A plastic greenhouse provides an extended growing environment by capturing solar energy. Its main vulnerability is poor insulation and a lack of thermal mass compared to rigid glass or polycarbonate structures. Single-layer plastic sheeting allows heat to escape rapidly, especially after sunset. For gardeners seeking to lengthen their cultivation season or protect frost-sensitive plants, supplemental heating is necessary. Efficient heating strategies must first minimize heat loss before warming the internal air.
Maximizing Heat Retention
The foundation of efficient plastic greenhouse heating is minimizing the rate at which warmth is lost. Heat loss occurs through two avenues: conduction through the plastic skin and air infiltration through gaps. Sealing all tears and gaps in the plastic sheeting is the most immediate step, as unsealed openings create drafts that rapidly exchange warm indoor air for cold outdoor air.
A second, interior layer of insulation can reduce conductive heat loss by trapping a layer of still air. Horticultural bubble wrap, which features larger air pockets than standard packaging material, is a popular choice. The air sealed within the bubbles acts as an insulating barrier, similar to double-pane windows, slowing the transfer of heat out of the structure overnight. Affixing this material to the interior frame while leaving a small air gap between it and the outer plastic sheet increases its effectiveness.
Using an internal polyethylene liner instead of bubble wrap achieves a similar double-skin effect. This method may require a small fan to keep the two plastic layers separate and prevent condensation buildup. This air pocket increases the material’s insulating value (R-value), helping to maintain a consistent temperature gradient. While these interior linings are translucent, they reduce the total amount of sunlight reaching the plants, a trade-off for improved heat retention. Positioning a reflective material, such as a foil-backed insulation board, on the north-facing wall can reflect available light back to the plants while insulating the coldest side of the structure.
Passive Heating Strategies
Passive heating methods utilize natural processes and materials to store and release thermal energy. This approach leverages the sun’s energy to create a thermal battery inside the greenhouse. Materials with high specific heat capacity, such as water, stone, and concrete, are employed as thermal mass to absorb heat during sunny daylight hours.
Placing large, dark-colored containers of water, such as 55-gallon drums painted black, in a sunny location is an effective technique. Water has one of the highest specific heat capacities, meaning it can absorb substantial heat energy without a large temperature increase. The black color maximizes solar absorption during the day, and the stored warmth is slowly radiated back into the greenhouse air overnight, which helps moderate temperature swings. Concrete blocks or stone pathways laid inside the structure function similarly, absorbing heat from the sun and releasing it slowly after dark.
Heat generated by microbial decomposition through composting is another passive technique. An active compost pile constructed from organic materials like fresh manure, straw, or wood chips can reach internal temperatures of 130 to 160 degrees Fahrenheit (55 to 70 degrees Celsius). This biological process releases a gentle, consistent warmth that can elevate the air temperature by a few degrees and reduce frost risk. A common application is creating a hotbed, where a deep layer of fresh, composting material is placed beneath a raised growing bed, providing direct warmth to the root zone of plants.
Active Heating Systems
When passive methods are insufficient to protect tender plants, particularly in very cold climates, active heating systems become necessary. These systems rely on an external power source or combustible fuel to generate heat. Electric heaters are simple to install and operate at nearly 100% efficiency in converting electricity to heat, but their running costs are typically high. Fan-forced electric heaters circulate warm air quickly, providing rapid temperature recovery, while radiant electric heaters warm objects and surfaces directly rather than the air.
Combustion heaters, which primarily use propane or natural gas, often have a lower fuel cost than electricity for the same amount of heat generated. These heaters are less efficient, operating around 80% efficiency, with some heat lost through the exhaust. They produce carbon monoxide and water vapor, requiring safety precautions. Using a flued heater that vents exhaust gases outside is recommended to protect plants from toxic gases like ethylene and the user from carbon monoxide exposure.
For targeted warmth, heating cables or mats can be placed beneath seed trays and planting flats. This provides localized bottom heat directly to the root zone, which is efficient for seed germination and propagating cuttings without heating the entire volume of air. When using any electrical system in a damp greenhouse environment, all cords and connections must be managed safely. Utilizing ground-fault circuit interrupters (GFCIs) prevents electrical hazards. The choice between electric and gas balances the higher initial and operational cost of electric against the ventilation and safety requirements of combustion-based fuels.