Gladiolus flowers, known for their tall, striking spikes of color, are a favorite in summer gardens. Although often called a bulb, the gladiolus grows from an underground storage organ called a corm. This corm is a solid, swollen stem base that functions as a food reserve, allowing the plant to survive dormancy and regrow. Whether to remove the corm from the soil each year depends almost entirely on local winter conditions.
Determining If Lifting is Necessary
The need to dig up gladiolus corms is directly related to how cold the soil gets during winter. Most common varieties are tender and cannot tolerate prolonged freezing temperatures. Temperatures dropping to 28°F (-2°C) or colder for an extended period will fatally damage the corm.
Gardeners in warmer climates (USDA Hardiness Zones 8 through 10) can typically leave their corms in the ground year-round. A thick layer of protective mulch is often sufficient to insulate them from occasional cold snaps.
In colder regions (Zone 7 and below), corms must be lifted to survive the winter. Leaving corms in cold or wet soil also risks rot. Excess moisture combined with cold soil creates an environment conducive to fungal diseases that can destroy the corm before spring.
The Process of Lifting and Curing
Corms should be removed in the autumn, ideally after the foliage has turned yellow or brown following a light frost. This yellowing indicates the plant has transferred its energy into the new corm for storage. Waiting too long risks a hard freeze damaging the corms before they can be dug up.
To lift the corms, use a garden fork to gently loosen the soil in a wide circle around the plant. Carefully pull the entire clump from the soil and shake off loose dirt; avoid washing them. The stalk should then be cut down to about one to two inches above the corm.
The next step is curing, which involves drying the corms to seal the outer skin and prevent rot. Spread the corms in a single layer in a warm, dry, and well-ventilated location, such as a garage or shed, out of direct sunlight. This drying period lasts for one to two weeks, or until the outer husk feels crisp and papery.
Storing Gladiolus Corms Over Winter
Once cured, corms require preparation before long-term storage. First, separate the new corm from the shriveled mother corm, which is a flat remnant at the base of the new growth. This older corm should be discarded as it can become a source of rot or disease.
Clean the corms of any remaining dirt and inspect them for damage. Discard any that feel soft, mushy, or show mold. Some gardeners lightly dust the corms with a fungicide or sulfur powder to deter pests during dormancy. Do not remove the papery husk, as it provides a protective layer.
Gladiolus corms must be stored in a cool, dry, and well-ventilated environment that remains above freezing. The ideal temperature range for winter storage is between 35°F and 50°F (2°C and 10°C), which often means an unheated basement or a root cellar. Corms should be kept in breathable containers to ensure air circulation and prevent moisture buildup, such as:
- Mesh bags
- Paper bags
- Cardboard boxes lined with newspaper
Replanting in the Spring
The annual cycle is completed when spring arrives and the stored corms are prepared for replanting. Wait until the danger of the last hard frost has passed and the soil temperature has warmed to at least 55°F (13°C). Planting too early in cold, wet soil can cause the corms to rot.
Before planting, inspect the corms to ensure they are firm and healthy. For a continuous display of blooms, corms can be planted in succession every two weeks until early summer.
Plant the corms with the pointed side facing up, typically four to six inches deep depending on size. Spacing them about five inches apart allows for adequate air circulation and growth. Providing well-drained soil and full sun sets the stage for a successful season of summer blooms.