Irises are a garden favorite, though most common varieties, particularly bearded irises, grow from thick, fleshy, underground stems called rhizomes, not true bulbs. Division is a standard maintenance practice necessary to ensure the health and continued vigorous blooming of these plants. Over time, rhizomes multiply and become severely overcrowded, which significantly reduces the plant’s ability to produce flowers. Dividing and replanting new sections provides the space and resources necessary for the irises to thrive.
Recognizing the Need for Division and Optimal Timing
A mature iris clump signals its need for division through several noticeable changes in its growth pattern. A primary sign is a significant reduction in flower production, often occurring every three to five years. The clumps may also exhibit a “donut” pattern, where the center becomes woody and barren while new growth is pushed to the outer edges. Overly crowded rhizomes will also push each other up and out of the soil surface, making them vulnerable to sun damage and moisture loss.
The optimal window for dividing bearded irises is in the mid-to-late summer, specifically four to eight weeks after the spring blooming period has finished. This timing, typically late July through August, is ideal because the plants are entering a semi-dormant state. Dividing irises during this period gives the newly separated sections enough time to establish new feeder roots before winter dormancy. Replanting in late summer greatly increases the likelihood that the divisions will flower well the subsequent spring.
The Process of Lifting and Separating Rhizomes
The division process begins with proper preparation, starting with gathering the necessary tools. These include a sharp spade or garden fork, a clean knife or pruners, and a sterilizing solution such as a 10% bleach mixture or rubbing alcohol. Use the spade or fork to carefully dig around the entire clump to loosen the soil, making deep cuts outside the root zone. Once the soil is loosened, gently pry the entire clump out of the ground.
After lifting the clump, gently shake off the loose soil or use a stream of water to clean the roots and rhizomes thoroughly. Cleaning the soil away allows for a clear inspection to identify healthy sections and any signs of disease or pests. The next step involves separating the rhizomes. It is important to discard the old, central “mother” rhizome, as this section is woody, non-productive, and will not bloom again.
Healthy divisions, known as “increases,” are the young, plump rhizomes growing off the sides of the mother section, each having a fan of leaves and a network of roots. Use the sterilized knife to cut these increases away from the clump. Ensure each division is firm and free of soft, mushy spots that could indicate bacterial soft rot or iris borer damage. If a borer is found, the affected area must be cut away and destroyed, and the cutting tool sterilized before making further cuts.
Finally, prepare the divisions for replanting by trimming back the foliage into a fan shape, reducing its height by approximately one-third to one-half (usually to about 6 inches). This reduces moisture loss through transpiration and minimizes the chance of wind rocking the newly planted sections before they establish a strong root system. It is also beneficial to trim the long roots back to about an inch or two to encourage the rapid growth of new feeder roots after planting.
Replanting and Establishing New Irises
Successful re-establishment depends on preparing the new planting site, which must have well-draining soil and receive at least six hours of full sun daily. Irises planted in perpetually wet soil are susceptible to rot, so amending heavy clay soil with compost can improve drainage.
When preparing the hole, dig it wide enough to accommodate the roots and create a small soil mound in the center. The most important detail is the depth of the rhizome, as irises do not tolerate deep burial. Place the rhizome horizontally over the mound with the roots draped down the sides, ensuring the top remains at or just barely below the soil surface. In heavy clay, leave the top of the rhizome partially exposed to sun and air, which encourages flowering.
After placing the rhizome, backfill the hole, firmly tamping the soil around the roots to anchor the division securely. For optimal growth, space the replanted rhizomes approximately 12 to 24 inches apart, which allows for several years of growth before the next division is needed. Immediately after planting, give the irises a deep watering to settle the soil around the roots. Keep the soil consistently moist but not soggy for the first few weeks as new roots begin to form.