The iris is a popular perennial known for its elegant, colorful blooms. Achieving success relies heavily on the correct planting procedure, particularly the space provided between each plant. Proper spacing ensures adequate air circulation, which helps prevent common diseases like bacterial soft rot, and guarantees sufficient access to soil nutrients and sunlight for robust growth and flowering. Determining the correct planting distance begins with understanding the subterranean structure of the specific iris variety.
Understanding Iris Types and Their Growth
Irises are classified based on whether they grow from a rhizome or a true bulb, which directly influences how they spread and how far apart they should be planted. The most recognized type, the Bearded Iris, develops from a rhizome—a thick, fleshy, horizontal stem that grows at or just below the soil surface. This structure multiplies by sending out new growth shoots laterally, resulting in clumps that expand outward over time.
In contrast, Beardless Irises, such as Siberian or Dutch Irises, grow from true bulbs or deeper-set rhizomes. Dutch Irises grow from a bulb that multiplies vertically and less aggressively on the surface. Siberian Irises grow from thin, fibrous rhizomes that form dense, cohesive clumps rather than spreading widely. This vertical or tight clumping habit allows these varieties to tolerate closer planting arrangements than their bearded relatives.
The growth pattern dictates the need for surface exposure, particularly for Bearded Irises. Their spreading rhizomes require ample sunlight and air circulation directly on their surface to prevent rot and promote flower bud formation. Beardless types do not rely on sun exposure to the same extent, making them less sensitive to tight groupings and allowing them to be planted more closely together.
Calculating Planting Distance
The specific planting distance for irises is determined by the variety’s growth habit and its need for air circulation and light. For Bearded Irises, which grow from surface rhizomes, generous spacing is necessary to accommodate lateral expansion and ensure good ventilation. Position Bearded Iris rhizomes between 12 and 18 inches apart to allow for future growth and prevent crowding. Crowding can lead to reduced bloom count and the spread of fungal diseases.
Planting Bearded Irises too closely causes them to compete for resources and obstruct the sunlight required to “bake” the rhizome tops, which is important for flower bud formation. While 12 to 18 inches provides an immediate, visually full effect, a wider spacing of 18 to 24 inches can be used to prolong the period before division is required.
Beardless Irises, including Siberian, Dutch, and Japanese varieties, thrive with tighter spacing. Siberian Irises can be planted approximately 8 to 12 inches apart, as they form upright, dense clumps and do not rely on surface rhizome exposure for flowering. Dutch Irises, which are true bulbs, can be planted even closer, often just 2 to 4 inches apart, allowing for massed displays. When planting in groups, arrange them in triangular or circular clusters while maintaining the minimum spacing requirement from the center of one rhizome or bulb to the next.
Ensuring Proper Depth and Orientation
Beyond horizontal spacing, the vertical placement of the iris is equally important for health and flowering. For Bearded Irises, the planting depth is extremely shallow, as their rhizomes should be exposed to sunlight. Plant the rhizome so its top surface is either barely visible at the soil line or covered with no more than half an inch of soil.
Planting Bearded Iris rhizomes too deeply is a common error that inhibits flowering and predisposes the plant to rot due to trapped moisture. To plant, create a small mound of soil in the center of the hole. Center the rhizome on this mound with the roots spread downward and outward, orienting the fan of leaves to point away from existing clumps to direct new growth outward.
In contrast, true bulb irises, such as the Dutch Iris, must be planted deeper for stable root development and cold protection. These varieties require a planting depth of 3 to 5 inches below the soil surface. When planting any iris bulb, the pointed end should face upward, and the soil should be firmly tamped down around the roots before watering thoroughly.
When and Why to Divide Irises
Even with correct initial spacing, irises will eventually outgrow their area due to natural multiplication. Bearded Irises should be divided every three to five years, as the expanding network of rhizomes creates overcrowding. This crowding leads to a decline in the number and size of blooms and increases the risk of disease due to poor air circulation.
The optimal time to divide Bearded Irises is in the late summer or early fall, six to eight weeks after blooming. This timing allows replanted sections enough time to establish new roots before winter dormancy. The process involves lifting the entire clump, cutting the leaves back to one-third of their height to reduce transplant shock, and separating the healthy, firm outer rhizomes. Discard the older, spent, or diseased sections from the center, and replant the vigorous new divisions according to initial spacing requirements. Siberian Irises are more tolerant but should also be divided when vigor declines, preferably in early spring or late summer.