. Grow with annuals and perennials to fill the gaps left by their leaves, which wither by summer.
Landscape uses: Smaller bearded irises are perfect in rock gardens and along paths and beds. For mid- to late-spring bloom, plant taller ones in a perennial border, or in a separate bed to provide optimum conditions. They also look splendid among garden ornaments and along patios. Siberian and Japanese irises are good choices for borders and wet sites, such as along a stream or the edge of a pond, although they prefer slightly drier conditions in winter. Reticulated irises look at home among rocks, naturalized in thin grass, or at the front of borders.