To understand a geranium's relationship with frost, one must first consider its origins. The common garden geranium (Pelargonium) is not a true geranium but a tender perennial native to South Africa. This heritage means it is evolutionarily programmed for warm, sunny, and frost-free conditions. Its cellular structure is not equipped to handle freezing temperatures. When water inside the plant's cells freezes, it expands, forming sharp ice crystals. These crystals pierce and rupture the cell walls, a catastrophic event from the plant's perspective. Once thawed, the damaged cells leak their contents, leading to the characteristic symptoms of frost damage: blackened, mushy, and wilted leaves and stems. This is not a simple case of dormancy but rather a severe physical injury that is often fatal.
Geraniums thrive in temperatures between 65°F and 75°F (18°C - 24°C) during the day. Their tolerance for cooler weather begins to drop significantly as temperatures approach freezing. A light frost, defined as temperatures between 29°F to 32°F (-1.7°C to 0°C), will likely damage the most tender new growth and flowers. However, a hard frost or freeze, where temperatures drop below 28°F (-2°C) for an extended period, is almost always lethal to the entire plant. The plant's metabolism grinds to a halt, and the widespread cellular damage described above occurs. Therefore, from the plant's point of view, frost is not something to be survived but something to be entirely avoided.
Since geraniums cannot protect themselves from freezing conditions, they rely on the gardener to alter their immediate environment—to create a protective microclimate. The most effective strategy is physical barrier protection. Covering the plants with a blanket, burlap, or a frost cloth before nightfall is highly effective. This method works by trapping the residual heat radiating from the soil, keeping the immediate air around the plant several degrees warmer. It is crucial that the covering extends all the way to the ground and is secured to prevent heat from escaping. Plastic coverings are not recommended as they can trap moisture and, if they touch the foliage, transfer the cold directly to the plant, causing more harm than good.
For potted geraniums, the simplest protection is to move the containers indoors to a sheltered location when frost threatens. A garage, shed, or porch that stays above freezing can provide sufficient protection for a short cold snap. For geraniums planted in the ground that are prized varieties, another option is to lift them from the soil, pot them, and bring them indoors for the winter. They can be kept as houseplants in a sunny window or forced into dormancy by storing the bare roots in a cool, dark place. Mulching heavily around the base of in-ground plants with a thick layer of straw or leaves can help insulate the root zone. While the top growth may die back, this protection can sometimes allow the plant to regenerate from the crown in spring, but this is not a guaranteed outcome and depends heavily on the severity of the winter.
