From a botanical perspective, the poinsettia (Euphorbia pulcherrima) is a photoperiodic plant native to the Pacific coast of Mexico, where it enjoys a relatively stable, warm climate. Its temperature requirements are therefore specific and crucial for maintaining its health and vibrant bract coloration throughout the holiday season and beyond. The ideal temperature range is a reflection of its tropical origins.
During its period of active growth and display, which for most consumers is from late November through the winter months, the poinsettia thrives in daytime temperatures between 65°F and 75°F (18°C - 24°C). This range is ideal for several physiological reasons. It supports efficient photosynthesis, the process by which the plant converts light into energy, without causing excessive stress. It also promotes proper respiration and transpiration rates, allowing for healthy cellular function and nutrient transport. Nighttime temperatures can be slightly cooler, ideally dropping to around 60°F - 65°F (15°C - 18°C). This mild dip mimics the plant's natural environment and helps to prolong the life of the colorful bracts (modified leaves) and the tiny central flowers, called cyathia.
Poinsettias are highly sensitive to sudden temperature changes and drafts, which can be more damaging than a steady, slightly non-ideal temperature. Exposure to cold drafts from doors, windows, or air conditioning units, or direct contact with hot, dry air from heating vents or radiators, can cause significant physiological stress. This stress manifests as rapid and severe leaf drop (abscission). The plant interprets a cold draft as a signal of a damaging frost, triggering an abscission layer to form at the base of the leaf stem to conserve resources. Similarly, hot, dry air drastically increases the rate of transpiration, desiccating the leaves and bracts and causing them to wilt, curl, and drop prematurely.
For an enthusiast wishing to keep the plant alive and encourage it to rebloom the following year, temperature management becomes even more critical after the holiday season. As spring approaches and the bracts fade, the plant enters a resting phase. During this time, slightly cooler temperatures are acceptable. However, to initiate the flowering process for the next season, precise temperature control is essential during the bud development period (typically October through December). Nighttime temperatures must be consistently maintained at or above 60°F (15°C), as cooler nights can delay or completely inhibit the development of the colorful bracts, regardless of perfect light conditions.
Straying outside the recommended ranges has direct and visible consequences. Temperatures consistently below 50°F (10°C) will cause chilling injury, leading to root damage, leaf blackening, and massive leaf drop. Brief exposure to near-freezing temperatures (below 40°F / 5°C) is often fatal. Conversely, prolonged exposure to temperatures above 80°F (27°C), especially when combined with low humidity, accelerates the plant's metabolism excessively, leading to wilting, nutrient deficiencies, and a shortened overall lifespan of the display. The vibrant red or other colored bracts will fade and deteriorate more quickly under high heat stress.
