During the winter, protecting your iris rhizomes from the cold ensures that the plant will bloom in the spring. For those who live in warmer climates where the soil seldom freezes, Greg Stack from the University of Illinois recommends leaving the rhizomes in the ground, covered with a layer of mulch. Other options for winter storage of iris rhizomes exist for those who have already dug up their irises or for those who live in areas where the soil freezes.
Rinse off any dirt from the rhizomes and pat dry. Dust with powdered sulfur to protect against rot and insects, as recommended by North Dakota State University.
Use a pencil to poke several holes in a plastic bag. Alternatively, fill an open box with peat.
Place the iris rhizomes into the plastic bag or bury them in the peat and keep the container in the refrigerator until after the last frost before replanting.