The success of a winter greenhouse operation hinges on the precise management of interior temperature. This controlled environment must counteract freezing outdoor conditions to sustain plant life and growth. Achieving the correct temperature is not a single number, but rather an appropriate thermal zone tailored to the specific plant species being cultivated. The ideal environment balances the needs of the plants with the significant energy demands of heating an enclosed space during the coldest months. Effective temperature control begins with understanding these plant-specific requirements before implementing any heating or retention strategy.
Defining Necessary Temperature Zones
The required winter temperature is dictated by the plants’ tolerance, dividing a greenhouse into three primary zones based on minimum overnight temperatures.
Cold Greenhouse Zone
The Cold Greenhouse zone is the least expensive to maintain, requiring a minimum night temperature between 35°F and 45°F. This range is suitable for overwintering dormant plants like geraniums, fuchsias, or citrus trees. It also supports harvesting hardy cool-season crops such as kale, carrots, and spinach. Plant growth is minimal, but survival is secured by staying safely above freezing.
Cool Greenhouse Zone
The Cool Greenhouse zone elevates the minimum night temperature to 45°F to 55°F. This climate actively supports the growth of cool-season vegetables like lettuce, Swiss chard, and broccoli, and is suitable for starting semi-hardy seedlings. The higher temperature encourages slow but steady growth, allowing for a continuous winter harvest and early spring transplants. Maintaining this zone demands a moderate increase in heating costs compared to frost protection.
Warm Greenhouse Zone
The Warm Greenhouse zone requires the highest energy expenditure, setting the minimum night temperature at 60°F or above, often aiming for 65°F to 70°F. This environment is necessary for temperature-sensitive tropical plants, orchids, and warm-season crops like tomatoes, peppers, and cucumbers. These plants require sustained warmth to thrive and produce fruit. This zone mandates the most robust heating and insulation measures.
Passive Heat Retention Strategies
Reducing heat loss is the first line of defense against winter cold, often proving more cost-effective than generating more heat.
Insulation and Sealing
Insulating the glazing material is a highly effective strategy, often involving lining the interior with horticultural bubble wrap. This material creates a trapped air space that significantly slows down heat transfer, cutting heat loss by up to 45% without completely blocking sunlight. Structural improvements, such as sealing air leaks around vents, doors, and seams with weather stripping, prevent warm air from escaping. Utilizing twin-wall polycarbonate sheeting instead of single-pane glass also provides a superior insulating barrier due to internal air channels.
Thermal Mass
Incorporating thermal mass materials helps stabilize the internal temperature by absorbing solar energy during the day and releasing it slowly at night. Large water barrels, painted black for maximum heat absorption, are a common form of thermal mass due to water’s high specific heat capacity. Positioning these containers in direct sunlight allows them to function as thermal batteries, buffering the temperature drop on cold nights.
Active Heating Systems
When passive retention is insufficient to meet the required temperature zone, active heating systems are necessary to maintain a stable environment.
Electric Heaters
Electric heaters are a popular option for smaller greenhouses, offering precise thermostatic control and safety without producing combustion byproducts. Forced-air electric fan heaters are especially useful as they circulate the air. This circulation helps eliminate cold pockets and reduces the risk of fungal disease by preventing stagnant, damp conditions.
Gas and Fuel Heaters
For larger structures or where electricity costs are prohibitive, propane or natural gas heaters provide a high heat output, often measured in British Thermal Units (BTUs). Unvented models release combustion gases and significant moisture into the greenhouse, necessitating careful ventilation to prevent carbon monoxide buildup and excessive humidity. Vented gas models, while more expensive to install, exhaust these byproducts safely outside. Kerosene or paraffin heaters are a lower-cost option for supplemental heat but also require daily ventilation to manage humidity and introduce fresh air.
Direct Heat
Horticultural heating cables or heat mats can be used to warm the soil directly. This method is highly efficient for seed starting and rooting cuttings without needing to heat the entire volume of air in the greenhouse.
Monitoring and Maintaining Stable Temperatures
Maintaining temperature stability involves using the right tools to monitor and regulate the environment, preventing both freezing and overheating.
Monitoring and Control
Use an accurate digital thermometer with high/low temperature memory to track daily fluctuations and identify cold spots. Placing sensors at the plant canopy level provides the most relevant temperature reading for the plants. Connecting the active heating system to a reliable thermostat ensures heat is only supplied when the temperature drops below the set minimum, conserving energy. A remote monitoring system or a high-low temperature alarm can send an alert if conditions move outside the acceptable range, allowing the grower to intervene before cold damage occurs.
Ventilation and Contingency
Even in winter, proper ventilation is necessary to prevent the temperature from spiking dangerously high on sunny days, which can rapidly stress plants. Strategic opening of vents or the use of automated louvers allows for air exchange and manages humidity levels that increase with winter heating. Having a contingency plan, such as a battery-powered backup heater or thermal blankets, is an important final step in protecting the greenhouse contents during a power outage.