Irises are popular garden perennials, celebrated for their colorful blooms. Though often called “iris bulbs,” these plants grow from thick, fleshy, underground stems called rhizomes. Irises require periodic division, typically every three to five years, because overcrowded rhizomes lead to a decline in health and flower production. Dividing and replanting rejuvenates the plants and ensures robust blooming in subsequent seasons.
Visual Cues Your Irises Need Dividing
Irises provide clear visual signals when the clump is too dense and requires division. The primary indicator is a noticeable reduction in the number of flowers produced. Overcrowding forces the plant to expend energy competing for resources, impacting its ability to generate flower stalks.
The clump’s physical structure changes as it ages. An overcrowded stand often develops a “donut hole” appearance, where the center becomes woody, bare, and unproductive. This central portion is the old, spent mother rhizome that no longer produces blooms, leaving new growth concentrated on the outer edges.
Rhizomes may also push up out of the soil, becoming tangled and exposed. This surface-level growth, sometimes called “sunbathing,” leaves the plant vulnerable to sun-scald and makes the clump unstable. Observing these signs confirms the need for division.
Optimal Timing for Lifting and Transplanting
The time to dig and divide irises is a key factor for success, occurring after the spring bloom concludes. For most common varieties, like bearded irises, the ideal period is four to six weeks after flowering ends, typically mid-summer through early fall (July to September). This timing is chosen because the plant enters a semi-dormant phase, reducing the shock of transplantation.
Dividing during late summer allows the rhizomes to establish a healthy root system before the first hard frost. Establishing feeder roots is necessary for the plant to anchor itself and store energy for winter. This period often presents drier weather, which minimizes the risk of bacterial soft rot—a disease that thrives in wet soil and can be fatal to freshly cut rhizomes.
While late summer is safest for bearded irises, other types (Siberian or Japanese) can be divided in early spring as new growth begins. However, the late summer/early fall window is the most reliable method for maximum establishment and ensuring blooms the following spring. Complete replanting at least six weeks before the expected date of the first killing frost.
The Process of Preparing Iris Rhizomes
Once the optimal time arrives, preparation begins by carefully lifting the entire iris clump from the ground. Use a garden fork positioned about 6 to 8 inches away from the foliage to gently pry the mass upward, minimizing damage to the roots and rhizomes. After lifting, shake or wash excess soil off the rhizomes.
Separating the individual rhizomes can be done by hand or with a sharp, clean knife. Only keep the younger, firm, healthy rhizomes that have a fan of leaves and a good root system for replanting. The old, woody, leafless “mother” rhizome in the center must be discarded as it will not bloom again.
Inspect each healthy division for pests or disease, such as iris borers, which leave soft, mushy spots. Trim away any damaged tissue until only clean, white, fleshy material remains.
The foliage is then trimmed into a fan shape, typically 6 to 8 inches tall, to reduce water loss and prevent the division from rocking in the wind. Trim the long roots back to a few inches to facilitate easier planting and encourage new feeder root growth.
The cut surfaces can optionally be left to air-dry or “cure” for a day or two in a cool, shaded place to allow the wounds to callous over. Some gardeners also choose to dust the fresh cuts with a fungicide or soak the rhizomes briefly in a diluted bleach solution to provide extra protection against fungal diseases before they are placed back into the soil.
Techniques for Successful Replanting
Successful replanting requires careful site consideration and a specific technique to ensure the rhizome is exposed. Irises demand full sun (a minimum of six to eight hours daily) and well-drained soil, as excessive moisture leads to soft rot. If the soil is heavy clay, amending it with compost or grit before replanting will improve drainage.
Shallow planting is the most important technique for promoting future blooms. Dig a shallow hole, create a small soil mound in the center, and position the rhizome over the mound. Spread the roots downward into the surrounding trench. The rhizome’s top half should be exposed or barely covered with a thin layer of soil—a practice sometimes called “sunbathing.”
Planting rhizomes too deeply is a common mistake that prevents flowering. Space individual divisions 12 to 18 inches apart to allow for future growth and air circulation, which helps prevent disease. After firming the soil around the roots, apply a thorough initial watering to settle the soil and eliminate air pockets.