How to Heat a Small Greenhouse | Keep Plants Alive in Winter

To reliably heat a small greenhouse (up to 100 square feet) you first insulate and add thermal mass, then use a 1,000–2,500-watt electric heater or a 10,000 BTU propane heater with a plug-in thermostat to hold temperatures above 45°F.

The cheapest electricity bill in winter is the watt you never burn. Before plugging in anything, seal air leaks, double-layer the glazing with 6-mil painter’s plastic, and stack water barrels along the north wall to soak up daytime sun. That gets you 5°F–10°F of free warmth every night. For the remaining lift, you need a heater sized to your cubic feet, not your square feet.

Calculate Your Heat Needs by Cubic Feet

Multiply length × width × height, then match that number to the heater’s BTU rating.

Oversizing can cost more up front, but undersizing means losing plants on the coldest night. Zone 5 winters (down to -10°F) demand a heater rated for the lowest expected temperature, not an average. For tested recommendations on specific models, check our heater for small greenhouse roundup.

The Best Heating Methods Ranked by Cost and Effort

Not every greenhouse needs a $200 propane unit. Here are the most practical options from cheapest to most reliable.

Passive solar plus thermal mass (free after setup)

Fill 55-gallon barrels (painted black) with water and stack them along the north wall in direct sunlight. The sun warms the water during the day; it radiates heat back at night. For mild winters, you need 2–4 gallons per square foot of floor space. Pair with straw bales stacked against the exterior walls for extra insulation. This combo alone can keep a mini greenhouse above freezing in most climates.

Compost hotbed (free heat, moderate work)

Excavate 18–24 inches deep inside the greenhouse, layer twigs, leaves, and manure, then top with 6–8 inches of soil. The decomposing material generates heat at root level for up to several months. A compost thermometer helps you monitor the temperature. Works best for ground-level propagation beds.

Electric fan heater with thermostat (reliable, $50–$150)

Place the heater low (heat rises) and pair it with a plug-in thermostat to cycle on only when temperatures drop below your target. At 1,500 watts on a standard household outlet, this handles most small greenhouses down to freezing. If humidity is high, build a small roof or shield over the heater to keep moisture out of the electronics.

Propane milkhouse heater with external thermostat (best for off-grid, about $200 total)

A basic propane “milkhouse” heater costs around $30, but it lacks a thermostat. Combine it with an external plug-in thermostat (about $30–$50) to automate on-off cycling, and you get a reliable system for about $60–$80. Both require ventilation to prevent humidity buildup and carbon dioxide accumulation.

Four Mistakes That Wreck Small Greenhouse Heating

Running a heater without a thermostat. This wastes power and can overheat plants. A $20 plug-in thermostat saves its cost in one cold month. Ignoring air leaks. Cold air seeps through unsealed cracks; caulk and weatherstripping are cheaper than extra BTUs. Improper heater placement. Electric fans on the floor, not the ceiling. Radiant heaters work best pointed at thermal mass. Wrong sizing. An undersized heater might run nonstop and still fail on a 15°F night—calculate cubic feet, not emotional optimism.

Safety note for propane: Propane heaters produce humidity and CO₂. Always provide ventilation—crack a roof vent or leave a small opening—to prevent plant rot and ensure safe oxygen levels. If the greenhouse is wet, choose a radiant or oil-filled heater instead of an electric fan model, which can suck moisture into its components.