Let’s talk about division. Not the division you learned in math class. The kind that involves separating or splitting herbaceous perennials to make more plants. Perennials, such as bearded irises (Iris x germanica), grow fuller and more beautiful with each passing year. Eventually, they outgrow their allotted space or become overcrowded. Growing cheek by jowl in cramped quarters can cause them to decline, which results in reduced growth and few, if any, flowers. Often these crowded conditions compromise their health as they surrender to troublesome diseases and insects.
A general rule of thumb is to divide spring and summer blooming perennials in the fall and early winter. Divide fall-flowering perennials in early spring when the new shoots are a few inches high to avoid interrupting flowering. For bearded irises (Iris x germanica), divide them six to eight weeks after bloom, or from July through September, but no later than six weeks before the first frost.
Here’s a step-by-step approach to dividing and replanting bearded irises.
1.With a shovel or spading fork, loosen the soil a few inches away from the clump of horizontally creeping underground stems called rhizomes. Watering the bed the day before will make digging easier.
2. Lift them and tease them apart. Then, gently shake the rhizomes to remove the soil to get a better look at them.
3. Separate the rhizomes so each section has at least one bud or fan of leaves. Sometimes young rhizomes will grow from the sides of older, spongy rhizomes. Use a sharp, sanitized knife to cut the young rhizomes away from the older sections. Discard the older pieces and any sections that are undersized or damaged by insects (notably iris borers) or diseases. (Don’t think twice about throwing away the old rhizomes. They bloom only once and then become a flowerless food reservoir). If you cut into infected growth, sanitize your knife or pruning shears.
4. Make sure each division consists of a firm rhizome with a fan of healthy leaves. Trim leaves to about one-third to one-half of their height to reduce moisture loss.
5. Lay the trimmed plants in a shady spot for a few hours to allow the cut ends to dry and heal.
6. Replant the bearded irises in a sunny, well-drained location. Form a cone of soil in the planting hole. Make it high enough so that when you spread the roots around the top of the cone and press it firmly into the soil, the rhizome rests just above ground level on clay soils. For lighter sandy soils, the top of the rhizome can be placed 1/2 inch or less below ground level. Space the divisions about 12 to 18 inches apart and in groups of three to form a natural-looking clump. Position the fan of leaves pointed away from other plants in the group. While closer spacings will create luscious floral displays sooner, they will require more frequent dividing.
7. Water the divisions to settle soil around the roots. Apply a shallow layer of mulch but avoid covering the rhizomes. In the absence of rain, water them weekly to help the roots become established.
For more information on growing these flowering rainbows of color, see HGIC 1167, Iris, and Bearded Iris.