Bearded iris clumps 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 no flowering and often weakens the plants making them more susceptible to insects and diseases.
Iris division is best done after blooming. Lift each clump by gently prying it loose from 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 roots under low pressure. Discard old rhizomes from center sections.
Trim existing leaves to a neat fan shape and then dig a hole to replant so that rhizome will be set no deeper than one inch. Check to be sure that all leaf fans face the same way so developing rhizomes do not grow together. Spread roots out evenly and firm the soil around the rhizome to eliminate major air pockets.
For more information, see the following Colorado State University Extension fact sheet(s).
For more information, see the following Planttalk Colorado™ script(s).
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Updated Monday, June 15, 2015