Gladiolus, with their towering flower spikes, are a classic feature of the summer garden, offering a dramatic array of colors. These plants grow from underground structures known as corms, which are thickened stem bases that store energy. Most gladiolus cultivars are tender and require special handling after blooming to ensure a healthy display the following year. Successfully overwintering corms involves a multi-step process: encouraging the corm to recharge its energy, removing it from the ground, preparing it for dormancy, and storing it under specific conditions. This care is essential for gardeners in cooler climates where corms would not survive freezing temperatures if left in the soil.
Cutting Back the Flower Spike and Foliage
The first step after the flowers fade is to remove the spent flower spike, a process often called deadheading. Cut the stalk just above the highest remaining leaf. Removing the spent spike prevents the plant from diverting energy into seed production, focusing that energy instead on recharging the corm below the soil line.
The remaining green foliage must be left intact after the flower spike is cut down. These leaves perform photosynthesis, converting sunlight into sugars that are transferred down to the corm for storage. Cutting the foliage prematurely will starve the corm, resulting in a smaller and weaker flower spike the next season. The leaves should only be cut back to about one or two inches above the neck of the corm once they have naturally yellowed and begun to die back, which usually takes six to eight weeks after the last bloom.
Timing and Technique for Lifting Corms
Once the foliage has fully withered and turned yellow or brown, the corms are ready to be lifted for winter storage. This should occur before the first hard frost penetrates the soil, as frost can damage the corms and compromise their viability for the next season.
In regions prone to freezing winters (colder than USDA Zone 7), lifting the corms is mandatory to prevent them from rotting in cold, wet soil or being destroyed by frost. Use a garden fork rather than a spade to gently loosen the soil several inches away from the base of the stem. This technique minimizes the chance of accidentally spearing or bruising the corm, which can introduce pathogens and cause rot during storage.
After loosening the soil, the entire plant can be lifted carefully by grasping the remaining stem and pulling the corm from the ground. Leave a small piece of the stem attached—around one to two inches—to act as a handle during the initial drying process. For gardeners in warmer climates (Zone 8 and above), corms can often remain in the ground year-round, though lifting is sometimes recommended every few years for division and inspection.
Curing and Cleaning Corms for Dormancy
Immediately following lifting, the corms must be cured to prepare them for long-term dormancy. Curing involves allowing the corms to dry in a warm, well-ventilated location, away from direct sunlight, for one to three weeks. An ideal curing environment maintains a temperature between 70°F and 85°F (21°C and 29°C). This warm, dry period allows surface wounds to heal and helps prevent the spread of fungal diseases.
Once the corms are fully cured, the cleaning process begins. Brush off withered roots and any remaining loose soil, but leave the protective husks intact. The old, shriveled mother corm will be attached to the base of the newly formed, larger corm. This spent mother corm should be gently snapped or peeled off and discarded, as its energy is depleted.
Small, bead-like baby corms, known as cormels, may also be attached. These cormels can be separated and saved for propagation, though they require several seasons of growth before they are large enough to produce flowers. If fungal or rot issues were a concern, the cleaned corms can be lightly dusted with a commercial fungicide powder before storage to protect them from pathogens.
Winter Storage Conditions
The final step is placing the cleaned and cured corms into a dormant environment until spring planting. The storage area must be cool, dark, and dry, with sufficient air circulation to prevent moisture buildup and mold growth. The optimal temperature range for storage is between 40°F and 50°F (4°C and 10°C), though temperatures as low as 35°F (2°C) are acceptable, provided the corms do not freeze.
A cool basement, a root cellar, or an unheated, insulated garage often provides the necessary conditions. The corms should be stored in a medium that allows air to pass freely around them, such as:
- Mesh bags
- Paper bags
- Old nylon stockings
- Cardboard boxes layered with newspaper or peat moss
Never store them in airtight plastic containers, which trap moisture and encourage rot. Check the stored corms monthly throughout the winter, removing any that show signs of shriveling or developing soft, mushy spots indicative of rot.