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 irises is in July and August.
Irises grow from thick, 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 hard 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 irises are fertile, well-drained soils and full sun. In poor soils, incorporate well-rotted manure, peat, or compost into the soil prior to planting.
When planting 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 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 28, 1996 issue, p. 110.