When your irises start to look overcrowded, chances are it's time to separate the bulbs, or rhizomes, to divide up your plants. To separate paper iris bulbs, wait until two or three weeks after blooming or until late summer and plan ahead exactly where the new plants will go so you have a few new areas prepared for planting. Letting family and friends know you're going to divide your irises is also a good way to find out who wants to take a few plants off your hands.
Dig up the bed area where your paper irises have become overcrowded by starting 8 inches away from the farthest out iris and working a shovel or garden fork under the roots and bulbs of the plants. Lift the clumps of irises from the soil and set them on the ground.
Gently brush off dirt clods with your fingers to give you a better view of the root system. Look over the paper irises to find individual plants with groups of two to three leaves close together.
Cut straight down through the rhizome at a point evenly spaced between two plants to sever the plants with a clean vertical slice. Use a sharp trowel or knife. Cut away as many individual plants as you can find and easily separate from the main clump of irises.
Clip the leaves of each new plant back to 4 to 6 inches with a clean, horizontal cut.
Put the new plants in a dry location away from direct sunlight for one or two days. During this time, check the cut surfaces of the bulb for signs of healing such as drying and hardening over at the sliced area.
Plant the paper irises wherever you want in your landscaping. You shouldn't need to separate the irises again for up to five years.