Irises typically bloom exotic flowers in late spring and throughout the summer. They are hardy in USDA hardiness zone 3 to 10. Irises have root systems that resemble bulbs and are often called bulbs, but they are in fact rhizomes. Since the bulbs (rhizomes) self propagate, every few years they must be dug up, divided and replanted. You can also replant iris bulbs if you just want to change their location.
Prepare the new planting bed in full sun or partial shade. Rake or till the top foot of soil. Mix in several inches of humus such as peat moss or compost, which will help with water drainage.
Plan--if possible--to dig up your iris bulbs in the fall, just after the foliage turns yellow or brown. Cut off the foliage first and leave just a few inches above the ground so you can easily handle and locate the bulbs.
Dig up the iris bulbs. The tips are usually just beneath the soil's surface and the bulbs extend about 4 inches deeper. Using a trowel may suffice, but because the fleshy roots can be tough, you may need a shovel to cut through them. Dig a couple of inches around the stems of the plant to clear the bulbs and to get a couple of inches of fleshy roots.
Divide irises at this point, if necessary. You can separate the bulbs with a knife to cut them. You can see where the bulbs have naturally split (and where the foliage has grown in different directions). Each new bulb should be at least 3 inches long with some fleshy roots attached.
Replant the iris bulbs about 8 to 10 inches apart from one another. Plant the bulbs about 4 inches deep with the tips just under the top of the soil. Fan out the flesh roots.
Backfill the soil and firmly pack it down to remove all the air pockets. Water well and cover the area with a couple inches of mulch, such as wood chips or bark.
Continue to water the bulbs about an inch of water a week when rain is scarce. Water in this manner until a couple days before the first freeze. Then stop watering the iris bulbs until spring.