A greenhouse functions as a controlled microclimate, modifying external weather conditions to optimize plant growth. Temperature is the most influential factor, governing the speed and efficiency of all biological processes. Maintaining the correct temperature involves managing a dynamic range that shifts between day and night cycles. This careful management determines plant health, yield, and overall success within the protected environment.
Optimal Daytime and Nighttime Temperatures
The ideal temperature for most greenhouse crops is a dynamic range, generally falling between 65°F and 80°F (18°C to 26°C) during the day to maximize photosynthesis. This warmth provides the energy for sugar production, fueling plant growth and development. Warm-season crops, such as tomatoes, peppers, and cucumbers, prefer the upper end of this spectrum, thriving closer to 72°F to 82°F (22°C to 28°C) when light levels are high. If temperatures exceed 95°F (35°C), fruit and flower setting can be compromised.
A necessary temperature drop must occur at night, typically to a range of 55°F to 65°F (13°C to 18°C). This cooler period reduces the rate of cellular respiration, which is the consumption of stored sugars. By slowing the breakdown of these sugars, more energy is conserved and transported to developing tissues like fruits and flowers.
The relationship between day and night temperatures is known as Differential Air Temperature (DIF), calculated as the difference between the Day Temperature (DT) and the Night Temperature (NT). A large positive DIF (DT >> NT) encourages internode elongation, leading to taller plants. Conversely, a negative DIF (NT > DT) or a zero DIF can suppress stem stretching and produce shorter, more compact plants, a technique applied in ornamental horticulture.
Cool-season crops, including lettuce, spinach, and many herbs, prefer a significantly lower overall temperature profile. Their daytime temperature may only need to reach 64°F to 72°F (18°C to 22°C), and they can tolerate nighttime temperatures as low as 50°F (10°C). Maintaining this cooler environment helps prevent bolting, which is the premature sending up of a flower stalk that can make leafy greens bitter.
Understanding Plant Thermoregulation
Temperature directly governs the activity of enzymes that catalyze photosynthesis and respiration within plant cells. Every plant species has a relatively narrow thermal range where its photosynthetic enzymes operate at peak efficiency. Plants are classified as limited homeotherms, meaning they possess mechanisms to stabilize their internal tissue temperature despite fluctuating air temperatures.
When the temperature rises too high, typically above 86°F (30°C), a plant’s metabolic machinery begins to suffer damage. High heat can cause the denaturation of proteins, including the enzymes necessary for photosynthesis, leading to a sharp decline in carbon dioxide uptake. The Photosystem II (PSII) complex, which initiates the light-dependent reactions, is highly sensitive to elevated temperatures and can become dysfunctional.
Plants attempt to cool themselves through transpiration, a process where water vapor is released through the stomata on the leaves, creating an evaporative cooling effect. This natural cooling mechanism is constrained by the available water supply and humidity in the environment. If the air temperature drops too low, below 50°F (10°C) for most warm-season crops, metabolic activity slows significantly. Prolonged exposure to cold can lead to cell damage and stunted growth, as the plant cannot efficiently mobilize stored energy for new development.
Managing Temperature Extremes
Maintaining a stable temperature requires proactive management against overheating and excessive cold, utilizing active and passive methods. Overheating is primarily managed by introducing cooler outside air and removing the hot air that accumulates at the top of the structure. Automatic roof and side vents facilitate natural ventilation, allowing hot air to escape through the ridge while drawing in cooler air from below.
For intense cooling, evaporative systems are effective, especially in drier climates. Pad-and-fan systems draw air through water-saturated pads, where evaporation absorbs heat before the air enters the growing area. Alternatively, fogging systems atomize water into fine droplets that flash-evaporate, lowering the temperature and increasing humidity.
Passive cooling is achieved by reducing solar radiation entering the greenhouse, typically through shade cloths. These cloths are installed externally and can block 30% to 50% of sunlight, providing significant temperature reduction without limiting the light needed for growth. Within the structure, horizontal air flow (HAF) fans prevent thermal stratification and ensure a uniform temperature at the plant canopy level.
To mitigate cold temperatures, heat retention and active heating systems are employed. Insulation is a highly effective passive measure, with materials like bubble wrap applied to the interior glazing to create an extra layer of air that reduces heat loss. Sealing gaps and drafts around doors and vents prevents the escape of warm air.
Thermal mass, such as water barrels painted black or concrete blocks, can be placed inside the greenhouse to passively store solar heat during the day. These materials slowly release warmth back into the air overnight, buffering the temperature drop and reducing the need for continuous heating. Active heating involves electric, gas, or oil heaters. Root-zone heating systems are a more energy-efficient option, warming the soil or hydroponic solution directly around the roots while allowing the ambient air temperature to be kept lower and still promoting optimal growth. Accurate temperature monitoring, using digital thermostats, is necessary to ensure these systems maintain the precise day and night differentials required for crop success.