Bearded irises are one of the most popular and widely grown perennials in the home landscape. While bearded irises are beautiful, they do require high levels of maintenance. One important chore is to divide bearded irises 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.
Bearded 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 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, cover the plants with several inches of straw or pine needles 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.