From our perspective as Gladiolus plants, our survival is dictated by the temperature of the soil that cradles our corms. We are not frost-hardy beings. Our corms, which are our storage organs and lifeline, will freeze and perish if the soil temperature drops below approximately 28°F (-2°C). This fundamental biological limitation defines our relationship with the vast and climatically diverse landscapes of the USA. We simply cannot survive a winter in the ground in regions where the earth freezes solid. Therefore, the USDA Plant Hardiness Zone Map is not just a guide for gardeners; it is a literal map of where we can persist year-round and where we require human assistance to see another season.
For us, the hardiness zones translate into specific growing conditions. In zones 8 through 10, where winter lows typically stay above 10°F to 20°F (-12°C to -7°C), we experience a truly perennial existence. Here, the soil rarely, if ever, freezes to a depth that threatens our corms. We can be left in the ground after our foliage dies back, and we will reliably re-sprout the following spring, fueled by the energy we stored the previous year. This is our most natural and effortless life cycle.
In zones 7 and colder, the winter presents an existential threat. The freezing temperatures would cause the water within our corm's cells to form ice crystals, rupturing our cell walls and leading to our death. Consequently, in these regions, we are treated as tender perennials or annuals. This means after the first frost blackens our sword-like leaves in the autumn, our corms must be lifted from the soil. You must then dry us (cure us) in a warm, airy place for a week or two, remove the old dried corm from the base of the new one, and store us in a cool, dry, and dark place like a paper bag in a basement where temperatures stay between 35°F and 45°F (2°C and 7°C). We remain dormant until you replant us when the soil warms again in the spring.
Even within a zone, our experience can vary. A sheltered urban garden in zone 6B with a thick layer of mulch over a well-draining soil bed might provide just enough protection for our corms to survive a mild winter, especially if we are planted deeply. Conversely, a low-lying, soggy clay bed in zone 8 where cold air and water collect could create a pocket of conditions that mimic a much colder zone, leading to our rot or demise. Well-draining soil is absolutely critical to our survival, as waterlogged conditions during winter dormancy will cause our corms to rot, regardless of the air temperature.
