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Horticulture and Home Pest News
Horticulture & Home Pest News is filled with articles on current horticulture, plant care, pest management, and common household pests written by Iowa State University Extension specialists in the Departments of Entomology, Horticulture and Plant Pathology.

Dividing Bearded Iris

This article was published originally on 7/1/2009

Bearded irises are very popular perennials found in many Midwest landscapes. The flower of the bearded iris consists of 3 inner segments (petals) known as standards. These segments are usually upright and make up the top part of the bloom. There are three outer segments (petals) that droop downward. These are appropriately named falls. The common name is derived from the fuzzy beard-like growth found on the iris falls. Bearded irises have one of the widest ranges of flower colors in the plant world. It would be hard not to have at least a few favorites due to the rainbow of color choices and combinations.
Bearded irises are colorful, long-lived and fairly easy to grow. Moderate levels of maintenance are required to achieve healthy attractive plants. One maintenance aspect is division, which should be done every 3 to 5 years. If not divided, the plants become overcrowded and flower production decreases. Crowded plants are also more prone to disease problems. The best time to dig, divide and transplant bearded irises is mid-summer (July and August). The storage organ of the iris is a rhizome. Carefully dig up the iris rhizome clumps with a spade. Cut the leaves back to 1/3 their original height. Wash the soil from the rhizomes with water. Next, cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome, and several large roots. Discard old, leafless center rhizomes and those that are soft or rotting.
Bearded irises perform best in fertile, well-drained soils in full sun. While they tolerate light shade, plants won't bloom as well. Bacterial soft rot is often a problem in wet, poorly drained sites. Wet, poorly drained sites can often be improved by initially incorporating organic matter, such as peat or compost, into the soil or planting the rhizomes in raised beds for better drainage.
When planting or transplanting bearded irises, dig a hole large enough to accommodate the rhizome and roots. Build a mound in the center of the hole. Place a rhizome on top of the mound and spread the roots in the surrounding trench. Then cover with soil. When planted, the rhizome should be just below the soil surface. The chances of rotting increases the deeper the rhizomes are planted in the soil profile. Water each plant thoroughly after planting.
To obtain a good flower display, plant 3 or more rhizomes of one variety in a group. Space the rhizomes about 12 to 24 inches apart. Point each fan of leaves away from the other irises in the group. Newly planted bearded irises are susceptible to injury their first winter. Repeated freezing and thawing of the soil during the winter months may heave the rhizomes out of the soil and damage or destroy them. To prevent damage to the newly planted rhizomes, apply a light layer of straw in late fall. Remove the mulch in early spring. The transplanted irises will bloom sparsely the first spring. The plants should be in full bloom in their second and third years.