Dividing Iris

close up blue iris with white centers and green leaves behind it

Iris are spring blooming perennials with sword-like foliage that bloom in a wide variety of colors. The most common species of Iris in Iowa are the Bearded Iris (Iris germanica or hybrids), the Siberian Iris (Iris sibirica), and the Japanese Iris (Iris ensata). All three of these species grow from rhizomes – a chunky brown stem that sits at ground level from which the leaves emerge. August is one of the best times to divide Iris plants that are crowded or not blooming as well as they used to. While it may be tempting to divide them later in the year when it feels cooler out, Iris are prone to a bacterial soft rot during cool, wet seasons and times of stress.

The process of division is very simple. Cut back the leaves to 3-4” above the rhizome. Use a shovel outside the edge of the clump and lift the clump out of the ground. Separate the individual rhizomes by pulling them apart or using a small knife. Remove the rhizomes that are showing signs of soft rot or have borer holes. If only part of a rhizome is affected, you can cut off the rot as long as 2” of rhizome remains. The remaining rhizomes should have at least one leaf clump. When replanting iris place the rhizomes roughly 4” apart at surface level. The rhizome should be half above ground and half below ground. Iris should be divided every 3-5 years to maximize bloom potential.

If you are replanting the iris in the same area they were lifted, check for iris borer in the soil. The small pinkish-white larva will bore holes in the rhizome then burrow underneath the rhizomes to pupate. The pupa form in mid-August and becomes a moth in late August or September that leaves groups of eggs at the base of the plants leaves. If you find signs of the iris borer, take steps to control it. The most effective control measure is to remove iris foliage during winter or early spring before the eggs hatch.

Prepared by Lindsey Smith, Plant Collections Curator

Photo caption: Iris sibirica ‘Caesar’s Brother’