From a botanical perspective, the gladiolus plant (technically growing from a corm, a type of bulb) is not inherently equipped to survive freezing temperatures in a dormant state. Its natural growth cycle involves a period of active growth, followed by a dormancy period that is triggered by environmental cues like shorter day lengths and cooler temperatures. However, this dormancy is a dry, resting state, not a freeze-tolerant one. The corm's structure is designed to store energy for the next growing season but is composed of soft, starchy tissue that is highly susceptible to cellular damage when frozen. Ice crystals form within the cells, rupturing their walls and leading to rot once temperatures warm, ultimately killing the corm.
The single greatest factor determining whether you can leave gladiolus corms in the ground is winter soil temperature. If soil temperatures in your region consistently drop below freezing (0°C or 32°F) and remain there for extended periods, the corms will freeze and perish. Even in marginally colder zones where the ground freezes briefly, the repeated freeze-thaw cycles can cause significant damage. Furthermore, cold, wet soil is a perfect environment for fungal and bacterial pathogens. A corm that might survive a mild chill in well-drained soil will almost certainly succumb to rot in cold, waterlogged earth, as its defenses are lowered during dormancy.
In United States Department of Agriculture (USDA) plant hardiness zones 7 and above (where winter lows typically stay above 0°F or -18°C), the risk of the ground freezing deeply is significantly reduced. In these regions, particularly zones 8-10, gladiolus corms can often be left in the ground year-round successfully. The soil remains sufficiently warm to protect the corm from fatal freezing. In these climates, the plant may not even experience a full dormancy and could retain some foliage year-round. However, even in warm zones, ensuring the planting site has excellent drainage is critical to prevent water from pooling around the corms during cool, rainy winter periods, which would otherwise lead to rot.
For gardeners in zones 6 and colder, attempting to leave gladiolus corms in the ground over winter will almost certainly result in the loss of the plants. The corms will not magically become cold-hardy. The metabolic processes of the corm are halted by the cold, and without the natural adaptations of hardy bulbs like tulips or daffodils, they cannot protect their cells. Come spring, instead of sending up new shoots, the corms will be a mushy, rotten mass in the soil. Furthermore, leaving them in the ground eliminates the opportunity to perform a crucial horticultural practice: dividing the corms. Each gladiolus corm is replaced by a new one each growing season and also produces smaller cormels. Lifting them allows you to separate these, discard the old depleted corm, and replant the new ones to propagate and maintain vigor in your flower bed.
