Iris is hardy plant so best time is fall,but they can be planted anytime. You can transplant them almost anytime they are not in bloom with great success,they take 2years to develop blooms again but they survive.
When the plant will become blooming,then we can cut the leaves and dig them up. to avoid transplanting in hot time. it is best time to transplant irises in winter season. when the irises dig them and divide and then transplant.
It depends on what city or state you are and wheater it is winter or summer time. The best time is when they stop blooming. You cut the leaves and replant them.
