A greenhouse is a structure designed to capture solar energy, creating a warm, protected environment that extends the growing season. This mechanism, often called the greenhouse effect, involves sunlight passing through the transparent covering and converting to heat, which is then trapped inside. While this solar gain is essential for early spring and late fall growth, it becomes a significant liability during hot weather. The challenge for any grower is managing this trapped heat to maintain a climate where plants can thrive, because once temperatures climb too high, the controlled environment quickly turns into a destructive oven.
Defining the Danger Zone
The temperature threshold that signifies “too hot” is not a fixed number but a zone where plant stress begins to occur. For most common greenhouse crops, including tomatoes, peppers, and cucumbers, growth is optimal in the \(70-85^{\circ}\text{F}\) day range, but problems begin above that upper limit. Generally, when air temperatures inside the greenhouse exceed \(90^{\circ}\text{F}\) (\(32^{\circ}\text{C}\)), most plants enter a state of heat stress where physiological processes slow down significantly.
Temperatures persisting above \(95^{\circ}\text{F}\) (\(35^{\circ}\text{C}\)) for several hours can cause severe damage, especially concerning reproductive functions like pollination. For many species, \(105^{\circ}\text{F}\) (\(40^{\circ}\text{C}\)) or higher is often lethal if sustained, leading to widespread cell death. This range is complicated by varietal tolerance; cool-season crops like lettuce and spinach experience stress above \(75^{\circ}\text{F}\), while heat-loving plants such as okra can tolerate the \(80-90^{\circ}\text{F}\) range more easily. The severity of the damage is a cumulative effect of the heat’s intensity, duration of exposure, and the rate at which the temperature rises.
The Physiological Impact of Overheating
The plant’s internal mechanisms are severely disrupted when the surrounding air temperature becomes too high. Heat primarily impairs photosynthesis, the process of converting light energy into chemical energy, by inhibiting the activity of key enzymes. As temperatures rise past the plant’s optimal range, these enzymes begin to lose their functional shape, a process called denaturation.
A plant’s first defense against heat is transpiration, similar to sweating, where water vapor is released through small pores on the leaves called stomata, providing an evaporative cooling effect. However, excessive heat often comes with low humidity, causing the plant to close its stomata to conserve water. This closure halts the cooling mechanism and simultaneously blocks the intake of carbon dioxide necessary for photosynthesis.
With stomata closed, the internal concentration of carbon dioxide drops, and the process of photorespiration begins to overtake photosynthesis. Photorespiration is a wasteful process that consumes energy rather than producing it, leading to a net loss of energy for the plant. Prolonged exposure damages the thylakoid membranes within the chloroplasts, the primary sites for light energy capture. This cellular damage can manifest externally as sunscald or leaf scorch, where the affected tissue turns pale or brown.
Immediate Mitigation Strategies
When an unexpected heat spike pushes the greenhouse into the danger zone, immediate measures are necessary to prevent crop damage. The primary action is to maximize passive ventilation to allow superheated air to escape. This involves opening all roof vents, side vents, and doors to create a chimney effect, where hot air naturally rises and exits through the top while cooler air is drawn in from below.
Rapid deployment of temporary shading is another reactive measure to instantly reduce solar load. A grower can quickly cover the exterior with a shade cloth, which can block \(30-70\%\) of the incoming light and significantly lower the interior temperature. Alternatively, emergency evaporative cooling can be implemented by activating misting or fogging systems. As the fine water droplets evaporate, they absorb heat from the air, providing a rapid cooling effect, particularly effective in drier climates.
Long-Term Temperature Management
Proactive measures prevent thermal spikes and provide consistent temperature control. Automated ventilation systems use powered exhaust fans to physically pull large volumes of air out of the greenhouse, replacing it with cooler outside air. These systems are often paired with motorized roof vents that open automatically when temperatures exceed a preset limit, such as \(77-86^{\circ}\text{F}\).
Incorporating thermal mass is a passive strategy that helps stabilize temperature fluctuations throughout the day and night. Materials like large water barrels, concrete flooring, or hay bales absorb heat during the day and slowly release it as the greenhouse cools after sunset. Permanent shading methods, such as applying a reflective whitewash to the exterior or installing a fixed shade cloth, consistently reduce the intensity of solar radiation entering the structure. These preventative design choices are more efficient than relying solely on reactive measures during a heat crisis.