There are more than 300 types of iris, in many shapes, colors and forms. But whether your cultivar is a bearded, Siberian or Japanese iris, the best time to divide your iris is in the spring after the last frost date or in the summer. According to the Cooperative Extension Organization, to get the best display of flowers, divide your irises every three to four years. If they're still not blooming well, they may need more sun.
Cut the leaves of your iris with scissors to one-third its original height.
Use a sharp-edged shovel or spading fork to dig up all of the rhizomes (the bulky, horizontal stems partially buried below the leaves).
Wash the rhizomes gently and discard any rhizomes that are rotted or mushy.
Cut the rhizomes apart at the joints. Make sure that each rhizome has some roots and leaves attached. You can discard any smaller rhizomes without leaves.
Replant your new iris in a sunny, well-drained site. Place it at the same depth as the original plant.