The gladiolus, often called the sword lily, is prized for its tall, dramatic spikes of colorful, trumpet-shaped flowers. To ensure a robust show of flowers the following season, care must continue after the last petal fades. The post-bloom period is when the plant stores energy for its next growth cycle. Proper attention to the remaining foliage and the underground corm is an important part of the annual gardening process.
Removing the Spent Flower Spike
The immediate step following the bloom cycle is removing the spent flower spike. This action, known as deadheading, prevents the plant from diverting energy into seed production. Allowing seed heads to form drains resources that should instead strengthen the underground corm.
Use clean, sharp shears to cut the flower stalk just above the highest leaf on the stem. It is important to leave a significant amount of the stalk and foliage intact, ensuring at least four sets of leaves remain below the cut. Making the cut at a slight diagonal helps prevent water from collecting on the remaining stub, reducing the risk of rot or disease entry. This pruning conserves the plant’s energy reserves.
Maintaining Foliage to Nourish the Corm
The remaining green foliage is responsible for manufacturing and storing energy for the subsequent year’s flowers. Through photosynthesis, the leaves convert sunlight into carbohydrates, which are stored in the corm. This energy accumulation determines the size and quality of the following year’s blooms.
During this period, the plant still requires attention to maintain healthy leaf function. While intense watering needed during the blooming phase can be slightly reduced, the soil should remain consistently moist, not waterlogged, to support corm development. A light application of a low-nitrogen fertilizer can assist with energy storage. The foliage must be allowed to yellow and die back naturally; this is the visual cue that the corm has absorbed all available energy. Prematurely cutting the green leaves will starve the corm, resulting in weakened or absent blooms the following season.
Lifting and Curing Corms for Dormancy
In colder climates (USDA Hardiness Zones 7 and below), gladiolus corms must be lifted to protect them from freezing temperatures. The optimal time is four to six weeks after flowering, or just after the first light frost causes the foliage to completely yellow and collapse. Waiting until the leaves have died back ensures maximum energy transfer to the corm.
To lift the corms, use a garden fork to gently loosen the soil in a wide circle around the plant, taking care not to damage the underground structures. Shake off any loose soil and trim the remaining dead foliage down to a stub of one to two inches above the corm. A new, plump corm will have formed on top of the old, withered mother corm. This old corm should be detached and discarded.
The newly harvested corms require “curing” before long-term storage. Curing involves drying them in a warm, dry, well-ventilated location, out of direct sunlight, for one to three weeks. This process allows the corm’s outer skin to seal and harden, creating a protective layer that minimizes moisture loss and resists fungal pathogens during dormancy. Small, rice-sized offsets called cormels can be separated from the main corm and saved for propagation, though they take multiple seasons to reach flowering size.
Preparing and Storing Corms for Winter
Once the corms are fully cured and feel firm and dry, they are ready for winter dormancy. Inspect each corm individually and discard any showing signs of damage, softness, or disease, preventing the spread of rot in storage. Healthy corms can be dusted lightly with an approved fungicide or insecticide powder to protect against mold and pests like thrips.
The chosen storage container must allow for air circulation to prevent moisture buildup, which is the primary cause of rot. Ventilated options include:
- Mesh bags
- Paper bags
- Old nylon stockings
- Cardboard boxes lined with newspaper
Avoid sealing the corms in non-porous plastic containers.
A cool, dry, dark, and well-ventilated location is necessary for successful overwintering. The ideal storage temperature is between 35 and 45 degrees Fahrenheit (2 to 7 degrees Celsius). This cool environment ensures the corms remain dormant until they are replanted the following spring. Labeling the container with the variety and color is helpful for organized replanting.