Irises are one of the showiest flowers in the spring garden. It's hard to believe that you can plant something that looks like a dehydrated stick and a few months later end up with ruffled blooms that look like butterflies on stems. Although the iris is a low maintenance flower, dividing and transplanting are essential to their long-term health. Learn the why, when, and how of iris transplanting.
Reasons to Transplant Irises
Bearded irises and Siberian irises both grow from fleshy underground stems called rhizomes, which are capable of producing the roots that will draw nutrients and water into the plant, as well as the shoots that will eventually form leaves and flower buds. Plants that grow from rhizomes can survive poor growing seasons because the rhizomes store starches and proteins that help the plant get by in lean times. Rhizomes also enable the asexual propagation of plants: Cut off part of a rhizome, replant it, and you will soon have a plant identical to the parent plant.
Plants that grow from rhizomes also spread by rhizomes, and irises are no exception. While some plants, like bamboo, spread rapidly (even invasively) by underground rhizomes, the iris spreads gradually over a few growing seasons. As iris rhizomes spread, they become crowded. This stresses the plants, and can even cause them to stop blooming. Irises that are stressed also become more susceptible to pests like iris borers. By dividing and transplanting your irises, you will rejuvenate the plants, and be rewarded with a greater number of healthy blooms in the spring.
When to Transplant
Transplanting irises takes some planning and effort, but the good news is that this isn't an annual chore. Irises need transplanting when they become crowded and perform poorly, on average every three to four years. If you notice one spring that the flower show isn't what it used to be, it's time to think about transplanting.
Like most plants, irises respond best to the stresses of transplanting when active growth is finished. Being dug up and broken apart interrupts an iris's growth cycle, so wait until the plants are finished blooming. When summer temperatures rise, the foliage will begin to brown and die back. By the end of July, iris plants will be receptive to the insult and injury of digging, cutting, and replanting. It's wise to accomplish your transplanting before September, as the return of fall rains can cause rot in newly divided plants.
How to Transplant
A large clump of iris rhizomes in need of dividing and transplanting can resemble an intimidating, Medusa-like mass. First, cut back the leaf fans by about a third. Dig around and under the entire clump, gently prying it from the ground. Don't worry if you break some of the rhizomes. After you've lifted the clump, brush the dirt off and inspect the rhizomes. Cut them apart with a sharp knife. If one rhizome seems to blend into the next, use the leaf fans as a guide to show you where to cut. The goal is to have one leaf fan for each viable rhizome piece.
Use this time to remove any rhizomes that show signs of bacterial rot or borer infestation. Good rhizomes will be firm, not soft. Any rhizomes that have borers may reveal pinkish caterpillars in July, but by late August the caterpillars will have left the rhizomes to pupate in the soil before they become moths. You can still break the life cycle after planting.
Discard old rhizomes from the center of the clump, and any rhizomes without leaf fans. Plant the remaining rhizomes in a site with full sun and excellent drainage. The top of the rhizome should be just visible at the soil's surface. Planting too deeply will encourage rot. Plant the iris rhizomes about two feet apart, with the leaf fans facing the same direction. Water once to help settle the surrounding soil and then don't water again.
After You Transplant Iris
After the first hard freeze, you can break the life cycle of the iris borer with good garden sanitation. The moths that emerged from the soil in the fall laid their eggs on the iris leaves. These eggs will hatch into new, hungry borers in the spring. Cut back your transplanted iris foliage to the soil level, and discard the foliage. You will be discarding the next generation of borers with this simple act, preparing your plants for a pest-free display of blooms in a few months.