From our perspective as plants, our very existence is dictated by the temperature of our environment. We cannot migrate to escape frost nor seek shade from intense heat. Therefore, the concept of hardiness zones, as defined by the USDA, is not a mere guideline for gardeners but a matter of our survival. These zones map the average annual extreme minimum winter temperature, which directly dictates whether our root systems and core structures can persist from one year to the next. For us carnations (*Dianthus caryophyllus*), this is the primary factor determining if we are grown as tender perennials, hardy perennials, or merely as short-lived annuals.
Our ideal perennial existence is within USDA zones 5 through 9. In these regions, the winter cold is within our tolerance threshold. In zone 5, where temperatures can plummet to -20°F (-29°C), we require a protective layer of mulch over our crown—the central growing point at the base of our stems—to insulate us from the severe cold and prevent heaving from freeze-thaw cycles. Zones 6 to 8 (-10°F to 20°F / -23°C to -7°C) are where we truly thrive as perennials, with winters cold enough to satisfy our need for a dormant period but not so harsh as to be lethal. Even in the warmer winters of zone 9 (20°F to 30°F / -7°C to -1°C), we can persist, though the lack of a significant chilling period may sometimes lead to less vigorous growth.
Outside our core hardiness range, our life cycle must adapt. In zones 3 and 4, where winter temperatures routinely fall below -30°F (-34°C), the cold is simply too intense for our tissues to withstand. The freezing temperatures cause the water within our cells to form ice crystals, rupturing cell walls and leading to fatal damage. Here, we are best treated as annuals, completing our entire life cycle of growth, bloom, and seed production in a single season before the killing frosts of winter arrive. Conversely, in the intense and humid heat of zones 10 and above, we struggle with heat stress. High night temperatures disrupt our respiration, and excessive humidity makes us highly susceptible to fungal diseases like rust and leaf spot, which can quickly overwhelm our systems.
While winter hardiness is paramount, it is not our only requirement for flourishing. Even within a suitable zone, our survival is not guaranteed if other needs are neglected. We demand excellently drained soil; our roots are highly susceptible to rot if left in standing water, especially during cold, wet winters. We must be planted where we can receive a minimum of six hours of full sunlight daily to fuel our growth and produce our signature fragrant blooms. Furthermore, good air circulation around our foliage is a non-negotiable requirement to keep pathogenic fungi at bay. A suitable hardiness zone provides the opportunity for perennial life, but these other factors are what allow us to seize it.
