The bougainvillea is a vibrant, woody, tropical vine native to the warm, frost-free climates of South America, particularly Brazil. It is celebrated for its showy, colorful bracts that surround inconspicuous flowers, creating a dramatic display. Because its physiological structure is optimized for warmth, the bougainvillea is extremely sensitive to cold temperatures. Any significant drop in temperature poses a direct threat to its survival.
The Critical Temperature Threshold
The lowest temperature a bougainvillea can tolerate without protection is the freezing point. Temperatures between 32°F and 40°F typically induce non-lethal stress, causing the plant to drop leaves and colorful bracts. This is a survival mechanism where the plant sheds surface area to conserve energy and moisture, entering a temporary semi-dormant state.
The truly dangerous threshold is a sustained temperature below 30°F. At this point, water inside the plant’s cells can freeze, rupturing cell walls and causing tissue death in the upper branches and foliage. Consecutive nights of freezing are particularly harmful because the cold penetrates deeper into the soil, potentially damaging the root system, which is often fatal.
Factors Influencing Cold Tolerance
A bougainvillea’s ability to withstand cold varies based on several environmental and plant-specific factors. The duration of cold exposure is paramount; a brief dip to 30°F is less damaging than twelve continuous hours at that temperature. Soil water content also plays a role, as slightly moist soil holds heat and insulates roots better than dry soil.
The plant’s location determines root protection; in-ground bougainvilleas have more insulation than container plants, which are exposed to cold air on all sides. Additionally, the presence of frost combined with low temperatures increases damage severity by rapidly drawing heat away from plant tissue. Older, established plants are generally more resilient than younger specimens due to their larger root systems.
Protecting Bougainvillea from Cold
Protection requires preemptive action when temperatures are forecasted to drop below 40°F. Container plants should be moved indoors to a sheltered location like a garage, greenhouse, or sunroom. If the plant is too large to move, apply a thick layer of organic mulch (four to six inches deep) around the base of the stem to insulate the root ball.
For in-ground plants, temporary coverings are necessary to shield the above-ground structure. Wrap the vine loosely with burlap or a specialized frost cloth, ensuring the material does not touch the foliage directly, as frost can transfer through contact points. Temporary heat sources, such as a string of incandescent holiday lights placed beneath the covering, can provide enough warmth to raise the temperature during severe cold dips.
Assessing and Recovering from Cold Damage
After a cold event, resist the urge to immediately prune away seemingly dead wood. The damage often looks worse than it is, and cutting back too soon can stimulate new, tender growth vulnerable to subsequent cold snaps. The proper time to assess damage is when the risk of freezing has passed and evening temperatures are consistently above 40°F, typically in mid to late spring.
To determine which parts of the plant are still alive, perform a simple scratch test on the stems. Gently scrape the bark with a fingernail or knife; if green tissue appears underneath, that section is viable. Prune only the wood that reveals brown or dry tissue, cutting back to the nearest section of healthy, green wood. Following pruning, provide a deep watering and a feeding of high-nitrogen fertilizer to encourage vigorous new growth.