Greenhouse temperature is the most important environmental factor, directly controlling plant growth, flowering, and overall crop health. Plants are highly sensitive to thermal conditions, with even minor fluctuations impacting their metabolic processes. There is no universal “correct” temperature for a greenhouse; instead, the ideal setting is a dynamic target determined by the specific crop and its stage of development. Effective management involves maintaining a precise thermal balance throughout the entire day and night cycle.
Establishing Ideal Day and Night Temperatures
Temperature management relies on understanding the Day/Night Temperature Differential, often shortened to DIF. This is the difference between the average temperature maintained during daylight hours and the temperature maintained overnight. This differential is driven by the physiological principle of energy conservation.
During the day, a higher temperature maximizes the rate of photosynthesis, which is the process where plants convert light energy into sugars for growth. However, plants also perform respiration constantly, which consumes these stored sugars to fuel metabolic processes. This consumption rate increases with temperature.
A cooler nighttime temperature slows the rate of respiration, preserving the sugars created during the day for growth. Therefore, a positive DIF—where the daytime temperature is typically 5 to 10 degrees Fahrenheit warmer than the nighttime minimum—results in better net growth and carbon efficiency. Consistent nighttime cooling is necessary to signal the plant to rest and consolidate the day’s energy production.
Adjusting Temperatures for Specific Crops
Optimal temperature settings are highly specific and depend on the plant’s natural climate of origin. Greenhouse plants can be grouped into thermal categories that define their ideal growing ranges. This classification allows for precise temperature control based on the current contents of the growing space.
Cool-season crops, such as spinach, lettuce, and cabbage, thrive in moderate conditions and are sensitive to heat. These plants generally prefer a daytime temperature below 75°F, with spinach doing best when the thermostat is set between 50°F and 65°F. Exceeding their upper thermal limit can cause the plant to “bolt,” or prematurely flower, resulting in a bitter taste and ending the harvest.
Warm-season crops, including tomatoes, peppers, and cucumbers, require higher temperatures to develop fruit successfully. Their optimal range is typically between 70°F and 85°F during the day. A nighttime temperature that drops below 55°F can cause significant stress. Sustained night temperatures above 75°F can cause the pollen to become sterile, leading to blossom and fruit drop.
For overwintering tropical plants like citrus trees and succulents, the primary temperature concern is maintaining a safe minimum to prevent plant tissue damage. While they may not be actively growing, a stable environment is necessary, often requiring a minimum nighttime temperature of 45°F to 50°F. Keeping the temperature in this semi-dormant range allows the plant to survive the winter without expending excessive energy on growth.
Managing Excess Heat (Cooling and Ventilation)
The greenhouse effect often causes internal temperatures to rise far above ambient outdoor temperatures, making cooling a primary management challenge. Passive ventilation uses natural forces to exchange air, relying on the principle that hot air rises. This is achieved by opening automatic ridge vents at the highest point of the structure, allowing heated air to escape, while cooler air is drawn in through lower side vents or roll-up sidewalls.
Active ventilation uses mechanical devices to move air forcefully and predictably. Exhaust fans are installed on one end of the greenhouse to pull stale, hot air out, drawing in fresh air through intake louvers or vents on the opposite end. Horizontal Air Flow (HAF) fans circulate air internally, eliminating pockets of stagnant air and ensuring a uniform temperature and humidity level throughout the plant canopy.
For extreme heat, evaporative cooling systems (‘swamp coolers’ or fan-and-pad systems) are employed. These systems use exhaust fans to pull air through a wet, saturated pad, where the evaporation of water removes significant heat energy from the air stream. This process can effectively lower the internal temperature by up to 10°C (18°F) below the outside air temperature, especially in drier climates.
Another passive cooling method involves the use of external shade cloth to reduce the solar heat gain before it enters the structure. For example, a 30% to 50% shade cloth is often used for warm-season vegetables to reduce heat stress while still allowing sufficient light for photosynthesis. Placing the shade cloth on the exterior of the greenhouse is significantly more effective, as it prevents the heat from being absorbed and trapped by the glazing material.
Protecting Against Cold (Heating and Insulation)
Minimum temperatures during cold weather are maintained using active heating and passive insulation techniques. Active heating systems provide reliable heat on demand, controlled by a thermostat set to the crop’s minimum desired night temperature. Forced-air heaters, fueled by propane, natural gas, or electricity, are common choices, with gas options often preferred for larger spaces due to lower operating costs compared to high-wattage electric units.
Electric heaters are convenient and provide precise temperature control, making them suitable for smaller greenhouse environments. Regardless of the fuel source, the heater should be placed to ensure even distribution of warm air, often requiring the use of circulation fans or specialized ducting. Vented combustion heaters are necessary when using gas fuels to prevent the buildup of combustion byproducts, which can be toxic to plants.
Passive heating and insulation techniques reduce the energy needed for active systems. Thermal mass objects, such as large, dark-colored barrels filled with water, absorb solar energy during the day and slowly radiate that stored heat back into the greenhouse air overnight. Applying bubble wrap or poly sheeting to the interior walls, particularly on the north side, adds an insulating layer that reduces heat loss through conduction and convection, further stabilizing the internal temperature during cold spells.