Gladiolus flowers are a summer garden favorite, but because they are tender plants, they will not survive the freezing temperatures of winter in most climates. The underground storage structure that produces the dramatic flower spikes is technically a corm—a solid, vertical underground stem—though it is often mistakenly referred to as a bulb. Unlike true bulbs, the gladiolus corm is a solid mass of stored nutrients that must be lifted from the soil and properly stored to ensure a vibrant display the following season. This process protects the plant’s energy source from freezing and excessive moisture.
Preparing Corms for Winter
The timing for removing the corms from the ground is important for their survival and depends on the plant’s natural cycle. Wait to dig until the foliage has begun to yellow and die back naturally, which allows the corm to absorb maximum energy for the dormant period. This usually occurs about six to eight weeks after the plant has finished blooming.
In colder regions, lift the corms shortly after the first light frost, as this kills the foliage without damaging the corm. Waiting too long risks a hard freeze, which can fatally injure the underground storage tissue. Once ready to dig, the green stalk should be cut back, leaving a stub of about one to two inches above the soil line for easier handling.
Digging Up and Initial Processing
The physical removal of the corms from the soil requires careful technique to prevent bruising, which can invite disease. Use a garden fork or spade, inserting it about six inches from the stalk to loosen the surrounding soil before gently lifting the whole clump. Avoid pulling directly on the remaining stalk, as this can separate the corm from the stem prematurely.
After lifting, gently shake off the majority of the clinging soil, but avoid washing the corms with water at this stage. The new, healthy corm has grown on top of the shriveled corm from the previous season. The spent mother corm should be carefully detached from the base of the new corm and discarded. Small, bead-like cormlets may be found clustered around the base; these can be saved for propagation, though they will take two to three years to reach flowering size.
Curing and Inspection
Curing is a two- to three-week phase that prepares the corms for long-term storage by drying the outer layers. This process encourages the formation of a protective, papery husk, which seals the corm and prevents rot during dormancy. The ideal curing environment is warm, dry, and well-ventilated, with a temperature range between 70°F and 80°F.
Spread the corms out in a single layer on a screen or flat surface, ensuring good air movement around them. After curing, inspect the corms for signs of damage, disease, or pests. Discard any corms that feel soft, mushy, or show visible signs of mold or rot, as these issues can spread to the healthy stock during storage. A light dusting with an all-purpose fungicidal powder is often recommended at this stage to prevent fungal infections, such as Fusarium, from developing over the winter.
Maintaining Dormancy: The Winter Environment
The final step is establishing the proper environment to maintain the corms’ dormancy throughout the winter months. The storage location needs to be cool and dark, with a consistently stable temperature between 35°F and 45°F. This temperature range prevents the corms from either freezing or prematurely sprouting before spring planting.
Low humidity is also necessary to discourage the growth of mold and mildew. Corms should be stored in containers that allow for adequate air circulation, such as mesh bags, paper sacks, or cardboard boxes lined with materials like vermiculite or peat moss. Check the stored corms monthly and promptly remove any that have become soft or moldy to safeguard the rest of your stock.