To prevent overcrowding, you need to know how to divide rhizomes. You should begin dividing irises when you can see rhizomes protruding from the soil. Or if you observe a decrease in blooming. It depends on the iris variety, but dividing irises is a job you need to do every two to five years. In a year when you are dividing you should leave the foliage and dig up rhizomes with care. You should be able to pull them apart using your hands while others you will need to sever using a knife. Healthy rhizomes are around one inch wide. They should have one or two leaf fans with a good root structure. If the rhizomes are soft, hollow or being to rot, it’s best to discard them. Rinse the roots and make sure they are free from pests and disease, in particular, iris borer worms before transplanting. Prune leaves to around five inches then re-plant with the rhizome raised and the roots outspread.