When and how do you divide bearded irises?
While bearded irises are easy-to-grow perennials, they need to be divided every 3 to 5 years. If not divided, plants become overcrowded and flower production decreases. Crowded plants are also more prone to disease problems. In Iowa, July or August is the best time to dig, divide, and transplant bearded irises.
Bearded irises grow from thick, underground stems called rhizomes. Carefully dig up the iris clumps with a spade. Cut back the leaves to 1/3 their original height. Wash the soil from the rhizomes and roots with a steady 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 all diseased or insect damaged rhizomes.
Bearded irises perform best in fertile, well-drained soils and full sun. In clay soils, incorporate compost, peat, or well-rotted manure into the soil prior to planting. 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 at least 3 rhizomes of one variety in a group. Space the rhizomes 12 to 24 inches apart.