Iris reproduce by rhizomes or bulbs
image by kconnors/morguefile
Depending on the species, irises reproduce from either bulbs or rhizomes--thick fibrous roots that grow along the surface of the ground. Rhizomes send out roots along the sides and bottom of the thickened center, while bulbs look more like onions, surrounded by a papery sheath. Dutch irises store food and energy in bulbs that send out roots from the bottom. Bearded iris and wetland species, like blue flag, produce rhizomes. Rhizomes and bulbs require the same basic care.
Dig out the rhizomes or bulbs in the fall once the foliage has died back on iris plants. Iris will rebloom in the same location if left in the soil, but always check the hardiness zone of those you are growing. Lift the rhizomes or bulbs with a garden fork and shake to remove any excess soil.
Layer the bulbs on a blanket or old newspaper in a shaded area and allow bulbs to cure outside for 2 to 3 days. If rain threatens, move bulbs inside to a cool well-ventilated area.
Inspect the bulbs for any signs of insect damage or disease and discard any damaged bulbs. Brush off any remaining soil.
Store bulbs in a box or bag filled with peat moss. Mist lightly, but avoid soggy peat moss. A little moisture prevents bulbs from shriveling, but too much encourages mold and disease. Place the box in a dry cool area that maintains temperatures between 40 and 50 degree F.
Check the health of bulbs every week or so to prevent disease. Look for signs of mold or shriveled bulbs. If bulbs shrivel, mist the peat moss lightly. Discard moldy bulbs and leave the container open to allow more air circulation if the soil or bulbs feel too moist.
Plant the bulbs again in the spring following the recommendations for your specific species and hardiness zone.