Gladiolus is a popular summer-blooming flower that grows from a specialized underground storage stem known as a corm. The practice of dividing these corms is necessary for propagation and maintaining plant vigor. Over the growing season, a new corm forms on top of the old one, often leading to the development of numerous small offsets called cormels. If left in temperate regions, corms can freeze or become overcrowded, causing a decline in flower size and overall plant health. Annually lifting and separating the corms ensures the continuation of healthy, robust blooms for the following season.
Timing the Lifting and Separation
The timing for lifting gladiolus corms is determined by the plant’s natural life cycle after blooming. After the flower spike fades, the foliage must be allowed to remain for six to eight weeks. During this time, the leaves continue photosynthesis, transferring energy to the newly forming corm beneath the soil. This energy transfer allows the new corm to mature, harden, and store nutrients required for the next year’s growth.
The corm is ready for lifting when the foliage naturally begins to yellow and die back, ensuring maximum energy storage. The lifting process must be completed before the first hard frost, which can damage the tender corms and compromise viability. In colder temperate regions (USDA Zones 7 and below), overwintering corms in the ground is not feasible due to the risk of rot and freezing, making this fall process mandatory.
Gardeners in warmer zones (USDA Zones 8 through 10) may leave corms in the ground year-round, but lifting and dividing every few years prevents congestion and disease. When lifting, use a garden fork to gently loosen the soil well away from the stem to avoid damage. Once the clump is loosened, cut the foliage back, leaving a short stub of one to two inches attached to the corm.
Step-by-Step Corm Division
Division can begin after the corms are lifted and excess soil is brushed away, often following a short drying period. The corm structure consists of the new, healthy corm developed on top and the shriveled, depleted old corm attached underneath. The old corm expended its stored energy to produce the plant and new corm, and must now be removed.
Separate the new corm from the old by gently twisting or breaking them apart at the point of attachment, easier once the corms have dried. The old corm is dark, flat, and withered, and should be discarded because it provides no benefit and can harbor diseases. Small cormels, which are tiny offsets clustered around the base of the new corm, should also be removed.
Cormels can be saved and grown separately to propagate new plants, though they typically require two to three years to reach flowering size. If slicing is needed to separate multiple growing points, use a clean, sharp knife. The cut surface must be allowed to dry and heal completely before storage to prevent the entry of pathogens. Only firm, disease-free corms should be kept; discard any soft, moldy, or shriveled material immediately to prevent the spread of infection.
Curing and Preparing for Storage
After division, curing and preparation ensure the corms survive winter dormancy. Curing is a drying process that allows the corm to form a protective, papery outer layer, effectively sealing the surface and reducing the risk of rot during storage. Spread the corms in a single layer in a warm, dry, well-ventilated area out of direct sunlight, ideally between 70 and 80 degrees Fahrenheit.
This curing period lasts two to three weeks, and corms should be turned occasionally for even drying. Once the outer husk feels firm and papery, the corms are ready for final treatment. Applying a light dusting of fungicide or insecticide powder protects the corms from mold, storage rot, and pests like thrips.
For successful winter storage, corms require cool, dry, and dark conditions, ideally maintained between 35 and 50 degrees Fahrenheit. Adequate air circulation is essential, so storage containers must permit airflow, such as mesh bags, cloth sacks, or cardboard boxes. Check the corms monthly throughout the winter, and promptly remove any corm exhibiting signs of decay or mold to safeguard the remaining stock.