Irises, particularly the common bearded variety, are stunning and relatively easy-to-care-for perennial flowers. However, their vibrant blooms will eventually slow down if they are not maintained, signaling that the underground root structures have become overcrowded. Irises do not grow from true bulbs but from thickened, horizontal stems called rhizomes, which store nutrients and spread new growth. To revitalize the plant and ensure continued vigorous flowering, periodically lift, divide, and replant these rhizomes every three to five years.
Optimal Timing and Necessary Tools
The ideal time to lift and divide iris rhizomes is during their semi-dormant period in late summer or early fall, typically six to eight weeks after blooming finishes. This window usually falls between July and September. Dividing irises at this time allows them to establish new feeder roots before the ground freezes, which supports the plant’s growth and flowering potential the following spring.
Before beginning, gather a few simple tools. You will need a sharp spade or a garden fork for gently lifting the clump; a fork is often preferred for minimizing damage to the rhizomes and roots. A pair of sharp, clean pruning shears or a knife is necessary for trimming the foliage and separating the rhizomes. A tarp or bucket is also helpful for collecting the lifted clumps and excess soil, keeping the work area tidy.
Lifting the Iris Clump
Prepare the above-ground foliage first, which helps reduce stress on the plant once the roots are disturbed. Use clean pruners to cut back the green, fan-shaped leaves to about one-third of their original height, typically leaving four to six inches of foliage. This trimming minimizes water loss while the plant focuses energy on re-establishing its root system.
With the foliage trimmed, begin excavation by inserting the spade or fork into the soil in a wide circle, starting six to eight inches away from the center of the iris clump. The goal is to loosen the soil and find the outer edge of the root mass without slicing into the rhizomes. Working around the circumference, gently lever the clump upward to lift the entire mass of tangled roots and rhizomes out of the ground. This careful technique minimizes breakage and preserves healthy, young roots.
Preparing the Rhizomes for Division
Once the clump is lifted, clean and inspect the rhizomes thoroughly. Shake off loose soil and use a stream of water to wash away remaining dirt, making the structure easier to see. The clump consists of older, spent rhizomes, often located in the center, and newer, plumper “increase” rhizomes branching off the sides. Only the younger, healthy increase rhizomes, which have a fan of leaves and robust roots, will produce flowers the following season; discard the older, woody, or soft central sections.
Use a sharp, sterilized knife or your hands to cut the rhizomes apart at the natural joints, ensuring each division retains at least one healthy leaf fan and a good network of roots. While separating the pieces, examine each rhizome for signs of pests, such as small holes indicating iris borer larvae, or soft, mushy spots suggesting bacterial soft rot. Any diseased or damaged sections must be immediately cut away and discarded away from the garden to prevent pathogen spread. If immediate replanting is not possible, allow the prepared rhizomes to cure in the shade for a few hours until a protective callus forms over the cut surfaces, which helps prevent rot.
Immediate Replanting or Storage
For immediate replanting, choose a site with full sun and well-drained soil, as irises are susceptible to rot in wet conditions. Dig a shallow hole and create a small mound of soil in the center of the planting space. Place the rhizome horizontally on this mound, spreading the roots down and out into the surrounding soil.
The rhizome should be planted very shallowly, with the top surface barely covered by soil or slightly exposed, as sun exposure is needed to encourage flowering. Position the divisions 12 to 18 inches apart to allow for future growth and air circulation, which helps deter disease. Water the newly planted rhizomes thoroughly to settle the soil around the roots, but avoid mulching, as it retains moisture against the rhizome.
If storage is necessary, the curing process should last a few days in a dry, shaded location. Once the cuts have dried and hardened, place the rhizomes in a container that allows for air circulation, such as a mesh bag or a ventilated cardboard box. Store them in a cool, dark, and dry environment, like a garage or basement, until planting.