Late July through the month of September is the optimum time to separate and transplant rhizomatous irises. Separating them at this time reduces the chance of bacterial soft rot, which is a devastating disease for the iris. Rhizomes are actually underground horizontal stems that store food for the plant. Rhizomatous irises grow in a clump, and you will be separating the rhizomes in this clump. You will be able to tell when your irises need to be separated by their lack of blossoms. More than likely you will need to separate them every 3 to 5 years.
Prepare a 10 percent bleach solution, which will be used to disinfect your cutting knife. Use a small container and add ¼ cup of bleach to 2 ¼ cups of water. Set this on the side.
Prune the iris leaves to approximately 1/3 of their height. Deposit the pruned leaves into the garbage or your compost bin.
Dig out the iris clump using your spade or digging fork. You want to lift out the entire clump of irises.
Use a sharp knife to separate the rhizomes. After each cut dip your knife into your bleach solution. This is to ensure that you are not passing on any diseases such as bacterial soft rot as you cut through the clump of irises.
Throw away any old rhizomes. You will want to replant only the smaller younger rhizomes, which are growing off of older stems.
Replant the irises immediately after separating, and water them thoroughly. They should be transplanted and watered as soon as possible to avoid shock to the plant.