A greenhouse is a controlled environment that allows growers to cultivate plants outside of their natural season by harnessing the sun’s energy. Temperature is the most important environmental factor influencing plant growth, development, and yield. Successful cultivation depends on establishing and consistently maintaining a stable climate. Effective temperature management ensures that biological processes like photosynthesis and respiration occur efficiently, which is the foundation for a productive growing operation.
Defining the Optimal Temperature Range
The optimal temperature range for most common greenhouse crops, including vegetables and flowers, falls between 64°F and 75°F (18°C and 24°C). Within this range, plants perform photosynthesis most effectively, allowing for rapid and healthy growth. Temperatures that are too low slow the plant’s metabolic rate, causing stunted growth and increasing the time needed to reach maturity.
Conversely, temperatures consistently exceeding 85°F (29°C) cause heat stress. High heat reduces the rate of photosynthesis, sometimes stopping it entirely, and leads to problems like wilting, flower drop, and poor fruit set. If the temperature exceeds 90°F (32°C), many plants struggle to survive. Maintaining temperatures within the recommended boundaries is necessary for maximizing the quality and quantity of the harvest.
The Importance of Day and Night Differentials
The ideal greenhouse temperature is not constant but fluctuates between day and night cycles. This difference, known as the differential (DIF), affects how a plant allocates energy. A positive DIF, where the day temperature is warmer than the night temperature, is preferred because it influences stem elongation and plant height.
The night temperature must be lower than the day temperature to slow down respiration, the process where plants use stored sugars for maintenance. Since photosynthesis only occurs during the day, a cooler night reduces the consumption of these energy reserves. This conservation allows the plant to use more energy for structural growth, such as forming new leaves and flowers. Typically, a night temperature drop of 5°F to 10°F (3°C to 5°C) below the daytime setting achieves this beneficial effect.
Adjusting Temperatures for Specific Crop Types
General temperature ranges must be tailored based on the specific crop, as different plants have varying base and optimal temperatures. Plants are broadly categorized into warm-season and cool-season types, each with distinct requirements.
Warm-season crops, such as tomatoes, peppers, and eggplants, require daytime temperatures between 70°F and 80°F (21°C and 27°C). Their minimum night temperature should not fall below 60°F (16°C), as cooler temperatures inhibit growth and delay fruit set. Overly high night temperatures can also cause flowering problems for these plants.
Cool-season crops, such as lettuce, spinach, and pansies, prefer a lower thermal environment. These plants perform best with daytime temperatures between 60°F and 70°F (15°C and 21°C), and they tolerate night temperatures as low as 50°F (10°C). Maintaining cooler conditions is important because high temperatures, especially above 80°F (27°C), cause leafy greens to “bolt,” or prematurely flower, making the leaves bitter.
Practical Strategies for Temperature Regulation
Maintaining the desired temperature requires a combination of structural and mechanical methods for both heating and cooling. During colder periods, active heating prevents temperatures from dropping below the plant’s base temperature. Systems like electric or gas heaters, or radiant heating, distribute warmth evenly throughout the structure.
Managing heat buildup is a major challenge, especially on sunny days when the greenhouse effect traps solar energy. Active cooling relies on ventilation, achieved through automated roof and side vents that open to release hot air. Exhaust fans are often used with these vents to pull warm air out and draw cooler air in, ensuring constant air exchange.
In warmer climates, evaporative cooling systems, such as fan-and-pad or fogging systems, are effective. These systems work by evaporating water, which draws heat out of the air and lowers the internal temperature. Shade cloth, available in various densities, is a simple tool used externally to block solar radiation and prevent heat entry. The entire process is often managed by automated systems that use sensors and thermostats to adjust equipment and maintain the pre-set day and night differentials.