The poinsettia, formally known as Euphorbia pulcherrima, is a popular houseplant associated with the winter holidays. Its vibrant foliage, consisting of modified leaves called bracts, makes it a festive indoor display. This plant is highly sensitive to cold temperatures because it evolved in the consistently warm, tropical regions of Mexico and Central America.
The Absolute Minimum Survival Temperature
The lowest temperature a poinsettia can tolerate without sustaining damage is 50°F (10°C). Exposure below this threshold, even for a short period, causes chilling injury, which is distinct from frost damage. The cold disrupts the integrity of the cell membranes, compromising the plant’s internal structure.
Damage occurs because chilling temperatures cause cell membranes to lose fluidity and become rigid, preventing the necessary movement of materials. This structural breakdown leads to the death of affected cells, resulting in irreversible damage to the leaves and stems. Even brief exposure, such as the trip from a store to a warm car, can initiate this process. If the temperature drops to the low 40s or below, the damage becomes more severe, potentially killing the plant outright.
Recognizing Cold Damage Symptoms
Visual signs of cold damage may appear immediately or take up to a full day to manifest. A noticeable symptom is the wilting and drooping of the true leaves and colorful bracts, making the plant look limp. This wilting is not a sign of thirst and will not be corrected by watering. The leaves may also begin to turn a gray-green hue before they fall off.
Defoliation, or leaf drop, is a common reaction as the plant attempts to shed damaged material. In more severe cases, the edges of the leaves and bracts may turn dark brown or black, indicating tissue death. If exposure was prolonged or near freezing, the stems may blacken and become mushy, signifying the plant is unsalvageable.
Maintaining the Optimal Temperature Range
For a poinsettia to thrive and maintain its vibrant color, it should be kept within a specific temperature range. Optimal conditions are daytime temperatures between 65°F and 75°F (18°C to 24°C). The plant benefits from slightly cooler nighttime temperatures, ideally in the low 60s, but the temperature must never drop below 50°F.
Proper placement is essential to maintain this ideal range. Poinsettias should be positioned away from temperature hazards, such as exterior doors or cold drafts from poorly insulated windows. Placing the plant directly against a windowpane can expose the foliage to conducted cold, causing localized leaf damage. Avoid placing the plant near heat sources like radiators or heating vents, as hot air can cause the leaves to dry out and drop prematurely.