A corm is a swollen, underground stem base that functions as a storage organ for food and nutrients, allowing the plant to survive periods of dormancy. Unlike a true bulb, which is composed of fleshy, layered leaves, a corm has a solid, internal structure often protected by a papery covering called a tunic. Plants like gladiolus, crocus, and freesia grow from corms, producing a new corm on top of the old one each season. Propagating these structures is an effective and economical way to increase your plant stock and ensure a greater abundance of blooms. This process involves harvesting, separating, curing, and storing the new corms and their offsets.
Timing and Harvesting the Parent Corms
The proper timing for harvesting corms is directly linked to the plant’s natural life cycle and its ability to store energy for the next growing period. Corms should be lifted from the soil only after the plant’s foliage has completely yellowed and died back, typically in late summer or early autumn. This natural dieback signals that the corm has finished its active growth and successfully transferred maximum food reserves into the underground storage organ. Harvesting before this point can result in immature corms that will not store well or bloom reliably.
It is important to complete the harvest before the first hard frost, which can damage the corms and lead to rot in storage. Use a garden fork, rather than a spade, to gently lift the corm clump, starting several inches away from the stem to avoid piercing the storage tissue. Once lifted, gently shake off the majority of the loose soil, taking care not to scrape or bruise the protective outer tunic. The remaining foliage should be trimmed back to within one to two inches of the corm, and the old, dried roots at the base can be clipped away.
Methods for Multiplying Corms
The most common method for increasing stock is through the natural production of cormels, which are small, secondary offsets that form around the base of the new corm. These cormels are essentially miniature versions of the parent corm and can be easily snapped off by hand once the main corm has been cleaned. Cormels represent the future generation of plants, but they will not bloom in the first season after planting due to their small size and lower stored energy reserves. They usually require one to three growing seasons to reach a mature, flowering size.
Separating the new corm from the shriveled, depleted parent corm is also a necessary step in propagation. The old corm, located directly beneath the new one, will be dry and brittle and can be discarded, as it will not grow again and only serves as a potential source for disease if left attached. For very large corms, division or “chipping” can be performed to create multiple new plants from a single specimen. This technique requires a clean, sharp knife to cut the corm into two or more pieces, ensuring each section retains at least one visible growth bud, or “eye.”
Cutting the corm creates a large, open wound, so the freshly cut surfaces must be treated immediately to prevent fungal and bacterial infections during the curing and storage phases. Dusting the cut surface with a commercial fungicide powder or a natural alternative like sulfur powder or cinnamon helps to seal the wound and inhibit the entry of pathogens. This protective layer ensures the survival of the propagule, especially since the corm’s internal tissue is highly susceptible to rot.
Curing, Storage, and Replanting the Propagules
After harvesting and separating the corms and cormels, they must undergo a curing period to heal any minor wounds and dry the outer layers. Curing involves spreading the propagules in a single layer in a warm, dry, and well-ventilated location, such as a garage or shed, away from direct sunlight. The ideal temperature range for this process is typically between 60 to 70 degrees Fahrenheit, and it should last for a minimum of one to three weeks. Curing allows the outer tunic to toughen, which significantly reduces the risk of rot during winter storage.
Following the curing process, non-hardy corms must be prepared for winter storage in a cool, dark environment that maintains a consistent temperature. A storage temperature between 35 and 45 degrees Fahrenheit is optimal to keep the corms dormant without freezing them. High humidity can cause mold and rot, while overly dry conditions can lead to shriveling, so a moderate humidity level is necessary. Corms should be stored in materials that permit air circulation, such as mesh bags, paper sacks, or open cardboard boxes layered with packing materials.
The final stage of the propagation cycle is replanting the stored stock the following spring, once the danger of frost has passed and the soil has warmed. The newly propagated cormels and divisions should be handled with care and planted with the growing point facing upward. Cormels require shallower planting, often only an inch or two deep, and can be placed closer together in a nursery bed to grow into flowering size. Mature corms and larger divisions should be planted at the standard depth, which is typically four to six inches, to provide stability for the tall flowering stalks.