From a botanical perspective, marigolds (Tagetes spp.) are classified as tender annuals. This designation is crucial to understanding their relationship with cold. As annuals, their entire life cycle—germination, growth, flowering, and seed production—is completed within a single growing season. Their "tender" nature means their cellular structure is highly susceptible to damage from cold temperatures. Unlike hardy perennials that have evolved mechanisms to survive freezing, marigolds are native to warmer, frost-free regions like Mexico and Central America. Their physiology is optimized for warmth and sunlight, not for coping with the formation of ice crystals within their tissues.
The primary threat frost and light freezes pose to a marigold is intracellular ice formation. When the temperature drops to 32°F (0°C) or below, the water within and between the plant's cells can freeze. Water expands as it freezes, and the sharp ice crystals act like tiny daggers, physically puncturing and rupturing the delicate cell membranes and walls. This process is catastrophic. Once these cell structures are compromised, they cannot perform their vital functions. The cell contents leak out, and the plant's structural integrity collapses. You will visually observe this as blackened, mushy, and wilted leaves and stems—the tell-tale signs of frost kill.
It is important to note that not all marigolds possess the exact same level of cold sensitivity. There is a slight spectrum of tolerance, often linked to their size and growth habit. The smaller, signet marigolds (Tagetes tenuifolia) are typically the most vulnerable, showing damage at the first hint of frost. The ubiquitous French marigolds (Tagetes patula) are also very tender. The larger, robust African marigolds (Tagetes erecta), while still firmly in the tender category, can occasionally withstand a very brief, very light frost, perhaps down to about 30°F (-1°C) for a short period, especially if they have been gradually acclimated to cooler fall temperatures. However, this is a marginal resistance and not a guarantee of survival.
The plant's potential for survival is directly tied to the severity and duration of the cold event. A light frost occurs when temperatures dip just slightly below 32°F (0°C) for a short time, often on a still, clear night. This might only damage the most exposed and tender new growth at the tips of the plant. A light freeze, where temperatures range from 29°F to 32°F (-2°C to 0°C) for several hours, will cause more significant damage to the entire plant, including leaves and flowers. A hard freeze, defined as temperatures below 28°F (-2°C) for an extended period, is almost always fatal. It causes such widespread cellular destruction that the marigold cannot recover.
If a cold event is forecast, protective actions can be taken. Covering marigolds with a frost cloth, blanket, or even a cardboard box before nightfall can trap geothermal heat radiating from the soil, often providing just enough insulation to keep the temperature around the plant above the critical freezing point. It is essential that the covering extends to the ground and is removed in the morning once temperatures rise. If a marigold is only lightly nipped by frost, with damage limited to the very tips of the leaves, it may continue to grow if warmer weather returns. However, if the stems are blackened and mushy, the plant has sustained fatal damage and will not recover.
