The warmth inside a greenhouse during winter is variable, determined by its structure, environment, and heating strategy. The fundamental principle is the greenhouse effect: solar radiation penetrates the transparent covering, is absorbed by interior surfaces, and then re-radiated as long-wave infrared energy. Since the glazing material is less transparent to this longer-wave energy, much of the heat is temporarily trapped, raising the interior temperature. However, this passive solar gain is rarely sufficient to maintain temperatures high enough for plant survival during severe cold or long, sunless periods. The final temperature depends entirely on the degree of insulation and whether supplemental heat is actively supplied.
Factors Influencing Passive Winter Warmth
The structure’s ability to hold warmth without mechanical heating relates directly to its construction materials and design. The glazing material’s R-value, a measure of resistance to heat flow, is the primary factor in heat retention. Single-pane glass has a low R-value of approximately 0.95. In contrast, multi-wall polycarbonate can offer an R-value of 3.2 or higher, significantly reducing heat loss. Double-pane glass, with an R-value around 2.0, provides a middle ground for light transmission and insulation.
Incorporating thermal mass is a key passive strategy to stabilize temperature fluctuations between day and night. Materials with high density and specific heat capacity, such as water, concrete, or stone, absorb solar energy during the day. Water is particularly effective, holding up to four times the heat of soil. This stored heat is then slowly released back into the structure during the night, preventing rapid temperature drops.
Strategic insulation and air-sealing further enhance passive warmth retention. Applying horticultural bubble wrap to the interior walls, especially at night, can create an insulating air layer that reduces heat transfer through the glazing. Sealing gaps in the foundation, around doors, and at vents prevents cold air infiltration and the escape of warm air. Placing the structure with a southern orientation and insulating the north, east, and west walls maximizes solar gain while minimizing heat loss.
Defining Greenhouse Temperature Zones
The required minimum winter temperature is defined by the needs of the plants being cultivated. Selecting a target temperature zone is the first step in determining heating requirements.
Cold Frame or Frost-Free Zone
This zone aims only to maintain a minimum temperature just above freezing, around 35°F (2°C). This protects dormant plants from lethal frost damage.
Cool Greenhouse Zone
This zone targets a minimum night temperature between 45°F and 50°F (7°C-10°C). This range is suitable for many hardy vegetables, herbs, and certain flowering plants like fuchsias. This temperature allows for slow growth while limiting energy expenditure for heating.
Temperate Greenhouse Zone
For more sensitive plants, the Temperate Greenhouse zone is required. This maintains a minimum of 55°F to 60°F (13°C-16°C).
Warm or Hot Greenhouse Zone
This zone is necessary for tropical or heat-loving crops, such as orchids, tomatoes, and cucumbers. It requires a minimum temperature of 65°F (18°C) or higher. Many common commercial crops thrive in an optimal range of 64°F to 75°F (18°C-24°C).
Strategies for Active Heat Generation
When passive measures are insufficient to maintain the target temperature zone, active heat generation becomes necessary, typically relying on mechanical systems. Forced-air heaters, fueled by propane, natural gas, or electricity, are common due to their rapid heating ability. These systems use a fan to quickly distribute warm air, which also helps control humidity and reduce the risk of fungal diseases.
A downside of forced-air heating is that heat rises, often leading to significant thermal stratification where the floor remains cold and the peak of the greenhouse is the warmest. Radiant heating systems offer a more efficient alternative by using hot water pipes or electric cables embedded under the benches or in the floor. This method directly warms the root zone and the plants from the bottom up, creating a more uniform temperature profile.
Alternative and backup heat sources can supplement the main system or provide heat in remote locations. Biomass heaters, which burn organic matter like wood pellets or chips, offer a sustainable option where fuel is readily available. Small wood stoves can also provide intense, localized heat, and even large, actively managed compost piles can generate a gentle, consistent warmth as a byproduct of microbial decomposition.