The genus Iris contains two distinct types of plants, meaning the question, “Is an iris a bulb?” does not have a simple yes or no answer. Irises are botanically classified by their underground storage organs, which can be either a true bulb or a thick, fleshy structure called a rhizome. This difference in their subterranean anatomy dictates their growth habit and specific planting and care requirements. Understanding this distinction is the first step toward successfully growing these popular flowering plants.
Understanding Iris Underground Structures
The difference between an iris bulb and a rhizome lies in their botanical classification as modified plant parts. A true bulb, like those found in tulips or onions, is a modified leaf structure, essentially a short, conical stem surrounded by layers of fleshy, scale-like leaves that store food reserves. Bulbous irises, such as the Dutch, Reticulata, and Spanish varieties, are typically smaller and bloom earlier in the season.
In contrast, a rhizome is a modified stem that grows horizontally, usually at or just beneath the soil surface. This specialized, creeping stem stores starches and proteins, and new growth, including roots and leaf fans, sprouts from nodes along its length. The majority of garden irises, including the well-known Bearded Irises (Iris germanica), as well as Siberian and Japanese Irises, are rhizomatous.
Planting and Maintenance for Rhizomatous Varieties
Rhizomatous irises, particularly the Bearded Irises, require a specific planting method that differs significantly from bulb planting. The thick, fleshy rhizome should be planted horizontally in the soil so that its upper surface is partially exposed to the sun and air. Planting the rhizome too deeply is a common mistake that can lead to rot and prevent the plant from flowering.
For planting, dig a hole and create a small mound of soil in the center; the rhizome is placed on this ridge with its roots spread out and facing downward. The top of the rhizome should be left barely visible, or at most, covered with no more than one inch of soil. They thrive in full sun, needing at least six to eight hours of direct light daily, and require well-draining soil to prevent the rhizome from sitting in moisture. Over time, rhizomes multiply and become crowded; to maintain healthy blooms, clumps should be dug up, divided, and replanted every three to five years.
Planting and Maintenance for Bulbous Varieties
Bulbous irises, which include varieties like Iris reticulata and Dutch Irises (Iris hollandica), are treated similarly to other spring-flowering bulbs. These varieties require deep planting, typically set three to five inches deep and spaced two to three inches apart. This depth protects the bulb and encourages consistent re-blooming.
The bulbs need a period of cold dormancy, or vernalization, to initiate flowering, which is why they are generally planted in the autumn. Bulbous irises demand well-drained soil, as excessive moisture can cause the bulb to rot. After flowering, the foliage must be left intact to die back naturally, allowing the leaves to photosynthesize and generate the energy needed for the underground bulb to form next year’s bloom.