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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 Irises

This article was published originally on 6/15/2001

Bearded irises are one of the most popular and widely grown perennials in the home landscape. The colorful, orchid-like flowers of bearded irises are composed of 6 segments. The 3 inner segments (petals), which are generally upright, are referred to as standards. The drooping, outer 3 segments are known as falls. The common name comes from the fuzzy growth or "beard" which runs down the center of each fall. Bearded irises are available in a wide range of colors, including white, yellow, orange, pink, wine-red, blue, and purple. The flowers of many varieties possess various combinations or blends of these colors. Bearded irises are commonly classified into dwarf bearded (plants are 4 to 10 inches tall), standard dwarf bearded (10 to 15 inches tall), intermediate bearded (15 to 28 inches tall), and tall bearded (over 28 inches tall). Bearded irises bloom in the spring. The dwarf bearded forms are the first to bloom, usually mid-April to early May. The tall bearded varieties are the last of the bearded irises to bloom, typically mid-May to mid-June. A few cultivars actually bloom again in late summer or early fall. Leaves are strap-like and grow in fan-shaped clumps. Gardeners can choose from several thousand named varieties.

While bearded irises are easy-to-grow, long-lived perennials, they need to be divided 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 in July and August.

Irises grow from thick, fleshy, underground stems called rhizomes. Carefully dig up the iris clumps with a spade. Cut the leaves back to 1/3 their original height. Wash the soil from the rhizomes with a stream of water. Then cut the rhizomes apart with a sharp knife. Each division should have a fan of leaves, a healthy rhizome, and several large roots. Discard the old, leafless rhizomes in the center of each clump. Also, discard all diseased and borer damaged rhizomes.

Ideal planting sites for bearded irises are fertile, well-drained soils and full sun. While they tolerate light shade, maximum flower production occurs in full sun. Bacterial soft rot is often a problem in wet, poorly drained sites. Wet, poorly drained sites can often be improved by incorporating organic matter, such as peat or compost, into the soil prior to planting. Raised beds are another option for gardeners with poorly drained soils.

When planting 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. Finally, water each plant thoroughly.

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, 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.



This article originally appeared in the June 15, 2001 issue, p. 72.

Year of Publication: 
2001
Issue: 
IC-485(14) -- June 15, 2001