Irises, recognized by their distinctive blooms, are hardy perennials that grow from thick, fleshy, underground stems called rhizomes. These rhizomes serve as food storage organs, sustaining the plant through dormant periods and enabling the emergence of new growth each year. Many gardeners seek to transplant or divide their irises to propagate new plants or to address overcrowding, which can lead to reduced flowering. The question of whether spring is an acceptable time for this process is common, as the plant is actively emerging from its winter rest. This article will focus on the implications and procedures for moving irises during the spring growing season.
Optimal Timing for Iris Division and Moving
The standard recommendation for transplanting and dividing rhizomatous irises, such as the popular bearded varieties, is late summer through early fall. This window typically occurs four to six weeks after the spring bloom has finished, generally spanning from July through September. Moving the plants during this period aligns with the iris’s natural semi-dormant cycle.
This timing is ideal because, after flowering, the iris rhizome is full of stored starches and proteins. The plant enters a phase focused on root regeneration rather than vegetative growth. This allows the newly separated rhizomes to quickly establish a robust root system in their new location before the ground freezes for winter. Furthermore, the drier conditions often found in late summer minimize the chance of bacterial soft rot, a common fungal disease that can infect and destroy the rhizomes.
Transplanting Iris During Spring Active Growth
While late summer is ideal, transplanting irises in the spring can be done if absolutely necessary. Spring is a period of intense active growth, which makes the plant more susceptible to stress from disturbance. Resources are being heavily directed toward pushing out new foliage and developing the current season’s flower stalks.
Uprooting the plant at this time causes a significant disruption to its water and nutrient uptake system, often resulting in transplant shock. The primary trade-off for a spring move is the near certainty of sacrificing the current year’s bloom. The plant must redirect all its energy away from flower production to healing its damaged root system and establishing itself. The milder spring temperatures can offer a benefit by reducing the risk of the rhizome drying out completely before roots can take hold.
Essential Steps for Spring Transplant Success
To minimize the severe shock of a spring transplant, handle the plants with care and prepare them properly. Begin by digging an ample circumference around the clump to preserve as much of the fine, feeder roots as possible. This is a departure from the fall method where roots are often trimmed. Gently lift the entire root ball and move it to a prepared site immediately.
Before replanting, cut the foliage back drastically, shaping the leaves into a fan that is only about one-third of their original height, or approximately 4 to 6 inches tall. This reduction in the leaf surface area reduces water loss through transpiration, allowing the rhizome to focus its limited resources on growing new roots. Plant the rhizome shallowly in the new bed, ensuring the top portion remains exposed to the sun and air, or is just barely covered with soil.
Immediate and Long-Term Care After Spring Transplant
Consistent and deep watering is the most important care requirement immediately following a spring transplant. Because the plant is actively growing, it requires more moisture than a fall-planted rhizome to support its existing foliage and encourage rapid root development. Water the newly planted rhizomes thoroughly to settle the soil around the roots. Maintain consistently moist soil for the first three to four weeks without allowing the area to become waterlogged, which can induce rot.
Avoid applying high-nitrogen fertilizers to spring transplants, as this encourages soft, leafy growth that is vulnerable to disease. Instead, a low-nitrogen fertilizer, such as a 6-10-10 formulation or a supplement like bone meal, supports root establishment without promoting excessive foliage. While you should not expect flowers this season, a successfully moved iris should settle in over the summer and reward you with a strong, full bloom display the following spring.