A greenhouse is a controlled environment designed to shield plants from harsh weather and optimize conditions for cultivation. While these structures effectively trap solar energy to maintain warmth, this mechanism also makes them highly susceptible to overheating. The rapid accumulation of heat is the greatest threat to success, making temperature management an absolute necessity. Uncontrolled high temperatures can quickly turn a thriving environment into a stress zone, causing irreparable damage to crops in a matter of hours.
The Mechanism of Rapid Heat Gain
The rapid temperature increase inside a greenhouse is due to the physical phenomenon called the greenhouse effect. This process begins when short-wave solar radiation, or sunlight, easily passes through the transparent glazing material like glass or polycarbonate. Once inside, this energy is absorbed by surfaces such as the soil, benches, and the plants themselves, causing them to warm up.
These warmed objects then re-emit the absorbed energy as long-wave infrared radiation, which is heat. Unlike the incoming short-wave radiation, the long-wave infrared radiation cannot easily pass back out through the glazing material. This trapping of thermal energy causes the air temperature inside the structure to rise quickly, often soaring far above the outdoor ambient temperature. Without a constant method for heat removal, the structure continuously accumulates energy from the sun.
Defining Critical Temperature Thresholds
What constitutes “too hot” is species-dependent, but generalized limits exist where plant stress begins. For many temperate crops, such as cool-season vegetables, sustained temperatures above 85°F (29°C) cause stress and slow growth. Heat-tolerant plants can withstand slightly higher temperatures, but performance drops significantly once the air temperature exceeds 95°F (35°C).
It is also important to consider radiant heat, which is the temperature of the plant surfaces, not just the ambient air temperature. Direct sunlight can cause leaf temperatures to be several degrees higher than the surrounding air. When air temperatures reach a sustained 90°F (32°C), heat damage becomes a serious threat to most greenhouse crops, inhibiting metabolic function.
Effects of Extreme Heat on Plant Health
Exceeding a plant’s thermal threshold triggers several harmful biological responses. One common sign is wilting, which occurs as the plant loses water through transpiration faster than its roots can absorb it. This rapid water loss causes a drop in turgor pressure, leading to the characteristic drooping of leaves and stems.
Heat stress directly impacts photosynthesis, causing the process to slow down or stop entirely above 95°F (35°C). Prolonged exposure to intense solar radiation can result in sun scald, where patches on the leaves turn white or brown as chlorophyll is destroyed. For fruiting plants, extreme heat can induce reproductive failure by sterilizing pollen, causing flowers to drop off before setting fruit and reducing yield. High temperatures can also trigger heat-induced dormancy, where plants cease vegetative growth to conserve energy for survival. Plants weakened by heat stress also become more susceptible to pest infestations and fungal diseases.
Practical Methods for Temperature Control
Managing heat accumulation requires a layered approach, with ventilation being the most immediate and impactful method for control. Effective ventilation ensures a high air exchange rate, achieved passively through roof and side vents or actively with exhaust fans. The goal is to rapidly replace the hot, trapped air with cooler air from outside, using exhaust fans to create a strong cooling draft.
Shading materials offer a proactive solution by reducing the initial solar energy entering the structure. Shade cloths, typically rated by the percentage of sunlight they block, can be deployed externally to prevent overheating. Alternatively, a temporary layer of whitewash can be applied directly to the glazing to reflect sunlight, reducing the internal temperature rise.
For environments that experience extreme heat, evaporative cooling systems provide a powerful supplementary method. These systems include foggers, misters, or fan-and-pad setups that rely on the principle of evaporation to cool the air. As water converts from a liquid to a gas, it draws heat energy from the surrounding air, significantly lowering the temperature inside the greenhouse. This method is effective in hot, dry climates where low ambient humidity allows for maximum water evaporation and cooling efficiency.