A greenhouse is a structure designed to manipulate the environment by capturing and retaining solar energy, creating a warmer microclimate than the external surroundings. A winter greenhouse’s exact temperature depends heavily on its specific design, construction materials, and the severity of the local climate. Because the structure is not a perfectly sealed system, the internal temperature fluctuates significantly between sunny daylight hours and cold winter nights. This warming effect relies on the physical process known as the greenhouse effect, which dictates how solar energy is transformed and held within the enclosure.
Passive Solar Gain and the Greenhouse Effect
The passive warming mechanism begins with the transmission of solar radiation. Shortwave radiation, including visible light, passes easily through the transparent covering material, such as glass or plastic. Once inside, this energy is absorbed by internal surfaces like the soil, plants, and structural elements. These surfaces heat up and re-emit the absorbed energy at a much longer wavelength, specifically as longwave infrared radiation.
The transparent covering is designed to be largely opaque to this re-radiated longwave infrared energy, preventing it from passing back out easily. This trapped energy elevates the internal temperature above the ambient outdoor temperature, acting as a heat trap. This passive heating is highly effective during clear, sunny days, sometimes raising the internal temperature by 20°F or more above the outside air. However, solar gain ceases immediately at sunset, requiring the structure to rely on stored heat and insulation to maintain warmth through the night.
Structural Factors Determining Heat Loss
Despite efficient passive solar gain during the day, the temperature inside a greenhouse drops significantly at night due to several heat loss mechanisms. The most straightforward pathway is conduction, the direct transfer of thermal energy through the covering materials themselves. Materials like single-pane glass or standard polyethylene film have high conductivity, meaning heat rapidly flows from the warm interior surface to the cold exterior surface.
Convection is another substantial source of heat loss, involving the physical movement of warm air away from the structure. In a greenhouse, this primarily occurs as infiltration, where warm internal air escapes through unintended gaps, cracks, vents, and door seals. A poorly sealed structure can exchange its entire volume of air multiple times per hour, quickly replacing warm air with cold outside air.
The third major heat loss mechanism is radiation, where heat radiates directly through the transparent covering into the cold night sky. This is particularly pronounced on clear nights, as the structure’s interior surfaces directly emit infrared energy. Structural integrity and material choice determine how quickly the collected daytime heat is ultimately lost.
Categorizing Winter Greenhouse Environments
The temperature a greenhouse maintains in winter dictates what types of plants can successfully survive or thrive, leading to three classification categories. An unheated or cold greenhouse maintains an internal temperature only 5°F to 10°F above the outside ambient temperature during the day. On cold nights, the temperature often drops near or below freezing, making it suitable only for overwintering hardy, dormant plants or cold-tolerant vegetables.
A cool greenhouse is minimally heated to maintain a nighttime temperature consistently above freezing, typically between 35°F and 45°F. This range is appropriate for cool-season crops, such as lettuce and spinach, and for protecting half-hardy tender plants. Maintaining this moderate temperature provides a sufficient buffer against sudden cold snaps without requiring excessive energy input.
A warm or intermediate greenhouse requires active heating to maintain a minimum temperature usually above 50°F to 55°F. This environment is necessary for growing tropical plants, propagating seedlings early in the season, or achieving out-of-season fruit production. This higher temperature zone demands a robust heating system and significantly greater energy expenditure to counteract continuous heat loss.
Methods for Supplemental Winter Heating
When passive solar gain is insufficient, supplemental strategies are used to stabilize the internal temperature, especially through the night. One non-mechanical method involves incorporating thermal mass, which utilizes dense materials to absorb excess heat during the day and slowly release it after sunset. Water is an effective medium for this purpose because its high heat capacity allows it to store a large amount of energy per volume.
Placing large containers of water or utilizing stone or concrete flooring helps to mitigate the rapid temperature drops that occur after dark. This passive heat bank creates a more stable thermal environment, reducing stress on plants. For environments requiring temperatures above freezing, active heating systems are employed, such as electric, propane, or natural gas heaters.
These active heaters should be paired with a thermostat to ensure they only activate when the temperature falls below the desired minimum set point, conserving energy. Beyond heating, reducing heat loss is accomplished by adding internal insulation, such as lining the walls with horticultural bubble wrap. This secondary layer effectively reduces heat loss by lowering the rate of conduction through the covering material.