Irises grow from modified stems called rhizomes, and they need to be divided periodically. It is easy to learn how to divide iris, but first it helps to understand how they grow. The rhizomes lay horizontally on or just below the surface of the ground; roots grow downward and leaves grow upward from them. As the plant grows it forms and ever larger cluster. If it is not divided it will choke itself out.
Divide the Plant
To divide an established iris cluster, dig up the rhizomes and cut each in two or three pieces. Leave about six inches of leaf on each so you will be able to tell how to orient the rhizomes when you replant them. Leave one-third to one-half of the original plant in place. Allow the divided rhizomes to dry for a day or two, then dust them with fungicide.
Plant the Divided Rhizomes
Choose a good location for your new plants; iris do best in moist, well-drained soil rich in organic matter, and they prefer full sun. Prepare the soil of the new flower bed by adding compost or manure if necessary. To plant the divided and dried rhizomes, simply lay them on top of the prepared soil. Make sure that the leaf side is facing up; they won’t grow if they’re upside-down. If you live in a very hot climate, partly cover the rhizomes with soil. Point the leaves in the direction in which you want the plant to grow.