Irises are popular flowers, but new gardeners often confuse their underground storage structures. While many assume all flowering perennials use bulbs, the most common Irises rely on a different structure. Popular Bearded Irises, known for their fuzzy lower petals, use a modified stem called a rhizome to store energy and survive winter. This structural difference dictates how the plant grows, is planted, and is cared for.
The Defining Structure of Most Irises
The majority of Iris varieties, including Bearded Irises (Iris germanica) and Siberian Irises, grow from a rhizome. A rhizome is a specialized, thickened stem that grows horizontally along or just beneath the soil surface. Unlike a true bulb, which is a modified leaf structure, the rhizome is a stem.
Rhizomes feature nodes, which are points along the stem where new shoots and roots emerge. This allows the rhizome to spread outward, producing new plant sections or “fans” of leaves, leading to dense clumps over time. The primary function of the rhizome is to store starches, proteins, and nutrients, enabling the plant to survive periods of dormancy, such as winter or drought.
For Bearded Irises, the fleshy rhizome must be planted shallowly, often with the top half exposed to the air. This shallow placement is necessary for the plant’s health. If the rhizome is covered too deeply, it can lead to rot and prevent proper flowering.
When an Iris Does Use a Bulb
While the rhizome is the defining structure for the most commonly grown Irises, a significant number of species grow from a true bulb. These are categorized as “Bulbous Irises” and use a different type of underground storage organ. The true bulb is a modified shoot, composed of layers of fleshy, underground leaves surrounding a central growing point, much like an onion.
Bulbous Irises are typically smaller and bloom earlier than their rhizomatous cousins, often providing color in early spring. Prominent examples include Dutch Irises (Iris hollandica), Spanish Irises (Iris xiphium), and Reticulata Irises (Iris reticulata). These bulbs are adapted for drier habitats and are planted deep beneath the soil, contrasting sharply with rhizomes.
The bulb’s structure efficiently stores the energy required for a rapid burst of growth and bloom early in the season. After flowering, the foliage typically dies back completely, and the plant enters a dormant phase during the summer months. This dormancy period distinguishes them from the rhizomatous varieties, which maintain their sword-shaped leaves longer.
Planting and Caring for Iris Rhizomes
Proper planting of an Iris rhizome is necessary for its long-term health and flowering success. The best time to plant or transplant rhizomes is in the late summer or early fall, allowing at least six weeks to establish a strong root system before the first hard frost. Newly planted rhizomes require a site that receives full sun, meaning a minimum of six to eight hours of direct sunlight daily.
The primary step is planting depth; rhizomes must be planted at or just barely below the soil surface. A common technique involves digging a shallow hole, creating a small mound of soil in the center, and positioning the rhizome on the mound with its roots spread out on either side. The top of the rhizome should remain partially visible or only lightly covered with soil.
This exposure to air helps prevent the fleshy rhizome from rotting in overly wet conditions. Good drainage is essential, and heavy clay soils should be amended with organic material before planting. Spacing the rhizomes 12 to 24 inches apart also promotes air circulation, reducing the risk of disease and maintaining bloom vigor.