A greenhouse is a controlled habitat designed to optimize plant growth by managing light, water, and atmospheric composition. Temperature control is the most important factor, directly influencing every physiological process from seed germination to fruit set. Since plants have vastly different thermal requirements, there is no universal “correct” temperature. The ideal warmth is a dynamic target dictated entirely by the specific crop being cultivated, requiring continuous, year-round climate regulation.
Establishing Temperature Zones Based on Plant Needs
Greenhouse cultivation is categorized into three temperature zones, defined by the minimum nighttime temperature required to prevent plant stress and maintain growth. The chosen zone determines the plant species that can be successfully grown and the energy investment needed. Maintaining the correct temperature allows plants to perform metabolic functions efficiently, leading to healthy development and high yields.
Cool Greenhouse
A cool greenhouse maintains a minimum nighttime temperature of 40°F (4°C) to 45°F (7°C). This environment suits plants that thrive in colder climates, such as hardy herbs and leafy greens. Cool-season annuals and winter vegetables like spinach and kale flourish in this lower thermal range. Daytime temperatures are typically kept between 60°F (16°C) and 70°F (21°C) to facilitate photosynthesis.
Moderate Greenhouse
The moderate greenhouse is the most common configuration, supporting a wide variety of crops with a minimum nighttime temperature range of 55°F (13°C) to 60°F (16°C). This zone is ideal for common garden vegetables, including tomatoes, peppers, cucumbers, and many bedding plants. Daytime temperatures generally range from 70°F (21°C) to 80°F (27°C). Maintaining this warmth is necessary for the successful flowering and fruiting of these warm-season crops.
Tropical Greenhouse
A tropical greenhouse requires the highest sustained temperatures, aiming for a minimum nighttime temperature of 65°F (18°C) to 70°F (21°C) or higher. This zone is reserved for sensitive tropical foliage, orchids, and tropical fruits like bananas or citrus. The elevated warmth mimics their native environments, with daytime temperatures often maintained between 75°F (24°C) and 85°F (29°C). Consistent temperature maintenance is a high priority, as these plants are susceptible to cold shock.
The Critical Role of Day and Night Temperature Differentials
Plant physiology dictates that a static, unchanging temperature is suboptimal because key metabolic processes occur at different rates during the day and night. Photosynthesis, which converts light energy into sugars, primarily occurs during sunlit hours when temperatures are naturally higher. Respiration, the process of consuming those stored sugars for growth and maintenance, occurs continuously but slows down at lower temperatures.
The difference between the day temperature (DT) and the night temperature (NT) is known as the temperature differential, or DIF. Allowing the temperature to drop at night conserves energy by slowing the rate of respiration. A cooler night temperature ensures the plant uses less stored carbohydrates for maintenance, leaving more energy for growth and flowering.
Manipulation of DIF controls plant morphology, which is useful in ornamental horticulture. A positive DIF, where the day is warmer than the night (DT > NT), promotes stem elongation, resulting in taller plants. This effect is regulated by the plant hormone gibberellin, whose synthesis is influenced by temperature changes.
Conversely, a negative DIF (NT > DT) or a zero DIF (NT = DT) suppresses internode elongation, resulting in shorter, more compact plants. A prolonged negative DIF can sometimes lead to undesirable side effects, such as reduced leaf area or downward curling of the leaves. Careful management of the temperature differential allows growers to steer plant growth and structure without relying on chemical growth regulators.
Strategies for Active Greenhouse Heating
When external temperatures fall below the minimum required for a chosen temperature zone, active heating systems introduce warmth into the structure. These systems rely on various fuel sources and distribution methods to maintain the set point temperature, especially during cold nights. Selecting a heating system balances initial investment cost with long-term fuel efficiency and responsiveness.
Forced-air heaters, which burn natural gas or propane, are a common and cost-effective option, distributing heat via fans and perforated plastic tubes. Hydronic heating systems circulate hot water through pipes under benches or beneath the floor, providing uniform, energy-efficient radiant heat. Electric resistance heaters offer precise control but can be expensive to run, especially in larger structures or regions with high electricity costs.
Passive methods minimize heat loss and reduce the burden on active heaters. Thermal mass, such as large water barrels or rock beds, absorbs solar energy during the day and slowly releases heat back into the greenhouse at night. Insulation is crucial; growers often use double-layer plastic film or thermal blankets pulled over crops at sunset to reduce heat transfer. Reliable digital thermostats and environmental controllers automate the process, ensuring the heating system activates promptly when the air temperature drops below the minimum set point.
Techniques for Preventing Overheating and Cooling
The greenhouse effect traps solar radiation, meaning preventing overheating during sunny periods is often a greater challenge than providing heat in cold weather. High temperatures cause heat stress, leading to flower abortion, poor fruit set, and reproductive failure in many plants. Successful temperature regulation requires dedicated cooling and heat mitigation strategies.
Ventilation is the primary method of cooling, achieved by exchanging the hot, stale air inside the structure with cooler outside air. Natural ventilation uses strategically placed roof and sidewall vents to allow hot air to escape and draw in fresh air from below. Mechanical ventilation uses powerful exhaust fans to pull air out, often aiming for a complete air exchange every minute to rapidly lower the temperature on hot, sunny days.
Shading is an effective technique to reduce solar heat gain before it enters the structure. External shade cloth, typically rated by the percentage of light it blocks, is deployed over the roof to intercept sunlight. Alternatively, temporary shading compounds, or whitewash, can be applied directly to the exterior glazing to reflect solar energy.
When ventilation and shading are insufficient, evaporative cooling systems actively lower the air temperature. These systems work on the principle that water evaporation absorbs heat from the surrounding air. The most common setup is the fan-and-pad system, where large exhaust fans pull air through wet, porous pads, cooling the air as it enters. High-pressure misting or fogging systems release fine water droplets that quickly evaporate, increasing humidity while providing a cooling effect.