The plant commonly sold as a geranium is actually a tender perennial belonging to the genus Pelargonium. These popular flowering plants are native to the warm, semi-arid regions of South Africa, making them highly intolerant of cold temperatures. Pelargoniums lack the natural defenses required to survive freezing conditions. Gardeners in most temperate climates must actively protect these sensitive plants from cold to ensure their survival through the winter.
The Critical Temperature Threshold
While 35°F (1.7°C) is above freezing, this temperature is dangerous for Pelargoniums and too cold for them to remain outdoors safely. The plant’s metabolic processes slow significantly once temperatures drop below 50°F (10°C). Prolonged exposure between 35°F and 40°F (1.7°C to 4.4°C) can cause chilling injury, a form of damage that does not require frost.
Chilling injury is often visible as leaf discoloration, such as a whitening or a reddish-purple hue. This stress weakens the plant, making it susceptible to disease and inhibiting growth for the following season. Irreversible damage occurs at or below 32°F (0°C), which is considered frost or freezing.
Frost causes water inside the plant’s cells to crystallize, rupturing the cell walls. This leads to the immediate, visible death of the tissue, often presenting as blackening, wilting, and a mushy texture. Even a short exposure to freezing temperatures can be fatal. The safe minimum temperature for active growth or rest is generally considered to be 40°F to 50°F.
Immediate Protection During Cold Snaps
When an unexpected, short-duration cold snap is forecast, quick action can save plants before the temperature drops too low. The simplest strategy for container-grown Pelargoniums is to move them temporarily to a sheltered, frost-free location. An unheated garage, a covered porch, or a shed can provide enough thermal protection for a few nights.
For plants too large to move or planted directly in the ground, temporary coverings are the best option. Covering the plants before sunset traps residual heat radiating from the soil. Materials like garden fleece, old blankets, or large cardboard boxes create an insulating layer.
Ensure the covering extends to the ground to maximize heat retention. Placing the plants close to a house wall also helps, as the building material absorbs solar energy during the day and slowly releases warmth overnight. Any opaque covering, such as a box, should be removed in the morning to allow the plant to receive light and prevent overheating.
Preparing Geraniums for Winter Dormancy
When sustained cold weather below 35°F is expected for the entire winter, a long-term overwintering strategy is necessary since Pelargoniums are not winter-hardy. Gardeners typically choose between keeping the plants actively growing indoors or forcing them into a dormant state.
Active Growth Indoors
To grow them as houseplants, carefully dig up the plants and pot them into containers just large enough for the root ball, pruning the tops back by about one-third to one-half. Place these potted plants in a cool, bright location, ideally with temperatures between 50°F and 65°F (10°C to 18°C). They require minimal watering throughout the winter, only when the soil is dry to the touch, and should not be fertilized.
Bare-Root Dormancy
Alternatively, the plants can be stored in a bare-root, dormant state. Before the first hard frost, plants are dug up, and excess soil is gently shaken off the roots. The stems are cut back significantly. The plants are then stored in a cool, dark, and dry location, such as a basement or root cellar, where temperatures remain consistently between 45°F and 55°F (7°C to 13°C).
The bare-root plants can be hung upside down or placed loosely in open brown paper bags or cardboard boxes to allow for air circulation. This method causes the foliage to dry out and drop, signifying dormancy. The roots will require a brief soaking once a month to prevent them from completely desiccating. This method keeps the plant’s root system alive until it can be repotted and revived in the spring.