A plastic greenhouse, often called a polytunnel or hoop house, provides a controlled environment that extends the growing season. These structures capture and magnify solar energy, creating a warm microclimate during the day. However, the polyethylene sheeting is a poor insulator compared to glass or rigid polycarbonate, leading to rapid heat dissipation after sunset. Maintaining a consistent, above-freezing temperature requires a comprehensive strategy. This approach must address structural flaws, store ambient heat, and introduce supplemental warmth when necessary to protect plants from overnight temperature drops.
Structural Adjustments to Minimize Heat Loss
Heat retention begins by securing the structure’s physical integrity to prevent existing warmth from escaping through drafts. Even small tears or gaps in the plastic covering allow a surprising volume of warm air to leak out. Specialized UV-resistant repair tape should be used immediately to patch any holes or splits in the film, creating an airtight seal.
Proper tensioning of the plastic sheeting is important, as loose material increases heat transfer and stresses the material. The greenhouse base must be anchored firmly to the ground to block cold air infiltration at the lowest point. Sealing this perimeter with soil, heavy timbers, or a buried skirt of plastic prevents drafts from chilling the root zone. For larger structures, internal partitions can isolate a smaller “hot zone” around temperature-sensitive plants, using an additional layer of plastic sheeting.
Utilizing Thermal Mass and Internal Barriers
Once the structure is sealed, the focus shifts to passively storing the solar energy collected during the day for release at night. This technique uses the principle of thermal mass, where dense, dark-colored materials absorb a significant amount of heat energy from the sun. Water barrels painted black or filled with dark water are highly effective, as water has a high specific heat capacity. Placing these barrels along the north wall or in sunny areas maximizes their absorption potential, providing a steady, gentle warmth after sundown.
Other materials, such as stone pavers, bricks, or even large rocks, can also function as thermal mass, absorbing heat and radiating it back into the space. A highly dynamic method involves placing an active composting pile inside the structure, as the microbial decomposition process generates substantial warmth. An actively decomposing pile can reach internal temperatures between 131 and 160 degrees Fahrenheit (55 to 71 degrees Celsius), which radiates significant residual heat into the surrounding air.
To further reduce heat loss through the plastic skin, internal barriers can be employed to create an insulating air gap. Lining the interior walls with bubble wrap, which traps air in small pockets, adds a layer of static insulation against the plastic film. At dusk, covering the plants directly with lightweight thermal blankets or horticultural fleece, known as row covers, creates a localized microclimate around the foliage. This secondary barrier prevents the direct radiation of heat from the plants and soil into the cold ambient air, offering protection from frost without touching the leaves.
Implementing Supplemental Heating Systems
When passive methods are insufficient during severe or prolonged cold weather, supplemental heating systems become necessary. Electric fan heaters are a common choice because they are thermostat-controlled and distribute heat evenly through forced air circulation. However, these systems increase electricity costs and may lower humidity levels, requiring careful monitoring for moisture-loving plants.
Fuel-burning options, such as propane or paraffin heaters, offer powerful, localized heat but introduce harmful combustion byproducts. Incomplete combustion can produce carbon monoxide, a colorless, odorless gas hazardous to humans and plants. The combustion process can also release ethylene gas, a potent plant hormone that causes symptoms like leaf yellowing, stunting, or the shedding of flowers and leaves.
Any unvented heater must be used with extreme caution, requiring continuous ventilation to mitigate the buildup of toxic gases and excess moisture. For maximum safety, only fully vented heating systems, which pipe exhaust gases completely outside the structure, should be considered. Solar-powered heating, such as solar air collectors, can capture and distribute heat during the day, but they do not provide warmth after sunset without a thermal storage system.