Bearded irises are popular perennials, valued for their stunning spring blooms and distinctive vertical foliage. They grow from thick, fleshy underground stems called rhizomes, which function as storage organs. While these plants are generally robust, they require periodic division to maintain their vigor and ensure consistent flowering. This necessary maintenance prevents overcrowding, which can compromise the health and productivity of the iris clump.
Recognizing When Bearded Iris Need Dividing
A significant reduction in the number or quality of blossoms is the clearest indicator that an iris clump is overcrowded and needs attention. Over time, the rhizomes multiply and spread, creating a dense mass that competes for nutrients and sunlight, leading to diminished flowering. You may also observe the rhizomes pushing themselves completely above the soil line, or notice a hollow, woody center in the clump where the oldest, spent rhizomes have ceased to produce new growth.
Crowded conditions make the plants more susceptible to common issues like bacterial soft rot and iris borers. Division is typically required every three to five years, though aggressive varieties may need it as often as every two years. Regularly dividing the clumps rejuvenates the plant, encouraging it to produce new, healthy growth that will flower prolifically in subsequent seasons.
Identifying the Optimal Timing Window
The best time to divide bearded irises is in the late summer through early fall, typically from July through September, after the spring blooming period concludes. This timing allows the newly separated rhizomes approximately four to six weeks to establish a healthy root system before the onset of winter dormancy and freezing temperatures. Transplanting during this period, when the weather is often drier, also helps to minimize the risk of fungal diseases, such as soft rot.
Attempting division in the spring interrupts the plant’s active growth cycle and sacrifices the current year’s potential flowers. A late summer division ensures the plant has stored sufficient carbohydrates from its summer growth to fuel the establishment of new roots. This period of semi-dormancy is when the plant is best able to handle the stress of being moved and divided.
The Step-by-Step Division Process
Use a garden fork or spade to carefully lift the entire clump from the soil, digging deeply around the perimeter to avoid damaging the roots. Shake off the loose soil to expose the network of intertwined rhizomes. If necessary, a gentle spray from a hose can wash away any remaining soil, making it easier to see the separation points and inspect for damage.
Separate the rhizomes by hand or by using a clean, sharp knife to cut them apart at the natural joints. Each new division must be firm, light-colored, and include a healthy section of rhizome, a good set of roots, and at least one fan of leaves. The old, woody, and leafless sections in the center, known as “spent” rhizomes, should be discarded as they will not produce flowers.
Inspect every piece for signs of disease, such as soft, mushy areas, or for the presence of iris borers. Use a sharp tool to cut away any compromised tissue until only solid, healthy white flesh remains. Next, trim the long, sword-like leaves into a fan shape, cutting them back to about four to six inches. This reduction in foliage helps limit water loss while the new roots are forming and the plant is recovering.
When replanting, prepare a shallow hole with a small mound of soil in the center. Place the rhizome on top of this mound, carefully draping the roots down the sides into the surrounding trench. Ensure that the top of the fleshy rhizome is either left slightly exposed to the air or barely covered with a thin layer of soil. Deep planting can lead to the rhizome rotting and will prevent the plant from blooming successfully.
Post-Replanting Care for New Irises
After the rhizomes are replanted, water them thoroughly to settle the soil around the newly positioned roots. Following this initial soaking, watering should be deep but infrequent, keeping the soil moist but not wet until new growth is clearly visible. Overwatering is a common mistake that can encourage the development of soft rot, which can quickly destroy the new rhizome.
The replanted irises require a location that receives full sun for at least six hours daily, as insufficient light will significantly hinder flowering. The slight exposure of the rhizome to the sun, known as “sunning the rhizome,” is deliberate and helps to prevent excess moisture retention around the stem. While new growth should appear relatively quickly, the new divisions may produce only a few flowers in the first spring, with full blooming resuming by the second season.