Answer: Traditionally, bearded iris are divided in early to mid summer and replanted immediately. I have also dug and divided them in early spring but that timing will usually cost a season of blooms. A fall planting can also be done, but it greatly increases the risk of frost heaving during the winter because the plants have not had enough time to root themselves thoroughly.
It is very faintly possible that bare-rooted iris would hold until next spring if you can store them cool (but well above freezing) and dry but not so dry that they shrivel up completely. However, it would really be better to plant them immediately in a temporary holding area. For this purpose, nearly any well-drained sunny spot will do. Another, less desirable, alternative might be to plant them temporarily in pots.
Generally, bearded iris require a sunny and well drained spot and do best with a soil pH near neutral. Your new permanent iris bed should be deeply prepared and well enriched with organic matter prior to planting, and the iris should be planted very shallowly to avoid rotting the rhizomes. (An inch of soil at most on top the rhizome is plenty. If your soil is heavy, plant them even shallower than that.) Water well after planting to settle the roots, and be sure they receive adeqaute moisture during the summer to root well. Remove any browning leaves this summer and clean out and destroy all the iris debris this fall to minimize disease problems. Apart from that, there isn't too much to do for iris with the possible exception of checking them for frost heaving during the winter.
Good luck with your iris!
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