Dividing Irises and Peonies
Lisa Olson, Master Gardener Intern
Now that the Peony and Iris blossoms are gone for the season it's the perfect time to dig and divide the crowded plants. This article will help you prepare these plants for next year's season of beautiful blooms.
The dog days of summer are upon us. It’s a great time to enjoy the fruits of your labors by harvesting fruits and veggies you have meticulously tended. Or maybe your garden presents a feast for your eyes, exploding with color or calming the senses with cool and shady textures. Before you get too comfortable sitting back and taking it all in, now is the perfect time to dig in and divide your crowded peonies and irises as their showy blossoms have faded away until next year.
The US Forest Service tells us there are approximately 280 species of irises in the world; this article will focus on the bearded iris. It is undeniably a popular choice among gardeners and it’s no wonder because of the rainbow of colors they provide in the garden. In fact, the origin of their name can be traced back to Greek mythology where Iris was the messenger traveling on a rainbow between the heavens and earth, thus the meaning of the name Iris is rainbow. Author Madison Moulton, a historian and political scientist, writes that cultivation of the iris plant can be traced back 2000 years to ancient Egypt. Throughout the centuries, it became a royal symbol, making its appearance on royal banners and coats of arms as the fleur-de-lis. In more recent times, the bearded iris, or Iris germanica, was named the state flower of Tennessee, and the fleur-de-lis is also the symbol of New Orleans. Now that you know the history of this garden beauty, it’s time to learn about how to divide this noble plant to keep it healthy and blooming in your garden for years to come.
The University of Illinois Extension reminds us that the hundreds of species of irises can be separated into two groups – rhizomatous and bulbous. The bulbous irises are planted in the fall along with other fall planted bulbs. The bearded iris, however, falls into the group of rhizomes. Rhizomes are underground stems that store food for the plant. The University of Minnesota Extension points out that during August, irises tend to go dormant, making it the perfect time to divide them. As irises mature, rhizomes keep producing more and more rhizomes and crowd themselves out. Benefits of regular division every three to five years include more profuse flowering, and it helps prevent problems like iris borer and soft rot. The University of Illinois Extension suggests dividing that often, especially if your iris patch is producing noticeably fewer blooms.
Using a shovel or pitchfork, carefully lift the plants out of the soil. The University of Wisconsin-Madison Extension offers the tip to water the ground thoroughly the day before if the soil is particularly dry to make lifting the plants easier. If possible, try to lift an entire clump and divide the rhizomes once you have lifted it and shaken or washed off the soil to inspect the rhizomes. If you notice any soft spots on the rhizomes or dark streaks on the leaves, iris borers are probably to blame. Extract and kill any caterpillars you find and use a sharp, clean knife or shears to cuts away any damaged parts. Any soft, smelly, or rotted material should be destroyed rather than placing it in the compost.
As Penn State Extension suggests, you can probably break apart the rhizomes with your hands, but if you need to use a knife, it is a good idea to dip it in a 10% bleach and water solution after each cut. The University of Wisconsin-Madison tells us to make sure each new plant division has a firm rhizome, is light-colored, at least 3 inches long with healthy roots, and has a fan of leaves. Penn State Extension adds that a good rhizome should be about as thick as your thumb.
While dividing your iris, it is recommended you cut back the leaves to about a third of their height. The University of Wisconsin-Madison Extension notes that this makes the plants easier to work with and helps reduce transpiration while the plant becomes reestablished. Using clean shears works fine for this step.
Now it is time to plant! Penn State Extension instructs us to dig a shallow hole 10 inches across by 4 inches deep and to build up a small mound of soil in the center. Set the rhizome on the mound with the roots cascading down the mound into the hole. Fill the hole with dirt leaving the top of the rhizome just visible. A common mistake is to plant irises too deep. Penn State Extension also recommends that for the most attractive display, plant single rhizomes or groups of three, 12 to 18 inches apart, with the fans of leaves all facing the same direction. Water thoroughly and do not add mulch to avoid rot.
Ella Fitzgerald and Louis Armstrong once sang, “Let’s Call the Whole Thing Off” when they couldn’t agree if it was a tomato or tomahto or potato or potahto. Here is a question for you: Do you call it a pee-uh-nee or pee-o-nee? According to dictionary.com, the correct pronunciation is… pee-uh-nee. With that settled, let’s learn about dividing peonies.
Unlike irises that thrive from frequent divisions, it is said that a peony plant left in the garden may well outlive the gardener! Peonies can go 50 to even 100 years without needing to be divided. However, that does not mean a gardener should never divide a peony plant. Reasons for dividing may be to have more peonies for free, to share, or to bring along part of a peony if a gardener is moving yet wants to keep a plant that has been in the family for generations.
Iowa State University Extension suggests that September is the best time to divide peonies. They recommend to first cut back the stems nearly to ground level. Next, carefully dig up the plant and then wash or shake the soil off the roots. Penn State Extension reminds us to remove any rotted material. The University of Illinois Extension notes that if dividing a large mature plant, you may dig up just half of the plant at a time. Use a sharp knife to divide the tuberous main root, called the crown, so that each wedge includes three to five pink or white-colored buds, or eyes, and one to two large main roots so that it can gather enough energy to provide for next year’s growth. Dig a generous hole and plant the new clumps so that the three to five “pink eyes’ are pointed side up and one to two inches below ground level, and space the plants two to three feet apart. Fill the hole back up with soil. University of Illinois Extension recommends for best results to add about an inch of organic matter such as compost or peat along with an inch of topsoil to cover the pink or white eyes. Be sure to water regularly throughout the fall if it doesn’t rain. It is also a good idea to apply mulch before winter.
By following these steps, you should have happy, healthy plants that will continue to provide you with beautiful flowers for years to come.
Photos credit: Susan Mahr, University of Wisconsin Extension (1, 2, 3, 4), Lisa Olson (5), University of Pennsylvania Extension (6)