To understand the relationship between geraniums and frost, one must first recognize their fundamental nature. Geraniums, particularly the common garden varieties (Pelargonium spp.), are not true hardy perennials in temperate climates. They are native to South Africa, a region characterized by warm, Mediterranean-like conditions with mild, wet winters and hot, dry summers. This evolutionary background is the key to their physiological response to cold.
From a plant's perspective, frost is not merely a temperature reading; it is a catastrophic cellular event. Geraniums, like all plants, are composed of cells filled with water. When the temperature plummets below freezing (0°C or 32°F), the water inside and between these cells begins to form ice crystals. These crystals are sharp and physically puncture the cell membranes and walls, which are the structural integrity of the plant. Once these membranes are ruptured, the cell's vital contents leak out. After the frost thaws, the damage becomes visible as blackened, mushy, and translucent leaves and stems. The plant is essentially suffering from massive internal damage, akin to countless microscopic wounds. This process is irreversible for the affected tissues.
It is crucial to distinguish between cold stress and a killing frost. Geraniums begin to experience distress well before the freezing point. Temperatures consistently below 7°C (45°F) can cause a condition known as chilling injury. At this stage, the plant's metabolic processes, such as photosynthesis and nutrient transport, slow down significantly. You may observe purpling or reddening of the leaves, a sign of anthocyanin pigment production in response to stress, and general stunted growth. This is the plant's warning signal. A true frost, however, is the lethal event described above. The plant's tolerance level is its absolute limit, and for most common geraniums, a hard frost (temperatures at or below 0°C for several hours) is fatal to the above-ground parts and often the entire plant if the roots freeze.
While most geraniums are frost-tender, there is a spectrum of cold tolerance within the broader geranium family. The common zonal or scented geraniums (Pelargonium) have virtually no natural defense against ice formation. Their strategy for survival in their native habitat is dormancy during the dry season, not the cold season. In contrast, a separate group known as "hardy geraniums" or Cranesbills (genus Geranium) are evolutionarily adapted to colder climates. These plants have developed mechanisms to manage cold, such as moving water out of their cells into intercellular spaces where ice formation is less damaging, and producing natural "antifreeze" proteins that lower the freezing point of their cellular fluids. This is why a hardy Cranesbill can die back to the ground in winter and regrow from its roots, while a tender Pelargonium cannot.
When a gardener brings a geranium indoors before the first frost, the plant experiences a sudden shift from a declining, cold-stressed environment to a stable, warmer one. From the plant's physiological viewpoint, this abrupt change can halt the progression towards dormancy and trigger weak, etiolated (leggy) growth as it struggles to adapt to lower light conditions. A more natural, though managed, approach is to force the plant into a true dormancy. By storing it in a cool, dark place like a basement, the plant is allowed to enter a state of suspended animation. Its metabolic activity drops to near-zero, and it sheds its leaves, conserving all its energy within its stems and roots. This state mimics a dry season in its native habitat and is the plant's best chance for conserving its life force to regenerate when warm conditions return.
