The striking, vibrant blooms of the iris make it one of the most celebrated perennial flowers. Achieving a successful display requires precise planting technique. Proper spacing, orientation, and depth determine the health of the plant and the abundance of its flowers. Understanding these requirements ensures the plants thrive in their chosen location.
Understanding Iris Types and Planting Timing
Iris plant material is divided into two distinct groups, though the term “iris bulb” is often used loosely. True bulbous irises (e.g., Dutch, Spanish, English varieties) grow from a nutrient-storing structure and must be planted in the fall. These bulbs require cold dormancy over winter to initiate spring growth and flowering.
The most widely grown irises, Bearded Irises, grow not from a true bulb but from a thickened, horizontal stem known as a rhizome. The rhizome serves a similar storage function but requires a different planting approach. Planting bearded iris rhizomes is best done in late summer or early fall, typically from July through September. This timing allows the roots to fully establish themselves before the ground freezes.
All irises demand a planting site that receives at least six hours of direct sunlight each day for optimal blooming. Well-draining soil is necessary, especially for rhizomatous types, which are highly susceptible to rot. Poorly draining, heavy soil can quickly lead to decay. If the existing soil is heavy clay, amending it with compost or grit before planting improves drainage and aeration.
Critical Spacing Requirements
Spacing between individual iris plants is the most important factor for long-term health and flower production. The required distance differs substantially between the two main types, reflecting their distinct growth habits. Planting irises too closely together leads to resource competition and poor air circulation, which reduces blooms and increases the risk of fungal disease.
Rhizomatous irises, such as the Bearded Iris, require generous spacing because they spread horizontally. These plants should be spaced approximately 12 to 24 inches apart (12 to 18 inches is a common recommendation). This distance is necessary to allow for adequate air movement around the rhizome, which helps prevent the development of soft rot. If planted closer for a dense visual effect, the clumps must be divided and replanted more frequently, typically every two to three years.
Bulbous irises, which include Dutch and Iris reticulata, can be planted much closer together to achieve a massed, clustered effect. These true bulbs should be spaced about 3 to 6 inches apart, depending on the size of the bulb. For smaller varieties like Iris reticulata, a spacing of approximately 4 inches is common. Since these bulbs grow vertically and do not spread laterally like rhizomes, they tolerate tighter proximity.
The Mechanics of Planting: Depth and Orientation
The placement of an iris in the soil is as significant as the distance between plants. For Bearded Iris rhizomes, planting depth is extremely shallow to ensure the structure receives warmth. The top surface of the rhizome should be barely covered with soil or, ideally, slightly exposed. Planting the rhizome too deep is a common error that can result in the plant failing to bloom.
When preparing the planting site for a rhizome, dig a shallow hole and form a small mound of soil in the center. The rhizome is centered on this mound, and its long feeder roots are spread out and downward along the sides of the mound. After covering the roots and firming the surrounding soil, the top of the rhizome should remain visible or covered by no more than half an inch.
In contrast, true iris bulbs require a deeper planting depth similar to other spring-flowering bulbs. They should be planted with the pointed side facing upward, ensuring the growing tip is correctly oriented. A general rule is to plant the bulb at a depth two to three times its height (typically 2 to 5 inches deep). Once planting is complete, a thorough initial watering settles the soil around the structures and eliminates air pockets.