The iris is named after the Greek goddess of the rainbow, with good reason since its showy flowers are available in so many beautiful shades. Properly cared for, the iris is a dependable and long-lived perennial, and its sword-like foliage makes a good contrast to rounded garden plants. Irises grow from fleshy rhizomes, which spread horizontally just below the surface of the soil. All varieties bloom in spring and early summer, although some newer reblooming varieties will even put on a second show in the fall.
Probably the best known are the bearded iris, which feature fuzzy "beards" on the falls, or lower petals, of the flowers. Sizes range from the tiny early blooming miniatures, with 2 to 8 inch tall stems, to the stately tall bearded varieties, with stalks up to 28 inches. There are also beardless iris, including Siberian iris, Louisiana iris and Japanese iris, with more grass-like foliage.
All irises, but particularly the bearded varieties, prefer a location in full sun. Most do best if provided with well-drained soil (the exceptions are Japanese and Louisiana irises, which prefer wet soil). Plant container-grown irises in the spring or early summer to give it time to become established before winter weather. When planting, the top surface of the rhizome should barely be covered with soil. If the soil is heavy clay, plant the iris even higher, leaving the top half of the rhizome exposed above the soil. Firm the soil around the rhizomes and water in well.
Feed irises with a complete fertilizer once or twice during the growing season, being sure not to let the fertilizer touch the rhizomes. Do not fertilize after mid-August so that the iris hardens off before winter. It is better to underfeed than overfeed bearded iris, say Karen Russ and Bob Polomski, horticultural specialists with Clemson University. In general, iris are drought resistant, and too much water can encourage rot. Give newly planted irises an inch of water a week until they are established; water established plants only during periods of high heat and drought. Prevent seed production by removing the flowers as soon as they fade; do not remove the foliage, however, until it yellows in the fall. This allows the plant to store energy for next year's flower production. Newly planted irises are more vulnerable during winter freeze/thaw cycles; covering them with a light layer of straw in the fall will help prevent their heaving from the soil.
After three to five years of bloom, irises become crowded and require division. The best time to do this is in July or August. Lift the entire clump with a spading fork and hose off the soil. Discarding the center portion of the rhizome, cut the vigorous growth at the edges into individual plants (called "fans") using a sharp knife. Some gardening books recommend cutting the leaves down to 1/3 of their usual height to neaten the newly divided plant.
If irises are not flowering, check to be sure they are receiving enough sunlight and an appropriate amount of fertilizer. Poor flowering can also be due to rhizomes that are planted too deeply or that have become crowded. The most serious pest affecting iris is the iris borer. Plants can also be troubled by bacterial soft rot, which causes the rhizomes to become mushy, and iris leaf spot, a fungus which causes the leaves to brown in spots. Practice disease prevention by immediately removing any yellowing leaves or soft rhizomes. Remove all the foliage in the fall, since disease and insects can overwinter in the leaves.