Dividing Iris



by Karen Weiland, Advanced Master Gardener




The iris is one of the oldest garden flowers and can often be seen as a remnant of a long ago abandoned garden.

Most iris spread by way of underground stems that are called rhizomes.  These rhizomes can become too crowded over a period of time and this crowding can be the cause of reduced blooming in the plant. 

There are times when gardeners wonder when is the best time to divide and transplant iris.  Dividing iris plants is a normal practice of iris care.  They usually need to be divided every three to five years and only after they have finished blooming.  Doing this will help rejuvenate the plants and encourage more flowers for future years.  Late summer through early fall is a good time to lift, divide and transplant iris.

To make it easier to handle the plant, cut the leaves back to about one third their height.  Begin by using a spade or fork to lift the rhizomes out of the ground.  You should not have to go very deep as iris rhizomes and roots grow rather shallowly.  Brush as much of the dirt as you can off of the rhizomes and roots and then use a sharp garden knife to cut and divide the rhizomes into pieces that are about four inches long.  You want nice, firm rhizomes with roots and a fan of leaves.  Dip the knife into a ten percent bleach solution after each cut to prevent the spread of disease.  You may come to a point where you have some rhizomes that have no leaf fans on them.  Just give those a toss into the compost pile.

There are two groups of Iris, rhizomatous and bulbous. Bulbous iris form a more typical bulb and include dutch and reticulate iris which are planted in October with other bulbs.

While you are dividing the rhizomes be sure to inspect them for rot, disease and bugs.  Iris borer is about the nastiest bug an iris can get.  The adult is a brownish moth that lays her eggs on the iris leaves in the fall.  These eggs will overwinter and hatch in April or May into caterpillars.  These caterpillars will then bore first into the leaves and then into the rhizome to feast and reach maturity.  About early August the caterpillars will move into the soil to pupate into a moth.  The iris borer will look like a pink caterpillar inside the rhizome.  If you find one give it a good squish or a stomp.  Bacterial soft rot will sometimes occur along with the borers.

Transplant your iris into a sunny, well drained spot.  Dig a hole about five inches deep, then pull together a mound of soil in the middle of the hole.  Set the rhizome on top of the mound allowing the roots to fall over and down the sides of the mound.  Cover the roots and rhizome with soil so that the rhizome is just barely below the soil surface and just slightly exposed.  Do not plant the rhizome too deep or it may rot.   Firm the soil and water gently and thoroughly.  I like to mix a root stimulator with my water when planting anything.  It really works well. Keep the rhizomes watered if rainfall is not adequate.

As always, Happy Gardening!

More information about gardening and related subjects is available online at www.hort.purdue.edu/ext/garden_pubs  The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange Co., 636-2111 in Noble Co., 925-2562 in DeKalb Co., and 668-1000 in Steuben Co.