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 foliar diseases. The best time to dig, divide, and replant 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 forceful stream of water from the garden hose. 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 insect damaged rhizomes.
Bearded irises perform best in fertile, well-drained soils and full sun. The planting site should receive at least six hours of direct sun per day. Plants that don't receive sufficient sunlight will not bloom well. 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 sphagnum peat moss 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.
Divided irises typically bloom sparsely the following spring. However, plants should be in full bloom by their second year.