A greenhouse is a structure designed to modify the immediate environment, extending the growing season. It works by capturing solar radiation, converting it into thermal energy, and trapping the resulting heat inside. Whether a greenhouse requires a mechanical heating system is conditional upon the interplay of several environmental and horticultural factors. The need for a heater depends on the external weather, the specific temperature requirements of the plants, and the structure’s intended purpose.
Factors That Determine Heater Necessity
The decision for adding supplemental heat is based on calculating heat loss potential versus the desired internal temperature. Geographic location, often referenced by the USDA Plant Hardiness Zone, is a primary consideration, as it indicates the average annual minimum winter temperature. Greenhouses in colder zones, where nighttime temperatures frequently drop below freezing, will likely require active heating to maintain a stable environment.
The species of plant being cultivated is equally important, as temperature requirements vary significantly. Cold-tolerant crops like winter greens and brassicas often survive near-freezing temperatures, requiring only protection from wind and frost. Conversely, tropical or tender plants, such as orchids or citrus, demand a much warmer minimum temperature, typically requiring the air temperature to remain above 55°F to 60°F.
The grower’s objective also dictates the necessity of a heater. A greenhouse used only for extending the shoulder seasons in spring and fall may not need heat. However, a structure intended for year-round production, seed germination, or overwintering sensitive plants requires a reliable heat source. An oversized heater can cause temperature fluctuations that stress plants, while an undersized unit may fail to maintain the minimum temperature during the coldest hours.
Active Heating Options
When climate and plant requirements demand supplemental heat, several mechanical options utilizing different fuel sources are available. Electric heaters are a popular choice for smaller greenhouses because they are easy to install and operate at nearly 100% efficiency without exhaust loss. These units should be greenhouse-specific, featuring waterproof casings and integrated thermostat controls for safe and consistent operation.
Gas and Combustion Heaters
For larger spaces or where electricity costs are high, heaters fueled by propane or natural gas offer a more powerful heat output. These combustion systems are often the most cost-effective option for heating a large volume of air, but they introduce safety and ventilation considerations. Combustion produces byproducts, including carbon dioxide and moisture; unvented heaters can also generate ethylene gas, which is harmful to sensitive plants.
Vented gas heaters address this by using a heat exchanger to separate exhaust from the greenhouse air, expelling fumes outside through a flue pipe. Indirect air heaters, powered by natural gas, liquid petroleum gas (LPG or propane), or kerosene, transfer heat via a separate chamber, ensuring clean air inside the growing environment. Kerosene heaters are an alternative for locations without plumbed gas or electricity, but they still require careful venting and monitoring to manage fumes and moisture.
Non-Mechanical Temperature Management
Many cost-effective strategies minimize heat loss and stabilize temperatures without relying on a powered heater. These passive techniques enhance the greenhouse’s ability to capture and retain solar energy. One effective method is the strategic use of thermal mass, which refers to materials that absorb heat during the day and slowly release it back into the environment at night.
Large containers or barrels filled with water, especially if painted dark, are excellent thermal mass options that can be positioned to maximize solar gain. Other dense materials like stone, concrete, or brick used for walkways or raised beds also function as thermal mass, reducing the severity of nighttime temperature drops. The greater the total mass, the more stable the internal temperature will remain.
Structural improvements and insulation are important for heat retention, especially at night or during cloudy days. Applying insulation to the north-facing wall, which receives minimal direct sunlight in the Northern Hemisphere, significantly reduces heat loss through conduction. This can be achieved using rigid foam panels or reflective materials. Double-layer glazing, such as two sheets of polycarbonate with an air gap, acts as insulation by trapping air and slowing heat transfer. Vents should be closed before sunset to trap warm air collected during the day. Proper orientation, typically with the longest side facing south, maximizes solar exposure.