Gladiolus are striking, tall flowering plants that produce flower spikes from a specialized underground storage organ known as a corm. While often referred to as bulbs, corms are solid stem tissue used by the plant to store nutrients. In cold climates where the ground freezes, these corms cannot survive the winter outdoors. They must be dug up, a process called lifting, to protect them from freezing temperatures and excessive moisture, ensuring survival and robust blooms the following season.
Identifying the Optimal Time for Lifting
The timing for lifting gladiolus corms is governed by the plant’s maturity and the threat of severe cold. After flowering, a waiting period is necessary to allow the corm to fully develop and replenish energy reserves. This period generally spans six to eight weeks following the final bloom cut or fading of the flower spike.
During this maturation phase, the plant transfers stored energy from its foliage down to the newly forming corm. The visual cue that this process is complete is the yellowing and browning of the leaves. Corms are considered mature when the leaves have largely wilted and turned brown, often coinciding with 30 to 60 days post-flowering.
Lifting the corms before the first hard frost is required for their survival. A light frost that causes the foliage to wilt is often beneficial, signaling the end of the growing season and prompting final nutrient transfer. However, a hard freeze, defined as temperatures dropping below 28°F (–2°C) for an extended period, can damage or kill the corm tissue by freezing the internal water. Gardeners in colder regions (USDA Zone 7 and below) must plan to lift corms in late summer or early fall, completing the task before the ground is threatened by deep freezing.
Proper Technique for Digging and Cleaning
The physical process of lifting begins by preparing the plant above ground to reduce moisture and disease risk. Once the foliage has turned yellow, the stalk should be cut back, leaving a stub of about one to three inches above the soil line. This trimmed stem acts as a handle during lifting and prevents the corm from drying out too quickly.
Use a garden fork or spade to loosen the soil, inserting the tool six inches or more away from the base of the plant to avoid piercing the corm. Gently pry the corm cluster from the ground, taking care to gather the small cormlets (cormels) that may have formed around the base. Immediately after lifting, brush or shake off the excess soil clinging to the corm and roots.
The newly formed corm must be separated from the shriveled, spent mother corm located beneath it. While separation is often easier after curing, removing the bulk of the old root mass and soil now minimizes potential disease. Healthy corms should be firm and show no signs of rot or pest damage; discard any soft or diseased material immediately to prevent contamination during storage.
Curing the Corms Post-Harvest
Curing is a short-term drying process that follows lifting and is distinct from long-term winter storage. This step hardens the corm’s outer husk, sealing in moisture and creating a protective barrier against fungal pathogens and rot during dormancy. The duration for this process ranges from 10 days to three weeks, depending on environmental conditions.
The ideal curing environment is warm, dry, and provides good air circulation, with a temperature range of 70–80°F (21–27°C). Corms should be laid out in a single layer on screens, mesh trays, or cardboard in a well-ventilated garage, attic, or shed, keeping them out of direct sunlight.
Once the corm’s outer layer feels dry, firm, and papery, curing is complete, and the corm is ready for final cleaning. At this point, the old, dried mother corm and any remaining roots can be easily snapped off the base of the new corm. Cormlets saved for propagation should be kept separate from the larger corms.
Conditions for Successful Winter Storage
After curing, the corms are prepared for an extended period of cool dormancy until spring replanting. Successful long-term storage requires a balance of low temperature, low humidity, and consistent air circulation to prevent sprouting or rot. The temperature should be cool, ideally maintained in a range of 35–45°F (2–7°C), which mimics natural winter conditions without freezing the tissue.
The storage area must remain dry and have a relative humidity of approximately 50 percent to discourage mold growth. Storage containers should prioritize breathability, such as mesh bags, paper bags, old nylon stockings, or perforated cardboard boxes. Storing corms in materials like peat moss, vermiculite, or sawdust can help manage moisture levels.
The containers should be placed in a dark location, such as an unheated basement, root cellar, or cool garage, ensuring the space does not freeze. Periodically check the stored corms throughout the winter to ensure their health. Immediately remove any corms showing signs of softness, mold, or decay to protect the rest of the healthy stock from disease spread.