Irises grow in USDA hardiness zones 3 to 10 and bloom large flowers in the spring and throughout the summer. Irises have roots structures called rhizomes, which multiply quickly. In several years, most iris beds become overcrowded, at which time thinning and replanting in a new location (or give them away) becomes necessary for the plants to continue to thrive.
Dig up iris plants. While you can do this at anytime, it is best to wait until after they are done blooming. The rhizomes are usually located just below the surface. Carefully use a shovel to dig a circle about 4-6 inches deep around the plant and then lift up to pull the plant and rhizomes out. Breaking some of the stringy roots is OK and expected.
Plant the irises in their new location, which should be in full sun and have well draining soil, so if needed add a little bit of organic matter such as compost. Dig a hole that is just deep enough for the rhizomes. The tip should be right at the soil's surface, just as before.
Backfill the hole with soil and tamp it down with your hands to get rid of any possible air pockets. Water the area well and continue to water them two or three times a week until cold weather arrives, at which time--if you haven't already--add several inches of mulch to help keep the rhizomes warm.