Gladiolus is a popular summer flower that grows from a subterranean stem structure known as a corm. Unlike true bulbs, the corm serves as the plant’s primary storage organ, holding the necessary energy for the next growing season. Corms are not reliably winter-hardy in regions where the ground freezes consistently, generally USDA Zones 7 and colder. Gardeners in these areas must lift the corms from the soil in the fall to ensure their survival and successful reblooming, protecting them from lethal freezing temperatures and excess moisture.
Determining the Optimal Time for Lifting
The timing for lifting the corms directly impacts the energy reserves available for next year’s blooms. Wait until the foliage has completely died back, signaling the end of the seasonal growth cycle. The plant must photosynthesize for six to eight weeks after the flowers fade, transferring maximum nutrients to the developing new corm. This storage process prepares the corm for dormancy.
Corms are ready for harvest when the leaves turn yellow or brown and begin to wither. In colder climates, this often coincides with the first light frost of the season. A light frost kills the foliage, initiating dormancy without damaging the corm, which is insulated underground. Waiting until after a light frost but before a hard freeze ensures maximum maturity and prevents the corms from deep freezing.
Proper Technique for Harvesting Corms
Once the foliage has fully died back, remove the corms carefully to prevent bruising. A garden fork is the preferred tool for lifting, as it minimizes the risk of slicing into the corms compared to a spade. Insert the fork several inches away from the stalk to loosen the soil in a wide circle around the plant clump. Gently lift the entire clump from the ground, avoiding pulling directly on the remaining stalk.
After lifting, shake off the loose soil immediately; do not wash the corms, as moisture encourages rot. Use clean shears to trim the remaining stem and foliage to a short stub, leaving only one or two inches attached above the corm. The clump contains the new, larger corm, the shriveled old mother corm, and small offsets called cormels. Detach and discard the old, spent mother corm, as it will rot in storage.
Curing and Prepping Corms for Winter
After harvesting, the corms require a period of curing, a drying process that toughens the outer skin and seals minor wounds. This hardening, known as suberization, is essential as it reduces moisture loss and protects against fungal pathogens during storage. Spread the corms in a single layer, typically on a screen or tray, in a warm, dry, and well-ventilated environment.
A suitable curing area should maintain 70 to 80 degrees Fahrenheit and be situated out of direct sunlight. Maintain this warm, airy condition for two to three weeks until the corms feel crisp and papery. Once cured, give the corms a final cleaning, removing remaining dried soil and shriveled husks. Inspect each piece closely, discarding any that feel soft, mushy, or show signs of disease before applying a dusting of garden fungicide or sulfur powder for added protection.
Long-Term Storage Conditions
Following curing, the corms are ready for long-term winter storage. The goal is to keep them cool enough to maintain dormancy but consistently above freezing. Ideal storage temperature ranges between 35 and 50 degrees Fahrenheit, with 40 to 42 degrees Fahrenheit considered optimal. The location, such as a cool basement or root cellar, must offer adequate air circulation and moderate humidity.
Store corms in materials that allow for air exchange to prevent moisture buildup, mold, or rot. Appropriate containers include mesh bags, perforated paper bags, or screen-bottom trays. They can also be layered in cardboard boxes, separated by materials like peat moss, sawdust, or shredded newspaper to regulate humidity and prevent contact. Periodically check the stored corms throughout the winter, removing any that are soft, dried out, or show signs of decay to prevent the spread of problems.