The popular perennial iris, known for its distinct flowers and sword-shaped foliage, is often mistakenly thought to grow from a bulb. The most common varieties, Bearded Irises, actually grow from a specialized, thickened stem called a rhizome, which stores food. Periodically digging up these rhizomes is necessary for continued healthy growth, typically to divide overcrowded clumps, rejuvenate plants showing diminished bloom, or prepare them for relocation. The timing of this process is the most important factor in ensuring the plant survives and thrives the following season.
Identifying the Ideal Time for Iris Division
The optimal time to dig up and divide Bearded Iris rhizomes is during their natural rest period after the spring bloom cycle concludes. This window typically opens in mid-to-late summer, spanning from July through September in most climates. Disturbing the plant during this phase of relative dormancy is less stressful.
This timing allows the newly separated rhizomes sufficient time to develop a robust system of feeder roots before the first hard frost. A minimum of six weeks is recommended for this establishment period. This time anchors the plant, allowing it to store energy for the winter and the following spring’s bloom. Dividing them too late risks the rhizomes being heaved out of the ground by freezing and thawing cycles before their roots can take hold.
This schedule is specific to the rhizomatous Bearded Iris. True bulbous irises, such as Dutch or Reticulata varieties, follow a different cycle and are typically planted in the fall to bloom in the spring. For the bearded variety, the summer division period capitalizes on warm soil temperatures that encourage rapid root production while the plant is less active above ground.
The Process of Lifting and Preparing Iris Rhizomes
The process begins by gently lifting the entire clump from the soil using a garden fork, working a few inches away from the rhizomes to avoid damage. The clump should be carefully pried up, and excess soil shaken or washed off to clearly expose the root structure. This allows for easy identification of the smaller, newer rhizomes, known as “increase,” which are best for replanting.
Next, separate the healthy, fan-bearing rhizomes from the older, spent mother rhizomes, which are typically found in the center and will no longer produce blooms. Each division selected for replanting must have a healthy fan of leaves and a good set of roots attached. Inspect each rhizome closely for signs of disease, such as soft, mushy spots indicating bacterial rot, or small holes suggesting iris borer infestation.
To prepare the divisions, the fan-like foliage should be trimmed back significantly, generally to about four to six inches. This reduction minimizes water loss through transpiration while the root system re-establishes. Any damaged or diseased sections must be cleanly cut away using a sterilized knife. The remaining healthy sections should be allowed to air-dry or “cure” for a day or two to promote healing of the cut surface.
Replanting Techniques and Initial Care
The new planting site must offer excellent drainage and receive full sun exposure, as irises are highly susceptible to rot in consistently wet conditions. To ensure proper planting depth, dig a shallow hole and build a small mound of soil in the center. The roots of the prepared rhizome are then spread out and draped over this mound.
The rhizome itself should be positioned so its top surface is just at, or slightly exposed above, the surrounding soil level. Planting the rhizome too deep is a common mistake that can lead to rot and prevent flowering. Once positioned, cover the roots with soil and firm the soil gently around the rhizome, ensuring the plant is stable and upright.
Proper spacing is necessary to prevent rapid re-crowding, so divisions should be planted at least 12 to 18 inches apart. Immediately after planting, water thoroughly to settle the soil around the roots. Following this initial soaking, watering should be minimal until new growth is visibly established, as excessive moisture during summer heat can compromise the rhizome’s health.