A greenhouse provides a controlled environment that extends the growing season by capturing solar energy and creating a microclimate. However, winter’s short days and frigid temperatures challenge the ability to maintain warmth. Deciding whether to heat a greenhouse is a balance between desired crop outcomes and the cost and effort of temperature regulation. Successfully growing produce or overwintering plants requires a specific strategy to manage the thermal demands of cold weather.
The Necessity of Winter Greenhouse Heating
The necessity of winter heating depends entirely on the gardener’s specific cultivation goals. For those simply overwintering dormant, hardy plants, such as perennials or bulbs, minimal supplemental heat is required. The structure alone provides protection from harsh winds and moisture, often keeping the interior just a few degrees above the outside air temperature.
A slightly higher level of temperature management is needed for extending the shoulder season or growing cool-weather crops like spinach, kale, and root vegetables. These plants tolerate near-freezing temperatures but benefit from minimal heat input to prevent frost and ensure continuous, slow growth. Maintaining the temperature just above the freezing point is sufficient for these hardy varieties.
Continuous heating is necessary only if the goal is to grow tropical, warm-weather vegetables or flowers during the winter months. Crops such as tomatoes, peppers, or orchids require consistent warmth to survive and thrive. This often necessitates a substantial energy investment to prevent plant cell damage and death from freezing.
Establishing the Ideal Temperature Zone
Once the need for heat is established, the target temperature must be defined based on plant requirements. The “Cold” zone is suitable for overwintering hardy plants and growing the most cold-tolerant vegetables, maintaining temperatures between 40 to 45°F (4 to 7°C). This zone focuses primarily on frost prevention rather than active growth.
The “Cool” zone is slightly warmer, targeting temperatures from 45 to 55°F (7 to 13°C), and is ideal for growing most leafy greens and brassicas like cabbage and broccoli. This range allows for slow but steady growth during the low-light winter months. Achieving this zone requires a modest amount of supplemental heat and good heat retention strategies.
The most demanding classification is the “Warm” or “Intermediate” zone, which requires a minimum temperature range of 55 to 65°F (13 to 18°C). This zone is necessary for the continuous production of heat-loving crops, such as tropical fruits, tomatoes, and certain flowers. Maintaining this consistently high temperature requires the greatest effort and involves the highest energy costs.
Active and Passive Heating Systems
Generating the required heat can be accomplished through both active and passive systems, which focus on heat generation and storage, respectively. Active systems involve mechanical equipment that consumes fuel or electricity to produce warmth on demand. Electric fan heaters are popular for their precise temperature control and uniform air circulation, helping to eliminate cold spots.
Alternatively, forced-air heaters that run on propane, natural gas, or kerosene offer a powerful heat source often used in larger structures. Hydronic heating systems, which circulate hot water through pipes embedded in the floor or placed along plant beds, provide a gentle, highly uniform heat distribution, though they require a higher initial investment.
Passive systems, conversely, rely on solar energy absorption and thermal mass to store and release heat without external power. Materials with high specific heat capacity, such as large water barrels painted black or rock beds, absorb solar radiation during the day. The stored heat is then slowly released into the greenhouse air at night, moderating the temperature drop and reducing the reliance on active heating.
Insulation and Heat Retention Strategies
Preventing generated heat from escaping is often a more cost-effective way to manage winter temperatures than increasing heater output. Heat loss occurs through the greenhouse covering materials and air leakage. Sealing gaps and cracks around doors, vents, and panel joints with weather-stripping or caulk is a fundamental first step in heat retention.
Foundation insulation, such as skirting the base of the greenhouse with foam boards, prevents heat from being lost to the cold ground. For the main structure, adding an internal layer of insulation is highly effective; for instance, lining the interior walls with horticultural bubble wrap creates a dead air space that reduces conductive heat transfer.
The strategic use of internal barriers also helps to concentrate heat around the growing area. Thermal curtains or blankets, often made of reflective material, can be drawn across the ceiling or around plant benches at night to trap heat directly over the crops. Creating a smaller, insulated growing space within the main greenhouse structure further minimizes the volume of air that needs to be heated.