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The Greener View: Dividing Iris and Iris Borer Problem

Jeff Rugg on

Q: We have some bearded iris that have spread out from their original planted spot and now form a circle with an empty spot in the middle. Can we divide them and replant them back into the spot they came from? It is the best location to see them from between some bushes.

A: Yes, you can divide them, and now is a good time to do it. Generally, we divide perennials in the fall or spring when they are dormant, but irises are an exception to the rule. They have large rhizomes that store food and water, allowing them to be moved in the summer heat without too much of a problem.

Some people divide irises every five years or so, while others have never done so at all. They are easy to dig up because the horizontal stem called the rhizome is at or near the surface, and there are only a few small roots down into the soil. After lifting them from the ground, you can wash them off or just break off any remaining clumps of soil.

Use a sharp knife to separate the rhizomes into pieces with two fans of leaves. Any leftover sections with only one fan of leaves are good for giving away or planting in new locations. The single fan sections may or may not bloom next year.

You can cut two-thirds of the leaf off before replanting. Mix in some compost to the soil where the iris will be planted. Plant the rhizome so the top is just at ground level and add a couple of inches of mulch on top.

While you are doing the transplanting, look for iris borers. Iris borers are the caterpillars of a night flying moth. The tiny caterpillars eat ragged holes along the edge of the leaf, and then work their way into the center of the leaf. The weakened leaf will hang over as if it is wilted, when it really has an insect problem. They then burrow down into the rhizome.


The caterpillar eats the inside of the rhizome. Not only is this a bad thing, but the iris can then get bacterial diseases that kill the plant. As the rhizome and root system die, the leaves look wilted again, but this time it is a combination of fewer roots and the diseases.

Watch for the top of a leaf to turn brown and start wilting. Follow that leaf down and see if it has any soft mushy areas down near the base. If a light tug pulls the leaf off, look inside it by holding it up to the light or opening it up. If the caterpillar is inside, just step on it. If it is not, check down at the leaf base to see if it has made it into the rhizome. Pry it out with a wire and step on it. If you cannot find the larvae, you may want to apply a systemic insecticide to the irises.

If the borer rhizome damage is not severe, go ahead and plant it. If there is a lot of soft rotting tissue, cut out that section of rhizome. Small amounts of rot can be scraped off. Then dip the rhizome in a one-part bleach to nine-parts water solution for a few minutes. Rinse it off and allow it to dry before planting it.


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