How to Heat a Greenhouse in Winter | Save Every Plant

Heating a greenhouse in winter requires matching the heater size to the structure’s volume and insulation level, with electric heaters for small spaces and propane or gas units for larger ones.

A cold snap hits, and the plants you babied since fall are one night away from frost damage. The fix for how to heat a greenhouse in winter starts with one honest calculation: your greenhouse’s cubic feet, not your gut feeling. Most growers lose heat through unsealed cracks and missing thermal mass long before the heater itself fails. Get those two things right, and even a modest 1,500W electric heater can keep seedlings alive through a Midwest January.

How Much Heat Does Your Greenhouse Actually Need?

The number on the heater’s box means nothing if it isn’t tied to your structure’s volume. Measure length × width × height in feet to get cubic feet, then factor in your local winter lows — a greenhouse in Minnesota needs roughly double the BTU of one in Georgia at the same size.

Thermal mass is the silent partner here. Black-painted water barrels placed where they catch winter sun absorb heat during daylight and release it slowly after dark. Plan on 2 to 4 gallons of water per square foot of floor area. A typical 130-square-foot floor could hold 260 to 520 gallons in barrels — enough to lift the overnight temperature by 5°F to 8°F without burning fuel.

Two Heater Routes: Electric vs. Propane

Electric heaters offer precision: a digital thermostat (like the one on the Bio Green Palma, around $250–$300) holds a steady 45°F without the temperature swings of a bare-bones unit. Propane models like the DynaGlo 10,000 BTU heater (about $170) are cheaper to run in most regions and push heat through a larger space fast, but they require separated combustion design to avoid carbon monoxide buildup — never use a standard propane heater inside an enclosed greenhouse without ventilation ports.

Insulation First, Heating Second

Skipping insulation turns every dollar of heating into a half-dollar of waste. Start with the smallest gaps: transparent silicone sealant around frame joints flexes with the wind and stops the drafts that can drain 50% of your heat. RHS’s heating efficiency guide notes that horticultural bubble wrap with large bubbles cuts heat loss by roughly 20% while reducing light by about 10% — a trade worth making. For nights when the forecast says single digits, cover your most tender plants directly with horticultural fleece.

Straw bales stacked against the outside walls act as natural insulation if packed tight with no air gaps. Some growers dig a 1- to 2-foot trench along the interior wall and fill it with fresh horse manure capped with soil — the composting generates steady 75°F heat below the plant bench for weeks.

Heater Placement Mistakes That Waste Fuel

Dropping the heater in a corner feels natural, but corners trap heat and force the unit to cycle harder. Mount the heater along the center of a long wall and aim the airflow across the length of the greenhouse. A small fan running on low keeps warm air moving past plants — without circulation, the ceiling can be 10°F warmer than the floor, which is the opposite of where seedlings need it. Keep the heater at least three feet from any plant, plastic pot, or bag of mulch. A grounded outlet is mandatory; extension cords cause voltage drop and create a fire risk in a damp environment.

Target Temperature: What to Set and Why

Most home greenhouses run well at 45°F to 50°F at night. Seedlings under lights need warmer patches — a heat mat under the tray is cheaper than heating the whole structure to 70°F. Set your thermostat on the cooler side and use a max/min thermometer placed at plant height to catch real swings; thermostat readouts are often off by 2°F to 3°F.

For a complete breakdown of shelter options built for cold weather, our roundup of the best DIY greenhouses for winter walks through frame materials, glazing, and sizes that make heating easier from the start.

The No-Fuel Heating Methods That Actually Help

Three passive strategies keep the heater from running all night. First, water barrels placed on the south-facing wall absorb direct light and release heat over six to eight hours — dark barrels beat light ones by a measurable margin. Second, a gravel or paver floor stores daytime warmth better than bare dirt, though less effectively than water. Third, a hot bed dug into the ground and filled with fresh manure generates biological heat for two to three months. None of these alone will carry a greenhouse through a polar vortex, but combined they cut run time on a 5,100 BTU heater by 30% to 40% on a typical January night.

Electric vs. Propane: Running Costs at a Glance

Electric heaters cost more per BTU in most states but have lower upfront hardware costs and need no fueling trips or ventilation worries. Propane heaters burn cheaper per BTU but demand separated combustion chambers, annual line checks, and a place to store tanks. Your local electric and propane rates determine which wins.

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