The gladiolus flower is a popular sight in summer gardens, though it is often incorrectly referred to as a bulb. These spectacular plants actually grow from a swollen underground stem known as a corm, which functions as a storage organ for starches and nutrients. Whether these corms can remain in the ground through the winter depends entirely on the climate conditions of the planting location. Corm survival is directly tied to the severity of the winter cold and the amount of moisture present in the soil during dormancy.
Climate Factors and Overwintering Zones
The primary factor determining if gladiolus corms must be lifted is the lowest temperature reached during winter. These tropical natives do not possess the cold hardiness required to survive freezing. Temperatures consistently dropping below 28°F will cause irreversible damage and lead to the death of the corm tissue.
Gardeners in USDA Hardiness Zones 8 and warmer generally have a climate mild enough to safely leave their corms in the ground year-round. In these regions, the soil temperature does not drop low enough to freeze the corms. Conversely, those cultivating gladiolus in Zone 7 and colder must remove the corms before winter arrives. The risk in colder areas is two-fold: the corms can be killed directly by freezing or succumb to rot from excessive moisture in cold, poorly drained soil.
Steps for Leaving Corms In-Ground
Even in warmer climates, some preparation is necessary to ensure successful overwintering. Once the foliage has yellowed and begun to die back naturally in the fall, it should be cut down. This signals that the plant has finished transferring energy back into the corm for next season’s growth.
A thick layer of protective mulch is highly beneficial, acting as an insulating blanket against unexpected cold snaps. Applying four to six inches of straw, chopped leaves, or shredded bark helps stabilize the soil temperature. This insulating layer is particularly important in the cooler end of the acceptable range, such as Zone 7, where it provides thermal protection.
Crucially, the planting area must have excellent soil drainage, as standing water or saturated soil is a major cause of winter mortality. Mulch must not trap excessive moisture around the dormant corms, which encourages fungal diseases and rot. If corms are left in the ground, they should also be periodically dug and separated every few years to prevent overcrowding and maintain plant vigor.
Lifting and Curing Corms for Storage
In areas that experience hard freezes, removing the corms from the ground is a necessary autumn task. The optimal time is a few weeks after flowering and after a light frost has caused the foliage to yellow. This timing allows the corm to fully mature and store energy for its dormant period. Use a garden fork to carefully loosen the soil about six inches away from the stem to avoid damaging the corm.
Once the corm is free of the ground, trim the remaining foliage back to a short stub of one to two inches. The newly dug corms must undergo a process called curing before storage. Curing involves drying the corms in a warm, well-ventilated, and dark location for two to three weeks. A temperature range of 70°F to 80°F promotes rapid drying and the formation of a protective outer layer, minimizing the risk of rot.
After the corms have cured and the outer husks are dry and papery, the old, shriveled mother corm at the bottom can be snapped off and discarded. This spent corm should be separated from the new, plump corm that developed on top. Small, attached offsets, known as cormels, can be gently separated and saved for propagation, maturing into flowering corms after one or two growing seasons.
Proper Storage Techniques
The corms must be moved to a location that provides consistent, controlled conditions for winter dormancy. The ideal storage environment is cool, dry, and dark, with a temperature maintained between 35°F and 50°F. This range keeps the corms dormant without freezing or encouraging them to break dormancy too early.
Good air circulation is important, as stagnant air leads to moisture buildup and the growth of mold or fungus. Corms should be stored in materials that allow for necessary air flow, such as mesh bags, old nylon stockings, or open cardboard boxes lined with newspaper. Avoid using sealed plastic bags or containers, which trap humidity and invite decay.
Before placing the corms into storage, dusting them with a garden fungicide helps protect against common storage diseases. Throughout the winter, check the stored corms periodically for signs of trouble. Any corms that feel soft, mushy, or show visible mold should be immediately removed and discarded to prevent the spread of disease.