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. Wash the soil from the rhizomes and roots with a steady stream of water. Then cut the rhizomes apart with a sharp knife.