Bearded Irises Grow from Rhizomes
Bearded irises grow from fleshy underground stems known as rhizomes, which produce the roots that 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. 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. Such plants also spread by rhizomes, and irises are no exception. While some rhizomatous plants, like bamboo, spread rapidly and even invasively, the iris spreads fairly gradually—one of its main virtues for gardeners. But, as iris rhizomes spread, they become crowded. This stresses the plants and can even cause them to stop blooming and become susceptible to pests, such as 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.
What About Other Types of Iris?
Not all types of iris are rhizomatous plants. The roots on the Siberian iris are quite different—fibrous masses that are much tougher and more difficult to lift and divide. Division of Siberian iris is a matter of forcefully cutting the fibrous masses into sections. (The process is similar for Japanese iris.) Other types of iris, such as the Dutch iris, grow from bulbs; division is a matter of separating out new bulbs produced by the clump. The process described here is particular to the bearded iris and similar types, such the crested iris.
Why and When to Divide and Transplant Iris
Iris roots can become woody and overgrown unless the clumps are lifted, divided, and replanted every three to four years. Overgrown clumps tend to get barren in the center, with foliage and flower stems that appear only around the outside of the clump. You can also lift and divide more often than this, such as when you want to share irises with others or when there are clear signs that iris borers have damaged your plants. Symptoms of borers include soft, mushy clumps or foliage that becomes yellow and soft while the flowers are still blooming. Left unattended, the rot that begins when borer worms drill into the rhizomes will gradually spread to consume the entire clump. Of course, irises are sometimes transplanted simply to move them to a new location. If so, it’s best to follow the same procedure: Lift and transplant after the plant has finished blooming and the leaves are beginning to dry out. Trying to transplant an entire clump in spring while preserving the season’s bloom rarely works.
Tools and Supplies You Will Need
ShovelSharp knifePrunersFungicide powder (optional)
Instructions
If you wish, you can divide the clumps into sections with a knife or sharp pruners. Even quite small sections will transplant successfully, provided they have at least some foliage and a few roots dangling from the fleshy rhizome section. It is possible to propagate dozens of individual plants from a single clump of iris. More often, though, gardeners divide into sections 4 to 8 inches long, each with four to six leaves. Some gardeners like to coat the cut faces of the rhizome with a fungicide powder before replanting to prevent disease, but it is not essential to do this. Dig a shallow hole just slightly deeper than the root portion of the iris, and then create a slight mound of soil in the center of the hole. A spacing of 12 to 24 inches between rhizomes is typical, but you can space them closer if you want faster results. However, closer spacing means you will probably need to divide again within a couple of years—these are fast-growing perennials. A traditional method is to plant in groups of three small rhizomes, with each group spaced about 24 inches from the next. 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.
