Bearded irises are among the easiest and hardiest perennials to grow, blooming year after year with little attention. Named after Iris, the Greek goddess of the rainbow, they are available in almost all colors except true red. A whole range of colors, sizes and forms are available that can give you a succession of blooms from mid-April to early July.
Iris have thick, fleshy, underground stems (called rhizomes) that store food produced by the sword-shaped, semi-evergreen leaves. The rhizomes grow best when planted at or slightly below the soil surface with feeder roots penetrating the soil below. Each year underground offsets develop from the original rhizome. Buds produce a large fan of leaves and several flower stalks. Success with iris depends on keeping the rhizomes firm and healthy. In general, this is done by providing the rhizome good drainage while the feeder roots below remain moist but not wet.
Site Selection and Preparation
A full sun exposure is preferred; however, some of the delicate pink and blue iris hold their color better in partial shade. Excessive shade will reduce or prevent flowering. Good soil drainage is essential to prevent rhizomes from rotting. It may be necessary to plant the rhizomes in raised beds, at least 6 inches high, to obtain proper drainage.
Iris will grow in many soil types but a light, loamy soil with a pH of 6 to 7 that has been amended with organic matter is preferred. A tight clay soil may keep the rhizome too wet and should have organic matter (pine bark, compost) incorporated to improve drainage. Manure is not usually recommended for iris but can be used if well-rotted and incorporated at least 6 inches deep into the bed and should not come in contact with rhizomes.
Fertilization of iris is important to obtain best results, but must be done in moderation. Nitrogen, potash, and phosphorus are essential for iris, but excessive nitrogen promotes lush growth that is more susceptible to rot diseases. At planting, incorporate a low-nitrogen fertilizer such as 5-10-10 into the soil. Taking and following the results of a soil test is the preferred method to determine fertilizer amounts.
The best time to plant bearded iris is July through September. This will allow them to become well established before winter. Container-grown iris can be planted in the spring. In a well-prepared bed, dig a shallow hole large enough to accommodate the rhizome or clump of rhizomes. Form a mound of soil in the center for the planting base. Make the mound high enough so the top of the rhizome is slightly above soil level. Spread the roots around the mound, fill with soil, and water. For a mass of color, plant at least three rhizomes (spaced 8 to 10 inches apart) or plant undivided clumps; point each fan of leaves away from the center of the group. Clumps should be spaced 18 inches apart. Mulch should be applied to fall-planted iris to reduce heaving during the winter.
Care and Maintenance
Before flowering, water plants often enough to keep the soil moist but not wet. Reblooming iris should be watered during the summer, while spring-flowering iris will tolerate drought. After flowers fade, cut flower stalks back to an inch or two above the rhizome to prevent seed formation. Plants that are growing well (good green foliage) may not need fertilizing. If you fertilize, apply ½ cup of 5-10-10 fertilizer per iris clump after flowering. Fertilizer can burn the rhizomes; it should be applied around but not directly on them. Reblooming iris should be fertilized in the spring as new growth begins and after spring flowering ends. Iris respond to shallow (1 to 2 inches) cultivation and should not be mulched. In early fall, cut leaves 6 to 8 inches from the ground, especially if foliage disease occur.
After 3 to 5 years, iris generally become crowded and should be divided. Iris can be divided any time, but many growers prefer to divide 4 to 6weeks after the flowering period. Cut the leaves to one-third their length. Dig the clump and wash soil off with a hose. Cut rhizomes apart so that each section has at least one healthy fan of leaves and firm, white roots. Older rhizomes may seem firm but should be discarded since they have limited flowering capacity.
Common Bearded Iris Problems
Poor flowering is normally due to planting in excessive shade, using excessive nitrogen fertilizer, or planting the rhizomes too deep. Limited flowering may also occur if plants become too crowded and need dividing.
Bacterial soft rot is the most serious iris disease. Bacteria enter through injuries or cuts to the rhizome. Soft rot causes the rhizomes to become mushy and have a disagreeable odor. Use of fresh manure or excess nitrogen, coupled with poor drainage, contribute to soft rot development. Dig up and destroy diseased rhizomes. If the rot is not extensive, cut off and destroy diseased plant parts.
Crown rot fungus causes a rot at the base of leaves where they join the rhizome and causes them to fall over. It is identified by reddish-brown “mustard seeds” which are produced by the fungus. Trim leaves to admit more sunlight and air movement to the rhizomes; carefully remove and destroy all diseased leaves.
Leaf spots After flowering, leaves may become dotted with small, brown spots. Bacterial leaf spot has a watery, streaked appearance. Water-soaked margins around the spot turn yellow. Fungal leaf spots are rust-colored, drier, and more confined. Since disease organisms overwinter on old foliage, cut and destroy leaves of infected plants in the fall. Spray with a registered fungicide during extended periods of high humidity or rainy seasons.
Mosaic is a viral disease that causes a mottling of leaves and flowers. It is transmitted by aphids. Remove and destroy infected plants and control aphids.
Iris borer The first symptoms of iris borers are small notches on the leaf edge or small accumulation of sawdust grass in early spring. Iris later develop loose, rotted bases and holes in rhizomes. Bacterial soft rot readily attacks borer-infested plants. Carefully remove and destroy old leaves, stems, and plant debris in the fall. A registered insecticide can be applied to the rhizomes in the spring as new growth occurs.