Gladiolus, often called “glads,” are popular summer-blooming plants prized for their dramatic, tall spikes of colorful flowers. Whether these plants return to the garden each year depends entirely on your specific location and the severity of the winter climate. Their annual return is dictated by how well their underground storage organs survive the cold and moisture of the dormant season.
The Core Answer: Hardiness and Climate
Gladiolus grow from a corm, a solid, vertically compressed underground stem that stores food, distinct from a true bulb. These corms originated from species native to South Africa, adapting them to mild, dry winters. They lack the cold hardiness needed to survive freezing temperatures in many regions.
The corm’s ability to survive winter is determined by your growing zone and temperature thresholds. Generally, the classic Grandiflora gladiolus hybrids are reliably perennial only in warmer climates, specifically USDA Hardiness Zones 8 and above, where consistent ground freezing is uncommon. A temperature drop below 28°F (a hard freeze) for an extended period will likely prove fatal.
In zones colder than Zone 8, freezing temperatures and wet winter soil cause corms to rot or die from frost. Gardeners in these regions (Zones 7 and below) must either treat gladiolus as annuals or manually lift and store the corms for the winter to ensure their return. Lifting and storing is often recommended even in Zone 7 to prevent loss and reduce disease risk.
Ensuring Their Return: Lifting and Storing Corms
For gardeners outside of consistently warm climates, lifting and storing the corms is the only way to guarantee flowers the following season, making gladiolus a “tender perennial.” This process begins in the fall, after the foliage has started to yellow and die back, typically following the first light frost. Waiting until this natural die-back ensures the plant has transferred sufficient energy to the corm for next year’s growth.
The corms must be carefully dug using a garden fork, placed several inches away from the stem to avoid damage. Gently shake off excess soil. After lifting, cut the stems back to a short stub, about one to two inches above the corm. The freshly dug corms are then laid out in a single layer in a warm, dry, and airy location to cure for two to three weeks.
Curing allows the outer husk to dry, becoming firm and papery, which creates a protective layer against rot during storage. Once fully cured, the old, withered corm from the previous year, which sits directly beneath the new corm, must be removed and discarded. The newly formed corm should feel solid and healthy; immediately discard any mushy or diseased specimens to prevent pathogen spread.
Optimal winter storage requires a cool, dark, and dry environment with good air circulation to maintain dormancy. The ideal temperature range for storage is between 35°F and 50°F. Corms should be placed in breathable containers, such as paper bags, mesh sacks, or cardboard boxes, avoiding plastic bags as they trap humidity.
Preparing for Next Season’s Bloom
Preparing the overwintered corms for their return to the garden in spring is the final step in the cycle. Before replanting, inspect the stored corms for any signs of mold, shriveling, or softness that may have developed over the winter. Healthy corms must be firm to the touch, indicating they are viable and ready to grow.
The corms should not be planted too early, as cold soil can delay growth or encourage rot. Planting should occur after the last anticipated hard frost, once the soil temperature has warmed to at least 55°F. For continuous flowering throughout the summer, gardeners can stagger their planting, setting new batches of corms into the ground every two weeks until early summer.
Gladiolus require a location with full sun and well-drained soil, as they are susceptible to rot if they sit in heavy, wet ground. Plant the corms at a depth of four times their height, with the pointed growing tip facing upward, which provides the necessary stability for the tall flower spike. The corm’s stored energy will be used to produce the dramatic summer blooms, completing the annual cycle.