Gardeners often face delays when planting iris roots, whether after dividing overcrowded clumps or receiving new plants. The viability of these underground structures outside of the soil depends entirely on the environment and the specific type of iris. Understanding how long the plant can survive requires recognizing the difference between the two main types of iris root systems.
Iris Structure: Bulbs vs. Rhizomes
Irises grow from two distinct types of underground storage organs: true bulbs and rhizomes. A true bulb, like those found in Dutch Iris or Iris reticulata, is a compact, layered structure similar to an onion. These bulbs are designed for a dormant, dry period and possess a protective outer tunic, which helps them resist desiccation and store for longer periods.
The more common Bearded Iris grows from a rhizome, which is a modified, fleshy underground stem that grows horizontally near the soil surface. This structure is much less protected than a true bulb and is highly susceptible to losing moisture and drying out quickly when exposed to air. Rhizomes are also prone to bacterial soft rot if they remain wet or are stored without adequate air circulation.
Determining the Maximum Time Out of Ground
The maximum time an iris can remain out of the ground is influenced by its structure and the surrounding conditions. A newly dug rhizome, such as a Bearded Iris, should ideally be planted immediately, but can generally survive for a period ranging from a few days to a few weeks if stored properly. However, if a freshly dug rhizome is exposed to direct sunlight, high heat, or strong air circulation, it can dehydrate and become non-viable within 24 to 48 hours.
Optimally stored rhizomes, kept cool and dry, can maintain viability for several weeks. The primary threat to rhizomes during storage is desiccation or rot, which is accelerated by high temperatures and fluctuating humidity. A firm texture indicates a healthy rhizome, while a soft or mushy feel suggests the onset of rot, necessitating immediate discarding.
Conversely, true iris bulbs are naturally suited for dry dormancy and can be stored for significantly longer periods, often weeks to several months, similar to tulips or daffodils. Species like Iris reticulata are often stored through the summer for replanting in the fall. This longer viability window is related to the protective layers and the lower moisture content of the bulb structure compared to the fleshy rhizome.
Essential Steps for Short-Term Storage
For rhizomes that cannot be planted immediately, preparation begins with a drying, or curing, process. After digging, the foliage should be trimmed back to a few inches. The rhizomes should then dry in a warm, dark, and well-ventilated area for one to two weeks. This curing allows the outer layer to harden and helps prevent soft rot.
After curing, store the iris rhizomes in a cool, dark, and dry location with consistent temperatures, ideally between 40°F and 50°F. Proper air circulation is paramount to prevent moisture buildup. Containers like shallow cardboard boxes with breathing holes or open-weave plastic bins are superior to sealed containers. Placing the rhizomes in a single layer also maximizes airflow.
Managing Humidity and Rot Prevention
To manage humidity, each piece can be individually wrapped in newspaper or placed on a layer of dry, breathable material like peat moss, sawdust, or wood shavings. It is important to avoid using airtight plastic bags, which trap moisture and create an environment where pathogens can quickly cause rot. Stored rhizomes should be checked every few weeks to ensure they remain firm and to remove any pieces showing signs of rot, preventing the spread of disease.