From a botanical and horticultural perspective, the life cycle of a gladiolus plant is fascinating and directly answers your question about its perennial or annual nature. The answer is not absolute and depends heavily on your specific US Hardiness Zone, as it dictates the winter conditions the plant's underground structure must endure.
Gladiolus plants are classified as tender perennials. They grow from a specialized underground storage organ called a corm, which is a solid, bulb-like structure. This corm is the plant's survival mechanism. During a single growing season, the plant uses energy from its leaves to photosynthesize and create a new, larger corm on top of the old one, which withers away. It may also produce smaller baby corms called cormels. In their native habitats, primarily South Africa and the Mediterranean, these new corms survive underground through the mild winter and sprout again the following spring, fulfilling their perennial life cycle.
The USDA Plant Hardiness Zone Map is the key determinant. It is based on the average annual minimum winter temperature. Gladiolus corms cannot survive freezing temperatures in saturated soil; they will rot and die. Therefore, their ability to behave as a true perennial is entirely dependent on your zone's winter lows.
For gardeners in the United States, the treatment of gladiolus falls into three categories based on hardiness zones:
Zones 7 and Below (Winter Lows below 10°F): In these zones, the ground freezes to a depth that will kill the gladiolus corms. Here, gladiolus are effectively grown as annuals. The corms must be lifted in the fall after the foliage has died back, dried, and stored in a cool (35-45°F), dry, and frost-free location over winter. They are then replanted the following spring.
Zones 8 (Winter Lows 10°F to 20°F): This is a transitional zone. In milder parts of Zone 8 with well-draining soil, a thick layer of winter mulch (4-6 inches of straw, pine needles, or shredded leaves) may provide sufficient insulation for the corms to survive in the ground. However, many gardeners in Zone 8 still choose to lift and store them to guarantee success the next year.
Zones 9 and Above (Winter Lows above 20°F): In these warmer zones where the ground does not freeze, gladiolus corms can reliably be left in the soil year-round. They will behave as true perennials, going dormant in the winter and re-sprouting each spring. In the warmest zones (e.g., parts of Zone 10), they may even retain some foliage year-round.
Even within a favorable zone, other plant-centric factors are crucial. Soil Drainage is paramount. A gladiolus corm sitting in cold, wet soil will succumb to rot far more easily than one in well-draining, sandy, or loamy soil. Microclimates in your garden, such as a south-facing wall that retains heat, can also create a pocket warmer than your overall zone rating, improving the chances of successful overwintering.
