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Planting and Growing Irises
(article reproduced from the AIS site)
In about 2 or 3 years, the new rhizomes will begin to crowd each other and you will want to divide the plant, cutting the newer parts of the rhizome free from the old, which may then be discarded. Unlike the other bearded irises, arils need to be transplanted annually.
You will have so many new rhizomes that you will share them with your friends. Perhaps you received your first rhizomes from a friend. When digging, keep all plants carefully labeled with their names, for sure identification. It is wise to keep diagrams of your planting area to double check individual labels on the plants.
This digging and separating is best done between one and two months after bloom season, usually in July or August. Soon after this the irises grow roots which help to hold the plant firmly during the winter in areas where freezing and thawing can result in heaving the rhizome out of the ground. If you live in this type of climate, a mulch of salt hay can be very beneficial.
Culture of the beardless irises differs somewhat from culture of the bearded irises. They should be transplanted in the fall or in early spring. The roots should never be allowed to dry out while they are out of the ground and they should be watered heavily after transplanting. They should be set slightly deeper than the tall bearded. Japanese iris should be planted in a distinct "depression" in heavy soil to assist in supplying moisture to the plant. Siberians and the Pacific Coast Natives can tolerate light shade but the Spurias, Japanese and Louisianas demand full sun. Louisianas and Japanese require moist conditions during the summer months while the Pacific Coast Natives enjoy a very low humidity and dry soil no matter how hot it may get.
All, except Louisianas, should be planted in a permanent spot where they can remain for many years as they resent being disturbed. Louisianas tend to "creep" and therefore, should be tended to every few years. All are heavy feeders and need to be fertilized regularly.