The iris is a perennial flower that spreads through underground tubers. In order to transplant the iris you must dig up the tuber and replant it in another location. Knowing when to do this can help ensure that you grow strong healthy irises wherever you plant them.
Transplanting your irises in early spring means digging up the tubers and replanting them before the iris begins its growing season. If you start digging and find green leaves beginning to shoot up it is best to leave them and wait until after the growing season. As long as the tubers aren't sprouting you can dig the tubers up, use a bulb cutter to separate them and replant the tuber pieces in a different area. These transplants are done to both thin out the patches of irises and to introduce irises to a different area.
In the fall after the blooms have stopped and the leaves start to turn brown and fall over is the ideal time to transplant irises. Cut off all the leaves and uncover the tubers so that you can see where you want to cut them to transplant. The ideal tuber piece to transplant will have at least two sprouts on it. This will ensure that when you transplant the tuber you will get growth in the spring from the already existing tuber formation, as well as any new growth that occurs once transplanted.
Irises are a very hardy flower and nearly impossible to kill. Cut your tubers with a sharp spade or bulb lifters and wash off excess soil clinging to the roots. These tubers that you lift will be fine sitting in a bucket for a few days if you aren't ready to plant them yet. If you do a fall transplant don't be surprised to see a few sprouts trying to come up even though it isn't the growing season. Thinning and transplanting these tubers seems to give them a growth spurt; they thrive in good soil and when they aren't overcrowded. Don't worry because nature will take over and the tubers will go into dormancy for the winter and begin their real growing season in the spring.