How to Heat a Greenhouse
in Winter

Passive solar is the foundation of any heated greenhouse — and it's free. Orient your greenhouse with the long side facing south (in the northern hemisphere) to maximize winter sun exposure. Use transparent glazing on the south-facing walls and opaque insulating panels on the north side. Solar energy enters through the glazing during the day and is stored in thermal mass (walls, floor, water barrels) to be released at night. A well-designed passive solar greenhouse can maintain temperatures 15–25°F above outdoor nighttime lows with zero energy input.

Best for: All greenhouses as a starting baseline. USDA zones 7–10 may need nothing else for cold-hardy crops.

Thermal mass is the most underused, most cost-effective greenhouse heating tool. Water has the highest heat capacity of any common material — a 55-gallon black barrel of water stores more heat than the same volume of concrete. Line the north wall with dark-painted 55-gallon barrels filled with water, or use black IBC totes if space allows. During the day they absorb solar heat. At night they radiate it back into the greenhouse. Studies show water barrels alone can raise minimum nighttime temperatures by 8–15°F in a sunny greenhouse.

Horticultural bubble wrap is one of the best-kept secrets in greenhouse heating. Applied to the inside of glazing with clips or binder clips, it adds roughly R-1 to R-2 of insulation and can cut heat loss by 30–50%. Use 3/4-inch (large bubble) horticultural grade — it transmits more light than standard packing wrap. Leave an air gap between the wrap and the glass or polycarbonate for maximum insulating effect. Remove in spring once outdoor temps stabilize to maximize light. A $40 roll can heat a 10×12 greenhouse almost as effectively as a $150/year electric bill.

An electric greenhouse heater with a built-in thermostat is the simplest active heating solution for small to medium greenhouses (up to about 200 sq ft). They're easy to install, precise with temperature control, and safe for enclosed spaces. The downside is running cost — electric heat is the most expensive fuel per BTU. A 1,500-watt heater running 8 hours/night costs roughly $45–$60/month depending on your electricity rate. For a small, well-insulated greenhouse this is manageable. For a large greenhouse in a cold climate, the bills become prohibitive. Always use a unit with IP44 moisture protection, a thermostat, and tip-over protection.

Propane heaters deliver more BTUs per dollar of fuel than electric, making them the preferred choice for medium to large greenhouses (200+ sq ft). A dedicated greenhouse propane unit with a thermostat and vented flue is the most reliable setup. Unvented propane heaters release CO2 and water vapor — plants don't mind the CO2 (it actually improves growth), but excess moisture promotes fungal disease. A simple vented unit with a 100-lb propane tank can heat a 12×20 greenhouse reliably through a zone 5 winter for about $80–$120/month in propane. Natural gas is cheaper if you have a line.

Hot water (hydronic) heating circulates heated water through pipes under benches or along the perimeter walls. It's the most efficient and uniform heating method for large greenhouses — heat rises from the floor level where roots are, rather than pooling at the ceiling. Commercial greenhouses overwhelmingly use this approach. For home growers it's a large upfront investment (boiler + pipe + labor) but pays back over time. A simple DIY version uses a small tankless hot water heater, a pump, and 1/2-inch PEX tubing along the north wall. Expect $500–$1,000 for a basic DIY setup.

Active compost generates significant heat — a well-managed pile reaches 130–160°F in the core. Jean Pain's bioreactor method uses a large pile of wood chips surrounding coiled pipes, with hot water circulated into the greenhouse. A simpler version for home growers: build a hot compost pile inside or adjacent to the greenhouse using straw, wood chips, and manure. The microbial activity generates enough heat to warm the immediate air by 5–15°F and keep the floor and root zone warm through winter. This method requires active management (turning the pile, maintaining moisture) but costs nothing if you have access to materials.

Best for: Growers with access to straw, wood chips, or manure who want a free supplemental heat source and finished compost in spring.

Seedling heat mats don't heat the whole greenhouse — they heat the root zone of your seedling trays and propagation areas directly. This is often all you need for starting seeds in an otherwise cool greenhouse. Most crops germinate fastest with soil temperatures between 70–85°F, and a heat mat maintains this without heating the whole structure. Used with a thermostat controller, a single heat mat draws 15–20 watts and costs less than $3/month to run. Use them for: starting tomatoes, peppers, basil, and cucumbers in late winter; rooting cuttings; and keeping seedlings warm during cold nights when the main heater is off.