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Knowledge to Grow by Gail Daniels - Dividing Iris


The iris is one of the oldest gardens flowers and has been around for at least 4,000 years. In Crete, priests and princes regarded the iris as a prize possession. The iris is native to the north temperate zone and several species have long been grown in Midwestern gardens. The most familiar type of iris is the bearded iris and includes more that 200 species. Species are separated into two major groups – rhizomatous and bulbous. Rhizomes grow horizontally underground stems that are used as food storage for the plant. Bearded, Beardless Siberian, and Japanese iris are in this group. Bulbous irises form a more typical bulb and include Dutch and reticulate iris. These are planted in October with other bulbs.

The best time to plant and transplant rhizomatous iris is late July through September. Dividing iris every 3-5 years can rejuvenate and expand your garden.

 When transplanting iris, first cut back the leaves to about ⅓ their height. Lift the entire clump with a spade or digging fork. Use a sharp knife to cut the younger, outward-growing rhizomes into sections. Leave as many roots and buds on each piece as possible. Dip the knife in 10 percent bleach after each cut. Discard the old central portions of the original rhizome and any sections that appear to be diseased or infested with iris borer.

Be sure when you divide the rhizomes to inspect them for soft rot and iris borer. Iris borer is the worst insect problem irises get. The adult borer is a brownish moth that lays eggs in the fall on the iris leaves. The eggs overwinter and hatch into caterpillars during April and May. The caterpillars first bore into the iris and leaves and by the end of July move into the rhizomes. In early August the caterpillars move from the rhizome to the soil to pupate into a moth.

When dividing iris, the iris borer will be a mature pink caterpillar inside the rhizome. The rhizome may look fine until your fingers push through to a mushy mess. Bacterial soft rot often accompanies iris borer damage.

 Replant the newly cut sections as soon as possible to avoid excessive drying.

Iris plants love a sunny, well-drained garden spot. They will tolerate half-day shade, but you will get much less blooms in full shade. They also need well-drained soil.

The depth is important. Iris rhizomes should be planted just below the top of the soil. The new plants will probably need watering about once a week until they grow a new root system.

Fall is an important time to control the iris borer. After the first hard frost, remove and destroy or bury the old iris leaves and plant debris to remove the eggs. In April and May you can look on the leaves of your iris and destroy any newly hatched caterpillars that may have been missed.



More information on iris is available in publication HO-75 available from the Purdue University Cooperative Extension Service. The Purdue University Cooperative Extension Service can be reached at 499-6334 in LaGrange County.