Bearded iris are known for their easy care, and they can fill a bed in only a few years. I suspect that yours are getting crowded. Bearded iris need to be dug and divided every 3-5 years to keep them growing well and not competing against one another for space, nutrients, and water. Divide them in late summer or early fall, following these steps. Carefully dig the clumps with a garden fork or spade, taking care not to chop into the rhizomes (the creeping, horizontal stems that look like roots). Divide the rhizomes by cutting them apart with a sharp knife. Slice off younger rhizomes that are growing off the largest, main rhizome. You will replant the babies and any large rhizomes that have buds or new shoots coming out of them.
Large, old rhizomes that have no buds can be tossed out, because they’re unlikely to rebloom. Wash the soil off the rhizomes so you can inspect each one for iris borer (a fat pinkish worm) or rot. If you find a borer, destroy it. Soft, smelly, or rotting plants should also be destroyed. Some gardeners like to wash their iris rhizomes in a 10 percent bleach solution to destroy disease or-ganisms, but that won’t help plants that are already rotting. Clip off the leaf blades so they’re 6-8 inches long. This reduces the stress that the plant goes through as it concentrates on growing new roots instead of trying to maintain long leaves. Replant the divisions, setting the rhizomes higher in the planting hole than the roots. A bit of the top surface of each rhizome should be just visible at the soil surface. Space the plants 12-18 inches apart. Water them well at planting, but do not continue to water unless the soil becomes dry. If some of the rhizomes were rotted, avoid replanting in the same area as they were growing.