Maintaining a stable, elevated temperature within a greenhouse is the central challenge of winter gardening. Plants require a specific temperature band to survive and continue metabolic processes. Successfully overwintering plants requires a multi-layered strategy that focuses first on preventing heat loss and then on generating supplemental heat. The primary goal is to minimize the temperature difference required by an active heating system, ensuring plant survival and managing energy costs.
Enhancing the Structure for Maximum Retention
Preparing a greenhouse for cold weather begins with structural improvements to reduce heat escape through the physical envelope. Heat naturally moves from warmer to colder areas, making any gap or thin covering a point of energy loss. Sealing all air leaks is foundational, as drafts allow warm interior air to be rapidly replaced by cold exterior air, straining heating efforts. Inspecting seals around doors, ventilation windows, and glazing panel overlaps is necessary, and these openings can be closed with silicone caulk, foam sealant, or weather stripping.
Glazing insulation is important, as standard single-pane glass or film provides little resistance to heat transfer. Many growers apply a secondary layer of material to create an insulating air pocket, which increases the material’s R-value. Horticultural bubble wrap, treated to resist UV degradation, is a popular choice because its trapped air bubbles provide insulation while allowing light transmission. For permanent structures, multi-wall polycarbonate sheeting offers superior insulation due to its manufactured air channels.
Insulating the structure’s base is beneficial, as an unprotected foundation or floor contributes to heat loss. In regions with deep frost, cold soil can creep inward, chilling the root zone of plants. Insulating the perimeter with rigid foam boards helps isolate the interior ground from the frozen exterior soil. Additionally, covering the north-facing wall with an opaque, reflective insulating material minimizes heat loss from the side that receives the least winter sun.
Utilizing Passive Heat Storage and Thermal Mass
After sealing and insulating the structure, the next strategy involves incorporating materials that store solar energy captured during the day for slow release at night. This technique, known as thermal mass, uses dense materials to absorb excess heat, buffering the temperature swings common in greenhouses. The high heat capacity of water makes it an effective thermal mass material.
Placing large, dark-colored containers, such as 55-gallon drums filled with water, in areas receiving maximum direct sunlight allows them to function as thermal batteries. These containers absorb solar energy throughout the day. As the greenhouse air temperature drops after sunset, the water slowly radiates the stored heat back into the environment. This passive release stabilizes the nighttime temperature, reducing the need for active heating.
An alternative method for generating passive warmth is using a large, actively decomposing compost pile inside the greenhouse structure. The biological process of decomposition generates heat as microorganisms break down organic matter. A well-managed compost heap generates a steady heat that radiates into the air and soil for weeks or months, offering the dual benefit of warmth and eventual nutrient-rich soil amendment.
Active Heating Systems and Safety Protocols
When passive retention methods are insufficient, a controlled active heating system must be employed. Electric fan heaters are popular for smaller greenhouses because they offer precise temperature control, and the fan aids in distributing heat evenly, minimizing cold spots. Electric units are considered the safest option, as they do not introduce combustion by-products or excess moisture into the environment.
For larger operations or where electrical capacity is limited, gas-fueled heaters burning propane or natural gas are utilized for their high heat output. These combustion systems require rigorous attention to safety, as they produce water vapor, carbon dioxide, and carbon monoxide. Adequate ventilation must be maintained to prevent the accumulation of toxic fumes, though this means some generated heat is lost to the outside.
Regardless of the heat source, the system’s operation should be governed by a reliable thermostat to prevent unnecessary energy consumption and overheating. Thermostats ensure the heater only activates when the temperature falls below a set minimum, typically around 45°F to 55°F for temperate plants. Using air circulation fans is also important to break up temperature stratification and ensure that the warm air reaches the plant canopy.
Safety Requirements
Safety protocols require that all electric heaters be properly grounded and kept away from water sources. Carbon monoxide detectors are mandatory when using any combustion heater inside an enclosed space.
Internal Protection Strategies for Vulnerable Plants
Even with a heating plan, localized protection for cold-sensitive plants provides a cost-effective safety net against cold snaps. Building small, temporary structures like cold frames or cloches over specific planting areas creates a secondary, insulated microclimate. These trap a layer of warmer air around the plant, offering additional frost protection.
Another targeted approach involves using electric heat mats placed beneath seedling trays or potted plants on benches. These mats apply warmth directly to the root zone, which affects a plant’s ability to absorb water and nutrients. Root zone heating allows the ambient air temperature of the main greenhouse to be kept slightly lower, saving on heating costs while ensuring plant health.
On the coldest nights, draping a layer of lightweight horticultural fleece or row cover fabric directly over vulnerable plants provides insulation. This material traps the heat radiating from the plants and soil surface without crushing the foliage. The fleece must be removed during the day to allow for light exposure and air circulation, but its use prevents frost damage during overnight lows.