The common garden iris, particularly the bearded variety, grows from a rhizome, which is a thickened, horizontal stem that stores nutrients. Although often called an “iris bulb,” understanding the plant’s structure as a rhizome is important for successful division and replanting. This separation process is necessary maintenance that ensures the continued health and abundant flowering of the plant. This guide provides steps for rejuvenating your iris plantings by correctly dividing and establishing new rhizomes.
Why and When to Divide Iris Clumps
Irises require division approximately every three to five years because the rhizomes multiply and become overcrowded. This congestion restricts air circulation and nutrient availability, leading to a reduction in blooms. Over time, the older, central rhizomes become woody and spent, often resulting in a “doughnut” pattern where only the outer edges produce healthy foliage and flowers.
The ideal time for this maintenance is in the late summer, typically from July through September, about four to six weeks after the main flowering period. Dividing the plants during this late summer dormancy allows the newly separated rhizomes time to establish a robust root system before winter. This timing also minimizes the risk of bacterial soft rot, which can be a concern if the process is done when the plant is actively growing.
The Step-by-Step Separation Process
Gather the necessary tools, which include a garden fork or spade, a sharp, sterilized cutting tool (knife or pruners), and a 10% bleach solution for sanitation. Use the garden fork to carefully lift the entire iris clump from the ground, inserting the tool several inches away from the base to avoid damaging the shallow-rooted rhizomes. Once lifted, gently shake or hose off the excess soil to expose the rhizomes for clear inspection.
Examine the clump to identify and discard the older, non-blooming sections, which are often woody, soft, or mushy. Keep the younger, healthy rhizomes, which are firm, light-colored, and have a fan of leaves attached. Separate these healthy sections, known as “fans,” by gently pulling them apart at the joints or using the sterilized knife to cut them, ensuring each division retains a good set of roots.
If using a cutting tool, dip the blade into the bleach solution between cuts to prevent the spread of diseases like bacterial soft rot. Each viable division should be approximately the thickness of a thumb and possess at least one healthy leaf fan. Finally, trim the leaves into a fan shape, reducing their height to four to six inches; this minimizes moisture loss through transpiration while the plant focuses its energy on root development.
Replanting and Post-Division Care
Select a replanting site that receives a minimum of six hours of direct sunlight daily and offers well-draining soil. Irises are highly susceptible to rot in overly wet conditions, so amend heavy clay soil with compost to improve drainage and aeration before planting. Dig a shallow hole, about four inches deep, and create a small mound of soil in the center.
Position the rhizome over this central mound, carefully spreading the roots down the sides into the surrounding trench. The top surface of the rhizome should be partially exposed to the sun or just barely covered with a thin layer of soil. Planting the rhizome too deeply is a common mistake that inhibits flowering and increases the risk of rot.
Space the individual rhizomes 12 to 18 inches apart to ensure adequate air circulation and allow room for future growth before the next required division. After planting, water the area thoroughly to settle the soil around the roots. Maintain consistent moisture for the first few weeks to encourage new root establishment, but avoid overwatering.
Do not apply mulch, as it can trap moisture against the rhizome and encourage rot. While the newly planted divisions are establishing, they may produce sparse or no blooms in the first year. They will be rejuvenated for a full display in subsequent seasons.