Bearded iris grow a little differently than most perennials -- they have rhizomes, or fleshy stems that grow along the ground. The best time to divide bearded iris is summer, when the plants are resting.
You should see each rhizome has roots growing out of the bottom and fans of leaves growing out the ends.
If your iris clumps are made of many rhizomes that are growing closely together, you can simply plant them farther apart.
But if you want to make even more, or you have really old plants, you can also divide the rhizomes themselves. To do this, break or cut the rhizome into pieces. Each piece should have at least one fan of leaves and roots growing out of the bottom.
If you wish, you can trim the foliage to keep it looking neat and tidy. Cut the fans back to about 3 inches tall.
It's also helpful to treat your iris rhizomes with a fungicide to stop mold or other diseases from attacking.
Continued on page 4: How Often to Divide Perennials