The iris rhizome is a specialized, thickened stem that grows horizontally, often just beneath the soil surface, functioning as a storage vessel for starches and moisture. This fleshy structure enables the iris plant to survive periods of dormancy or harsh conditions by storing energy reserves. Gardeners often remove these structures when dividing overcrowded clumps, relocating plants, or receiving bare-root stock. Successfully replanting these stems depends entirely on managing the time they spend exposed to the air, making understanding their survival limits important.
The Immediate Survival Window
A freshly dug iris rhizome can typically survive outside of the soil for a short period, generally ranging from a few days up to one or two weeks under standard conditions. The primary threat is the rapid loss of internal water content through desiccation. Survival time hinges on the rhizome’s ability to retain enough moisture to remain firm before its cells dry out irreversibly. Longevity also depends on preparation, specifically whether the rhizome is newly lifted or properly cured. Curing involves drying the rhizome slightly for a week or two, allowing cuts to callus over and forming a protective outer layer that strengthens it against rot and extends its shelf life.
Environmental Conditions That Affect Viability
The speed at which a rhizome loses its ability to grow is influenced by the immediate atmospheric conditions. High levels of humidity and moisture are detrimental because excess external dampness encourages bacterial or fungal rot. Since the soft, fleshy tissue is highly susceptible to decay, planting rhizomes too deeply or storing them in wet conditions can be fatal.
Temperature also plays a significant role in determining survival time outside the ground. Elevated temperatures dramatically increase the metabolic rate and hasten water evaporation, causing a rhizome left exposed on a hot summer day to shrivel rapidly. Conversely, cooler temperatures slow down biological processes, which preserves stored energy and moisture within the stem structure.
Direct sunlight and strong air circulation quickly compromise the rhizome’s health. Placing a rhizome in direct sun subjects it to intense heat and ultraviolet radiation, which damages plant tissues. Excessive air movement, such as a strong breeze, accelerates the drying process by stripping away moisture vapor. A protected, shaded location is better for preserving viability than an open, windy spot.
Maximizing Survival Through Temporary Storage
If immediate replanting is not possible, steps can be taken to extend the rhizome’s temporary survival for a few months. The most effective technique involves placing the rhizomes in a cool, dark environment consistently maintained between 40°F and 50°F. This temperature range minimizes metabolic activity and moisture loss, making a cellar, garage, or refrigerator suitable.
It is helpful to provide the rhizomes with protective material that absorbs moisture but allows for air exchange. Wrapping individual rhizomes in newspaper or placing them in a cardboard box layered with peat moss or dry sand is suitable. Avoid using sealed plastic bags, as these trap moisture and restrict airflow, leading to fungal or bacterial infection.
Before storage, ensure rhizomes are clean, with leaves and roots trimmed back, and cut surfaces are dry. Periodically check the stored rhizomes, discarding any that feel soft or show signs of mold. Although they can be stored for several months, replant them at least six weeks before the first hard frost to allow for root establishment.