If irises are grown in the right environment and given proper care, it is a normal part of their life cycle to be divided and replanted. It is how you keep them from overcrowding. If irises aren't divided, it can actually affect whether or not they produce blooms. But a lot of gardeners struggle with the concept. They wonder when it becomes obvious that their irises need dividing, when is the best time to transplant, and how they should go about moving them.
There are several indicators that your irises need dividing. When they stop producing the number of blooms they usually produced and also when the rhizomes begin to protrude out of the ground. Because of overcrowding below, rhizomes will fight over living space, pushing and shoving until a couple become dislodged and are forced too close to the surface. It will be easy to identify the root system as it pushes itself out of the ground being that it looks like a mass of cooked spaghetti.
Do not transplant irises in the spring. The best time to transplant is during the summer, right after the plant has finished producing blooms. Early fall is acceptable as well.
When you are dividing, dig up the clump of rhizomes, pulling out as much of the root system from the ground as possible. Afterwards, remove as much of the dirt as you can so you can see the different rhizomes. You may then begin breaking the clump apart. Each rhizome should be divided into three or four pieces, with each piece having at least one fan of leaves attached to it. In the center, there will be rhizomes that have no leaves. These can be thrown away. Once you have your separate rhizomes, you can replant them the same way as when you originally planted them.