Gladiolus are popular summer-blooming plants that produce tall, colorful spikes of flowers. Whether to remove them from the ground in autumn depends entirely on the winter temperatures of your specific location. These plants grow from a specialized underground stem called a corm, which stores energy. Unlike true bulbs, the survival of the gladiolus corm is compromised when ground temperatures drop too low, necessitating protective action.
Determining If You Need to Lift Corms
Gladiolus are classified as tender perennials, meaning they can survive multiple growing seasons only in climates where the ground does not experience prolonged freezing. The accepted hardiness range for leaving standard gladiolus corms in the ground without lifting is generally USDA Hardiness Zones 8 and warmer. In these milder zones, a heavy layer of organic mulch, such as straw or chopped leaves, often provides enough insulation to protect the corms from cold snaps.
Gardeners in zones 7 and colder must dig up their corms to ensure survival, as the freezing and thawing cycle of the soil can destroy the storage tissue. While some hardier varieties are known to survive in Zone 5 or 6 with deep planting and thick mulching, lifting remains the most reliable method for most gardeners in colder regions. Leaving the corms in the ground below Zone 7 risks losing the plants entirely to frost damage or rot.
Step-by-Step Guide to Lifting
The proper time to lift gladiolus corms is in the fall, after the first light frost but before the ground freezes solid. A light frost signals the foliage to die back, allowing the plant to send energy reserves back down into the corm for next year’s growth. Wait until the foliage has turned yellow or brown, typically about a month or two after the flowers have finished blooming.
Begin the process by cutting the stems down, leaving about one to two inches of foliage above the soil line. Use a garden fork to carefully loosen the soil about six inches away from the stem to avoid puncturing the corm, then lift the entire clump. Gently shake off the majority of the loose soil clinging to the roots and corm. Do not wash the corms with water, as this introduces moisture that encourages fungal growth during storage.
The corms must enter a curing period to dry out and form a protective outer layer before long-term storage. Spread the corms in a single layer on a drying rack or screen in a warm, dry, and well-ventilated location, such as a garage or shed. This curing process should take approximately two to three weeks at temperatures between 70°F and 80°F. The corm is adequately cured when its outer husk feels crisp, papery, and firm.
Preparing Corms for Winter Storage
Once cured, the old parent corm must be removed from the base of the newly developed corm. The original corm, which is shriveled and dark, should easily break away from the firm, new corm that grew above it. Inspect each corm for signs of damage, disease, or mold, and discard any that feel soft, mushy, or shriveled.
At this stage, you may notice small, bead-like structures called cormels attached to the base of the new corm. These cormels can be saved and planted separately to grow new gladiolus plants, though they usually take a few years to reach flowering size. Many gardeners dust the cleaned corms with a garden fungicide or sulfur powder before storage to prevent fungal rot and control pests like thrips.
The final storage environment must be cool, dark, and dry to keep the corms dormant until spring. Optimal storage temperatures range between 35°F and 50°F. Store the corms in a breathable container or material. Avoid using sealed plastic containers, as they trap moisture and encourage rot.
Breathable Storage Options
- Mesh bags
- Old nylon stockings
- Perforated paper bags
- Cardboard boxes mixed with a dry medium like peat moss or vermiculite