A greenhouse allows gardeners to extend their growing season by creating a microclimate that is warmer and more stable than the outside environment. This enclosed structure traps solar energy, a phenomenon often referred to as the greenhouse effect, which provides plants with the necessary warmth to thrive. However, this same heat-trapping mechanism poses the single largest threat to the plants within, as internal temperatures can quickly rise far beyond healthy thresholds. Understanding the physics of heat accumulation, the factors that drive temperature extremes, and the precise point at which plants begin to suffer damage is paramount to preventing a catastrophic crop failure. The question is not simply how hot a greenhouse can get, but rather, what is the maximum temperature plants can endure before they die.
The Physics of Solar Heat Accumulation
A greenhouse heats up through a two-step process involving the conversion of solar radiation. Sunlight, which is primarily composed of short-wave radiation (visible light), easily passes through the transparent glazing material, such as glass or polycarbonate. This incoming short-wave energy is then absorbed by all the surfaces inside the structure, including the soil, plants, and structural elements.
Upon absorption, the energy is re-radiated, but at a much longer wavelength, transforming into long-wave infrared radiation, which is perceived as heat. The glazing material, while transparent to the initial short-wave light, is largely opaque to this longer-wave infrared heat. This opaqueness effectively traps the thermal energy inside the structure.
This trapped long-wave radiation cannot easily escape, leading to a continuous accumulation of heat. The greenhouse structure acts as a thermal barrier, preventing the heat from dissipating back into the atmosphere, which causes the internal temperature to climb significantly higher than the ambient outdoor temperature.
Factors Determining Peak Internal Temperature
The maximum temperature an uncooled greenhouse can reach is determined by a complex interplay of environmental and structural characteristics. The most direct external influences are the ambient outdoor temperature and the intensity of solar radiation, with clear, sunny days causing the most rapid and dramatic temperature spikes.
The material used for the greenhouse covering significantly affects how much heat is retained; for example, glass or polycarbonate transmit light and trap heat differently due to variations in their transmissivity to long-wave radiation. Furthermore, the presence of thermal mass, such as a concrete floor or large barrels of water, absorbs heat during the day and releases it slowly at night. This can buffer temperature swings but also contribute to the daytime peak.
The physical dimensions of the structure also play a role, as smaller greenhouses possess a higher surface area to volume ratio, causing them to heat up much faster than larger structures. Finally, the greenhouse’s orientation and the angle of the sun’s exposure affect the total solar gain.
Temperature Thresholds for Plant Damage
Excessive heat exposure causes physiological stress in plants, ultimately leading to damage or death if sustained. When temperatures climb above a plant’s optimal range, the rate of photosynthesis begins to slow down, and respiration rates may increase, consuming more energy than is produced. A major consequence is the denaturation of enzymes, which are proteins that facilitate almost all metabolic processes.
Plants attempt to cool themselves through transpiration, a process where water vapor is released through stomata. If the temperature gets too high, or if water uptake is insufficient, the stomata close to conserve moisture. This closure prevents evaporative cooling and leads to a rapid increase in leaf temperature, which can result in leaf wilting or scorching. For many common greenhouse crops, significant heat damage, such as leaf senescence or flower drop, begins to occur when the air temperature surpasses 90°F (32°C).
The precise lethal temperature varies widely depending on the plant type. Cool-season crops like lettuce suffer stress when temperatures exceed 70°F (21°C), and will quickly bolt or become damaged at higher sustained temperatures. Conversely, warm-season crops like tomatoes can tolerate temperatures up to about 90°F (32°C). However, at higher temperatures, they may fail to set fruit due to poor pollination. Prolonged exposure above 100°F (38°C) is generally considered an extreme danger zone for the majority of greenhouse flora, leading to reduced yield and potential plant death.
Essential Strategies for Heat Mitigation
Managing internal temperatures is accomplished through a combination of active and passive cooling techniques designed to counteract solar heat accumulation. Ventilation is the most fundamental method, involving the exchange of hot internal air for cooler outside air. Passive ventilation relies on strategically placed roof and side vents, allowing the natural buoyancy of hot air to rise and escape while cooler air is drawn in at a lower level.
For larger structures or in hotter climates, active ventilation using exhaust fans is necessary to achieve a high air exchange rate, forcibly pulling hot air out and creating a consistent airflow. This air movement also helps prevent pockets of stagnant, overheated air from forming around the plants. A secondary, highly effective strategy is shading, which reduces the amount of short-wave radiation entering the structure.
Shading can be achieved using external shade cloths with various light-reduction percentages, or by applying a temporary coat of reflective material, often called whitewash, directly to the glazing. When these methods are insufficient, evaporative cooling is used, which takes advantage of the energy required for water to change phase from liquid to gas. Evaporative cooling systems, such as fan-and-pad setups or high-pressure fog/mist systems, introduce fine water droplets into the air. As this water evaporates, it absorbs latent heat from the surrounding air, significantly lowering the greenhouse temperature, a process particularly effective in dry climates.