The common Iris is a popular garden flower, but planting this perennial often leads to confusion regarding the correct depth. Irises are diverse plants, and planting depth is the most important factor for success and abundant blooms. The confusion arises because the term “iris” includes plants with two fundamentally different underground structures: the rhizome and the true bulb. Understanding this distinction is the first step in ensuring correct planting.
The Critical Difference: Rhizome Versus Bulb
Irises are propagated from one of two storage organs, each requiring an opposite planting depth. A rhizome is a thickened, horizontal stem that grows at or just below the soil surface, such as those found on Bearded and Siberian Irises. This structure stores food and moisture, and needs exposure to sunlight and warmth to trigger flower bud formation for the next season. Conversely, a true bulb is a tightly packed, layered structure, similar to a miniature onion. True bulb irises, such as the Dutch Iris, require deep planting for protection against temperature extremes and physical damage.
Planting Depth for Bearded Irises (Rhizomes)
The most common garden variety, the Bearded Iris, grows from a rhizome that must be planted extremely shallowly. Planting too deeply is the most frequent cause of a failure to bloom, often leading to rhizome rot. The goal is to ensure the top surface of the rhizome is exposed to the sun or, at most, barely covered with soil.
To plant a rhizome, dig a shallow hole wide enough to accommodate the roots, typically about 10 inches across and four inches deep. Create a small mound of soil in the center of the hole, allowing the roots to be draped down the sides. Position the rhizome on top of this ridge, with the roots firmly anchored into the soil on either side.
Backfill the hole, gently firming the soil around the roots while leaving the top half of the rhizome exposed. In very light, sandy soils or extremely hot climates, you can cover the rhizome with up to one inch of soil to prevent sun-scald. The rhizome must be near the surface because it needs solar radiation, which helps “bake” the rhizome and induce the physiological changes necessary for flower production.
Planting Depth for Dutch and Other True Bulb Irises
True bulb irises, including Dutch, Spanish, and Reticulata varieties, require a traditional deep planting method to thrive. These bulbs need the soil’s insulation to protect them from freezing winter temperatures and intense summer heat. Planting depth is determined by the size of the bulb itself, following the rule of planting at a depth two to three times the bulb’s height.
For most Dutch Iris and similar bulbs, plant them four to six inches deep, measured from the base of the bulb to the soil surface. The pointed end of the bulb must face upward to allow the shoot to emerge correctly. This depth ensures the necessary cold period for flower activation and provides stability for the tall stems. Deep placement also helps prevent the bulbs from splitting into smaller, non-flowering daughter bulbs, which is common with shallow planting.
Maximizing Success: Timing, Soil, and Spacing
Timing and soil conditions are important factors for success, regardless of whether you are planting a rhizome or a true bulb. Planting is best done in the late summer or early fall (July through September) to allow roots at least six weeks to establish before the first hard frost. New root growth is essential for anchoring the plant and sustaining it through winter dormancy.
Good drainage is non-negotiable for all irises, as waterlogged conditions quickly lead to rot in both rhizomes and bulbs. If your garden has heavy clay soil, amend it with organic material like compost or gypsum to improve soil structure and porosity. For bearded rhizomes, space them 12 to 24 inches apart for good air circulation and to prevent overcrowding, which reduces flowering. True bulbs can be clustered closer together, generally three to six inches apart, to create a dense floral display.
After planting, water the area thoroughly to settle the soil around the roots and eliminate any air pockets. Unlike most garden plants, Bearded Irises should not be mulched directly over the rhizome, as this traps moisture and increases the risk of fungal rot. Maintaining these surrounding conditions alongside the correct planting depth will set the stage for a successful bloom season.