The term “iris bulb” is frequently used when discussing the popular flowering plant, but for many garden varieties, this name is misleading. Irises grow from two fundamentally different underground structures: rhizomes and true bulbs. Knowing the physical appearance of the storage organ—which anchors the plant and holds its energy reserves—is crucial, as the structure determines how it must be planted for successful cultivation.
Distinguishing Between Iris Rhizomes and True Bulbs
Irises are classified into those that grow from rhizomes and those that grow from true bulbs. A rhizome is a modified underground stem that grows horizontally, often right at the soil surface, such as in the widespread Bearded Iris. In contrast, a true bulb is a modified bud consisting of layers of fleshy scale leaves wrapped around a central growing point, similar to an onion. These two structures require opposing methods of planting.
The visual difference is the most reliable way to identify the type of iris you are handling. Rhizomes are typically larger and more irregular, whereas true bulbs are smaller, globe-shaped, and possess a protective outer covering.
The Appearance of the Common Iris Rhizome
The most common garden varieties, such as the Bearded Iris, grow from a rhizome. This structure appears as a thick, fleshy, horizontal stem, not a true root. It typically measures several inches in length and is often knobby or segmented. When dormant, the surface is generally a light tan or yellowish-brown color, and the texture is dense and somewhat tuberous.
Fine, hair-like feeder roots sprout from the underside, anchoring the plant and drawing in moisture and nutrients. The top surface shows scars from previous leaf fans and often features visible growth points, or “eyes,” where new shoots emerge. The rhizome’s irregular, elongated shape is designed for expanding laterally near the soil surface.
Identifying True Iris Bulbs
True iris bulbs are found in varieties such as the Dutch Iris (Iris hollandica) and the Dwarf Iris (Iris reticulata). They present as a classic bulb shape, typically round or pear-shaped, and are considerably smaller than a rhizome, often only thumb-sized.
The structure is tunicated, meaning it is covered by a dry, papery outer skin, similar to a tulip or daffodil bulb. This tunic protects the inner layers of stored nutrients. The bottom of the bulb features a clear, flat basal plate where the true roots emerge. Unlike the horizontal rhizome, the true bulb is designed to grow vertically and remain buried deep beneath the ground.
How Appearance Guides Planting
The distinct appearance of these two structures directly guides successful planting orientation and depth. Because the rhizome is a fleshy stem, it must be planted horizontally and very shallowly. The top of the rhizome, where the growth buds are located, should be partially exposed to the sun or covered by only a thin layer of soil to prevent rot and encourage blooming.
Conversely, the true bulb must be planted with its pointed side facing upward, ensuring the root plate is directed down into the soil. The bulb’s layered structure requires it to be planted deep beneath the surface, typically four to six inches down, to provide insulation and proper anchoring. Planting a true bulb too shallowly or a rhizome too deeply are common mistakes, leading to desiccation or failure to flower.