The iris is prized for its intricate structure and wide color spectrum. Achieving successful blooms and ensuring the longevity of the plant hinges upon correct planting practices. Determining the appropriate depth for the subterranean storage organ is the single most important factor, and the method depends entirely on the specific type of iris being cultivated.
Distinguishing Iris Types
Iris planting instructions differ based on the two distinct types of root structures they possess. The first category, including the popular Bearded Iris, utilizes a specialized, fleshy, horizontal stem known as a rhizome. This modified stem grows along or just beneath the soil surface, serving as both a storage unit and a point for new growth.
The second category encompasses true bulb varieties, such as Dutch, Spanish, and English Irises. These plants form a layered, onion-like structure composed of concentric fleshy scales attached to a basal plate. Rhizome-based irises require very shallow planting, while true bulb varieties demand a significantly deeper placement within the soil. Understanding this fundamental anatomical distinction provides the necessary context for successful planting.
Planting Bearded Iris Rhizomes
Bearded Iris storage organs require exposure to sunlight and warmth to set flower buds. Gardeners must select a location that receives full sun and has excellent drainage to prevent moisture retention around the rhizome. The planting hole should be wide enough to allow the roots to spread out over a small mound of soil, positioning the rhizome directly on top of the mound.
These rhizomes must be planted incredibly shallowly. In warmer climates, the top half of the rhizome is often left completely exposed to the sun and air. In regions with intense heat or very cold winters, the rhizome can be minimally covered with no more than a half-inch to one inch of soil. The goal is to ensure the rhizome is anchored but not buried, allowing solar radiation to reach the top surface.
Proper spacing is also important for air circulation and future growth, requiring a distance of 12 to 18 inches between individual rhizomes. When placing multiple rhizomes, orient them so the fan of leaves faces away from the neighboring plant’s growth direction. This arrangement ensures that as the plants multiply and spread, they do not immediately crowd one another, promoting better flowering for several seasons.
The shallow planting depth is necessary for the physiological process that initiates blooming. A common technique is to plant the rhizome so the soil just barely covers the slender feeder roots, leaving the main body of the rhizome partially visible. This method supports the plant while fulfilling its sun-exposure requirement.
Planting Bulbous Iris Varieties
True bulb irises require deep planting to ensure both stability and protection. The typical requirement for Dutch, Spanish, and English Iris bulbs is a depth of three to five inches, measured from the top of the bulb to the soil surface. This depth anchors the bulb firmly against wind and rain, preventing the plant from being easily dislodged once the foliage emerges.
Planting the bulbs at this depth provides a buffer against rapid temperature fluctuations in the upper soil layers. This insulation protects the developing root system and new shoot from damaging freeze-thaw cycles, especially during the overwintering period. These bulbs also perform best in a slightly sandy loam soil that allows for quick water percolation away from the bulb surface.
For proper display and grouping, these bulbs can be planted much closer together than their rhizomatous cousins, typically spaced about three to four inches apart. Dig a hole to the required depth, place the bulb with the pointed end facing upward, and then gently backfill with soil.
Avoiding Common Planting Errors
The most frequent error when planting Bearded Iris rhizomes is placing them too deeply, which has direct and negative consequences on the plant’s health. Burial inhibits the necessary exposure to warmth, often leading to a phenomenon known as “blind” iris, where the plant produces foliage but fails to set flowers. Furthermore, burying the fleshy rhizome in moist soil encourages rot and fungal diseases, which can quickly destroy the plant.
Conversely, planting true iris bulbs too shallowly exposes them to various risks. A shallow placement can lead to the bulbs drying out quickly in warm weather, stressing the plant and diminishing flower quality. It also leaves them susceptible to being heaved out of the ground by frost or disturbed by small animals. Ensuring excellent drainage for all iris types, and avoiding the application of heavy mulch directly over Bearded Iris rhizomes, are preventive measures that support their long-term success.