The Dutch Iris (Iris hollandica) is a popular spring-blooming plant valued for its elegant, orchid-like flowers. Unlike the Bearded Iris, which grows from a rhizome, the Dutch Iris is a true bulb. This bulb structure functions as a subterranean storage organ, central to its life cycle and propagation. Dutch Iris bulbs do multiply, which is how the plant ensures its survival and expansion.
The Mechanism of Bulb Increase
Dutch Iris bulbs propagate through vegetative reproduction by forming new offset bulbs, or bulblets. The original mature bulb, often called the mother bulb, is typically expended after flowering. Before withering, the mother bulb initiates the growth of these offsets around its basal plate, the structure where roots emerge. These offsets are miniature clones, developing over the growing season using energy captured by the foliage.
When the foliage dies back, signaling dormancy, the new bulbs are mature enough to separate. The mother bulb is replaced by a cluster, usually including one or two flowering-size bulbs and several smaller bulblets.
Separating and Replanting New Bulbs
To encourage continuous blooming, lifting and separating the resulting bulb clumps is necessary every few years. The optimal time for this is in late summer or early autumn, after the foliage has naturally withered and turned yellow. This timing ensures the bulbs have entered their dormant phase. Carefully lift the entire clump from the soil using a fork, taking care not to damage the bulbs.
Once the clumps are out of the ground, gently separate the bulblets from the remnants of the mother bulb and from each other. Overcrowding underground is a primary reason for diminished flowering, as the bulbs compete intensely for nutrients and space. After cleaning off any loose roots and dried skins, the new bulbs should be replanted promptly in the fall.
The larger bulbs should be replanted four to six inches deep and four inches apart. Smaller bulblets will not flower immediately and can be planted slightly shallower in a dedicated nursery bed. This allows them to mature for a year or two until they reach the necessary size to produce a flower stalk. If temporary storage is needed, keep the bulbs in a cool, dry, and well-ventilated space until the autumn planting window.
Environmental Factors Affecting Success
Dutch Iris bulbs multiply reliably, but many new bulblets fail to bloom the following spring because they are too small. A bulb must reach a threshold diameter, typically around 8 centimeters in circumference, to store enough energy for flower production. Several environmental factors dictate whether these offsets successfully mature into flowering-size bulbs.
Soil drainage is a significant requirement, as the bulbs are susceptible to rot in waterlogged conditions. The planting site must offer excellent drainage, and amending heavy or clay soils with organic matter like compost is beneficial. Adequate nutrition is also necessary; a balanced, low-nitrogen fertilizer supports bulblet growth, though fresh manure should be avoided as it promotes only leafy growth. Consistent moisture during the active growth period helps the developing bulbs accumulate the reserves needed to form next year’s flower bud.