Irises are popular garden perennials, highly valued for their vibrant, showy blooms that arrive in the spring and early summer. The tall bearded iris (Iris germanica) is known for its carefree nature and ability to thrive in a variety of conditions. Irises are prolific multipliers, creating dense clumps that quickly expand their presence in a garden bed.
The Primary Mechanism: Multiplication via Rhizomes
The rapid spread of the bearded iris is due to its specialized growth structure, known as a rhizome. A rhizome is a thickened, horizontal stem that grows along or just beneath the soil surface, acting as a storage organ for starches and proteins. This structure allows the plant to survive harsh conditions and quickly fuel new growth.
New iris plants are produced asexually when smaller, lateral shoots sprout from the sides of the original “mother” rhizome. These new offshoots, often called “fans,” develop their own roots and eventually become independent plants. This vegetative reproduction process allows the iris to create genetically identical copies, leading to the formation of dense, expanding clumps.
The energy needed for this prolific multiplication is generated after the blooming season concludes. While irises can be grown from seed, the primary method of increase in the garden is through the steady, lateral extension of the rhizome structure. This continuous growth ensures the iris bed fills out quickly, requiring regular maintenance to maintain plant health and flowering performance.
Signs That Iris Clumps Need Dividing
Although the iris’s ability to multiply is beneficial for filling out a garden, this vigorous growth eventually leads to overcrowding, signaling the need for intervention. The most noticeable indication that a clump requires division is a significant reduction in the number of flowers produced. A healthy iris that stops blooming or produces sparse flowers is likely struggling for resources within a congested space.
Visual cues also help diagnose an overcrowded bed, with the most telling sign being the crowding of the rhizomes themselves. These surface-level stems may begin pushing each other upward and out of the soil as they compete for space, sometimes forming a visible mat. Older, established clumps often exhibit a “doughnut hole” appearance, where central plants have died out, leaving a ring of newer, healthier growth on the perimeter.
Failing to divide the plants every three to five years increases their susceptibility to pests like the iris borer and diseases such as bacterial soft rot. The dense nature of an old clump restricts airflow and creates moist, shaded conditions that favor these issues. An inspection might reveal soft, mushy, or discolored rhizome sections, indicating declining health due to overcrowding and subsequent disease.
Practical Steps for Dividing and Replanting
The ideal time to divide bearded irises is in mid-to-late summer, four to six weeks after the flowering period ends, allowing the newly separated plants enough time to establish roots before winter. Begin by using a garden fork to carefully lift the entire clump out of the soil, digging a few inches away from the rhizomes to avoid damage. Once the clump is lifted, shake off the loose soil or rinse it away to expose the rhizome structure.
Next, use a sharp, sterile knife or pruning shears to separate younger, healthier rhizomes from the old, central “mother” rhizome, which should be discarded. Each viable division should have a firm, creamy-colored section of the rhizome, attached roots, and a fan of leaves. Before replanting, cut the fan leaves back to about one-third of their height, leaving a sharp, inverted “V” shape. This trimming minimizes moisture loss and reduces the risk of the plant being rocked loose by wind.
To replant, select a site with full sun and well-draining soil, then dig a shallow hole large enough to accommodate the roots. Create a small mound of soil in the center of the hole and drape the roots over this mound. The rhizome should be placed with its top surface just at or slightly below the soil line, as planting too deeply encourages rot. Space the new divisions 12 to 24 inches apart, with the leaf fans pointing away from each other, and water thoroughly to settle the soil.