Gladiolus are striking summer flowers grown from a specialized underground stem structure known as a corm. These corms are not reliably cold-hardy in temperate climates, particularly in USDA Zones 7 and colder, where the ground freezes solidly. Because they are tender perennials, gardeners must lift the corms annually to protect them from freezing temperatures and excess winter moisture that can lead to rot. Storing them indoors ensures these vibrant plants can be successfully replanted the following spring.
Identifying the Optimal Time for Lifting
The timing for lifting gladiolus corms is tied to the plant’s natural life cycle and the onset of cold weather. The most important factor is allowing the foliage to fully mature after the flowers fade, which permits the plant to store maximum energy in the new corm. This maturation process generally takes six to eight weeks following the end of blooming.
Look for visual cues that the corm is ready, primarily when the leaves have begun to yellow and die back naturally. This signals that the plant has successfully moved its energy reserves into the corm for next season’s growth. Harvesting too early results in a less vigorous corm with insufficient stored energy.
A light frost often serves as a natural signal, killing the foliage without harming the corm underground. However, corms must be lifted before a deep, hard freeze occurs. A hard freeze (temperatures below 28°F/-2°C) can lethally damage the corm’s tissues, so dig them after the foliage dies back but before the ground freezes solid.
The Steps for Proper Excavation
Removing the corms from the soil must be done carefully to prevent bruising or nicking the storage tissue, which makes them susceptible to disease during storage. Use a garden fork rather than a spade, as the tines are less likely to slice the corms. Start digging about six to eight inches away from the main stem to loosen the soil in a wide circle.
Gently lift the entire clump, including the stalk, roots, and the attached corm mass. Once out of the ground, carefully shake off the majority of the clinging soil. The remaining foliage should be trimmed back, leaving a short stub of one to two inches of stem attached to the top of the corm. This stub assists in the curing process and provides a handle for later handling.
Curing and Preparing Corms for Storage
After excavation, corms require curing to dry and harden their outer skin before winter storage. Curing involves spreading the corms in a single layer in a warm, dry, and well-ventilated location, such as a garage or shed, out of direct sunlight. The ideal temperature range is between 70°F and 80°F (21°C to 27°C).
This drying period typically lasts two to three weeks, until the outer husk feels firm and papery. Once cured, clean and separate the corms: the new, healthy corm should be snapped away from the shriveled, spent mother corm, which is discarded. Small offsets, known as cormels, can be separated and saved for propagation.
Inspect each corm closely, discarding any that feel soft, mushy, or diseased. To protect them from fungal pathogens during dormancy, dusting with a commercial fungicide or sulfur powder is a common practice. Labeling the corms by color or variety ensures organization for spring planting.
Successful Winter Storage Conditions
The final step is moving the cleaned and cured corms to an environment that keeps them dormant, cool, and dry throughout the winter. The storage location must maintain a cool temperature, ideally between 35°F and 45°F (2°C to 7°C). This range prevents premature sprouting but remains safely above freezing. A cellar, unheated basement, or root cellar are suitable spots.
Storage containers must allow for air circulation to prevent moisture accumulation, which is the primary cause of rot and mold. Breathable materials are appropriate choices, including:
- Mesh bags
- Paper bags
- Old pantyhose
- Cardboard boxes
The corms can also be layered within a cardboard box using a dry, insulating medium such as peat moss, sawdust, or shredded newspaper. Stored corms should be checked periodically throughout the winter, and any that appear soft, moldy, or diseased must be removed immediately to protect the remaining healthy stock.