Bougainvillea is a tropical plant known for its vibrant, paper-like bracts, but its South American origin makes it sensitive to cold temperatures. This perennial vine is not frost-hardy, and its survival is threatened whenever temperatures drop near the freezing point. Temperatures falling below 32°F (0°C) cause ice crystals to form within the plant’s cells, leading to tissue damage that manifests as wilting or browning foliage. Temperatures sustained below 30°F (-1°C) for several hours can cause severe damage to the outer branches and may kill the plant down to the root crown. Proactive protection is necessary, ideally beginning when the forecast drops below 40°F (4°C), to shield this tender plant from damaging cold.
Long-Term Mitigation and Placement
Strategic placement reduces the risk of cold damage by utilizing natural microclimates in the landscape. Planting bougainvillea against a south-facing wall or structure is beneficial because these surfaces absorb solar energy during the day and radiate heat back toward the plant at night. This placement also offers a shield from cold northern winds, which can be particularly damaging when combined with low temperatures.
In regions that experience frequent, extended hard freezes, growing bougainvillea in containers is a more reliable long-term strategy. A potted plant is much easier to relocate indoors when cold weather threatens, ensuring its survival through the winter months. Preparation for cold weather should begin in late summer by ceasing nitrogen-rich fertilizer applications to discourage tender new growth that is susceptible to freezing.
Reducing the frequency of watering in the late fall encourages the bougainvillea to enter a semi-dormant state, which improves its cold tolerance. To protect the root system of in-ground plants, an insulating layer of organic mulch, such as straw, shredded bark, or leaves, should be applied. This layer should be four to six inches deep and cover the area around the base, but it should be kept a few inches away from the main stem to prevent moisture buildup and potential rot.
Immediate Action When Frost is Forecasted
When temperatures are predicted to drop below 35°F, temporary protection measures must be implemented to preserve the plant’s foliage and stems. For in-ground bougainvillea, the most effective method is to create a protective canopy using breathable materials like burlap, frost cloth, or old blankets. These materials trap the heat that naturally rises from the soil without allowing moisture to build up and freeze on the leaves.
The covering must extend fully to the ground and be secured with stakes or heavy objects to prevent wind from blowing it off and cold air from seeping underneath. A helpful technique to increase the temperature beneath the cover is to string a strand of old-style, incandescent Christmas lights through the branches, as these bulbs produce a small amount of heat. Non-LED lights can raise the temperature by several degrees, providing a localized thermal barrier against the cold.
Potted plants should be moved into a sheltered location, such as a garage, shed, or basement, that remains above freezing temperatures. If moved indoors for the winter, the plant should be placed in a cool area, ideally between 40°F and 50°F, to allow it to rest. Deeply watering the soil approximately 24 hours before the freeze event is beneficial, as moist soil holds and radiates heat more effectively than dry soil, creating a warmer microclimate around the root ball.
Assessing and Managing Frost Damage
After the cold threat has passed, the first step is to remove any temporary coverings to allow the plant to breathe and receive sunlight. Initial signs of cold damage include wilting, blackening, or browning of the leaves and outer stem tips, indicating that the plant tissue has been compromised by ice formation. It is important to resist the urge to immediately prune away the damaged material, even if it looks unsightly.
The frosted, dead foliage and stems act as a natural layer of insulation, helping to shield the underlying wood and root crown from any subsequent cold snaps. Pruning stimulates new, tender growth, which would be vulnerable to another unexpected late-season frost. A reliable practice is to wait until the danger of frost has fully passed, typically in late spring, or when evening temperatures are consistently above 40°F.
At the appropriate time, the extent of the damage can be determined using a scratch test on the branches. Scratching a small area of the bark with a fingernail or knife will reveal the cambium layer underneath; if the tissue is bright green, the branch is alive, but if it is brown, the tissue is dead and should be removed. For severe damage where the entire top growth is killed, the plant should be cut back almost to the ground to encourage robust new growth from the protected root system.