Iris is a genus of flowering plants recognized for its wide range of colors and distinct flower shape. Although often called a “bulb,” common garden varieties like the Bearded Iris grow from a specialized, thickened underground stem called a rhizome. The practice of digging and replanting, known as division, is necessary to maintain the plant’s health and vibrant flowering display. Dividing the rhizomes prevents overcrowding and rejuvenates the plant’s energy stores, ensuring continued vigor.
Optimal Timing for Dividing Irises
The successful division and replanting of irises centers on timing the process to align with the plant’s natural growth cycle. The optimal window for dividing most rhizomatous irises, especially the Bearded variety, is in late summer, from July through September. This period occurs after the spring blooming season has concluded, allowing the plant sufficient time to recover and store energy for the next year. Dividing them during this semi-dormant phase minimizes stress and reduces the risk of bacterial soft rot.
A clump requires division when it shows signs of diminished performance, such as a noticeable reduction in flowers. Overcrowding is the primary cause, leading to a woody, unproductive center and reduced air circulation, which increases disease susceptibility. Bearded Irises benefit from being lifted and divided every three to five years to prevent this decline and encourage robust blooming. Beardless types like Siberian or Japanese Irises prefer to be left undisturbed for longer periods and are only divided when bloom quality significantly decreases.
Preparing the Rhizomes for Replanting
Start by carefully digging up the entire iris clump using a spade or fork to minimize damage to the root system. Remove the soil, often by washing it away with a garden hose, to expose the fleshy rhizomes. Inspect all exposed rhizomes for damage, rot, or pests like the iris borer, cutting away soft or diseased sections until only firm, healthy tissue remains. Discard the old, leafless, and woody center sections of the clump, as they will no longer produce blooms.
Divide the main clump into smaller, healthy sections, ensuring each new division has a fan of leaves and a good set of roots attached to the rhizome. Trim the fan leaves back to a height of six to eight inches to reduce moisture loss and stress on the newly planted rhizome. Gardeners may apply a fungicide dust to the fresh cuts or allow them to air-dry for a day or two to form a protective callous before replanting. Trimming the old, long roots is also recommended to encourage the growth of new, vigorous roots.
Proper Replanting Techniques and Initial Care
Newly divided rhizomes require a planting location that receives at least six hours of direct sunlight daily, as insufficient light will limit flower production. Excellent soil drainage is equally important and can be improved by incorporating organic matter like compost into the planting bed. When planting Bearded Irises, create a small mound of soil within the planting hole. Center the rhizome over this mound with the roots spread out, ensuring the top of the rhizome remains at or slightly above the soil surface.
Planting the rhizome too deeply can lead to rot and prevent flowering. In regions with hot climates or extremely light soils, a thin layer of soil, no more than one inch, can be used to lightly cover the rhizome for protection. Rhizomes should be spaced 12 to 24 inches apart to allow for future growth and air circulation. After firming the soil around the rhizome, provide a thorough initial watering to settle the soil around the roots, then water sparingly until the plant shows active new growth.