Clumps of bearded irises should be divided and replanted before they become overcrowded. A single rhizome will branch many times over the years, developing into a crisscross clump, often choked with old leafless rhizomes. If it is not divided, the mass of leaves will exclude sun from the roots. This will lead to poor flowering or noflowering and often weakens the plants making them more susceptible to insects and diseases.
Iris division is best done after blooming, the same time of year that plantings should be made. Lift each clump by gently prying it loosefrom the soil. A spading fork is better for this than a shovel because it is less likely to cut roots and rhizomes.
Use a sharp, strong-bladed knife to trim younger rhizomes into sections that include healthy-looking roots and one or two strong leaf fans. Carefully wash soil off the roots under low pressure from a hose for ease of replanting. Discard old rhizomes from center sections.
Trim existing leaves to a neat fan shape and then dig a hole to replant so that the rhizome will be set no deeper than one inch. Check to be sure that all the leaf fans face the same way so that the developing rhizomes do not grow together. Evenly spread the roots and firm the soil around the rhizome to eliminate major air pockets.
For more information, see the following Planttalk Colorado™ script(s).