A greenhouse in July is easy. A greenhouse in January, when it's -22°C in Saskatoon and the wind is driving snow against the panels, is a different problem entirely. The structure that traps heat beautifully on a sunny afternoon loses it just as fast once the sun drops, and by 3 a.m. the inside temperature is tracking the outside temperature almost exactly unless you've done something about it.
The good news: you usually don't need to turn a backyard greenhouse into a tropical room. You need to keep it above a target threshold for whatever you're growing, and that's a far cheaper goal. This guide walks through the heating options that actually work in a Canadian winter, the wattage math behind them, and the passive tricks that cut your heating bill before you plug anything in.
Decide your target temperature first
Every heating decision flows from one number: the minimum temperature you need to hold overnight. Pick it before you shop for a heater.
- Frost-free (2°C to 5°C): Keeps hardy greens, overwintering perennials, and dormant fig or citrus alive. This is the cheapest target and what most Canadian hobby growers actually want.
- Cool (7°C to 10°C): Lets cold-tolerant crops like lettuce, spinach, kale, and herbs keep growing slowly through winter.
- Warm (16°C to 18°C): Needed for tomatoes, peppers, and tropical plants in winter. In most of Canada this is expensive enough that it only makes sense for a heated, well-insulated structure.
Holding a greenhouse at frost-free in Ottawa costs a fraction of what holding it at 18°C costs. Be honest about what you're growing. A lot of people heat to 18°C out of habit and pay triple for plants that would have been perfectly happy at 5°C.
Electric heaters: the default for most backyards
For a typical 162 sq ft backyard greenhouse, an electric fan heater is the simplest and safest choice. There's no combustion, no fuel to haul, and no moisture or CO produced indoors. A thermostatically controlled 1500W to 2000W greenhouse heater handles a well-sealed structure down to roughly -15°C outside while holding a frost-free target.
A few specifics that matter in practice:
- Get a model with a built-in thermostat, or add an external plug-in thermostat. A heater that runs constantly will bankrupt you; one that cycles on only when needed runs a fraction of the time.
- Use the fan even when not heating. Moving air prevents cold pockets and reduces the fungal problems that thrive in still, damp winter greenhouses.
- Run it on a dedicated GFCI circuit. Greenhouses are wet environments. Don't share a circuit with anything else, and don't run a heater off a household extension cord.
Rough cost math: a 1500W heater running at 50% duty cycle for 14 hours overnight uses about 10.5 kWh. At Ontario's roughly 13¢/kWh off-peak rate that's about $1.37 a night, or roughly $40 a month at frost-free. Push to 18°C and that same heater may run near-continuously, tripling the cost.
Propane and natural gas: more heat, more management
When electric can't keep up, or where electricity is expensive and gas is cheap, propane is the next step. A vented propane greenhouse heater puts out far more BTUs than a 2000W electric unit and will hold warmth in a larger or colder structure.
The trade-offs are real and you should plan for them:
- Combustion produces water vapour and carbon dioxide. A little CO2 helps plants, but unvented propane raises humidity sharply, which invites mould and botrytis. Pair it with active ventilation.
- Carbon monoxide is a genuine risk. Use heaters rated for greenhouse or indoor use, and install a battery CO detector at head height. Never use a patio or construction heater inside a sealed greenhouse.
- You'll be swapping tanks. In deep cold a 20 lb tank can also frost up and lose pressure. Larger growers run a 100 lb tank or a buried line.
For most backyard owners running a frost-free or cool target, propane is overkill. It earns its place in larger greenhouses, market gardens, or regions where winter routinely drops below -25°C.
Passive heat: free warmth you're probably ignoring
Before you size any heater, reduce the load. Passive measures cost little and run on sunlight, and they're the difference between a heater that sips power and one that runs all night.
- Thermal mass. Place 8 to 12 black-painted 20-litre water barrels or jugs along the north wall. They soak up sun during the day and release it slowly after dark, flattening the overnight low by several degrees. Water stores far more heat per litre than rock or concrete.
- Bubble wrap insulation. Lining the interior with horticultural bubble wrap (large-bubble UV-rated film) on the north and west walls cuts heat loss noticeably while letting light through. Leave the south-facing roof clear for maximum gain.
- Seal the gaps. Most heat escapes around the door and vents. Add weatherstripping and check that vents close tightly. A loose door can undo a barrel wall.
- Insulate the floor edge. A skirt of rigid foam around the base, buried a few inches, stops the perimeter from wicking cold up from frozen ground.
Stacked together, these can raise your overnight minimum by 4°C to 6°C, which often turns an impossible heating job into an easy one.
The polycarbonate advantage in cold climates
The greenhouse glazing itself does a lot of the work. Single-pane glass loses heat fast; 6mm twin-wall polycarbonate traps an insulating layer of air between two walls, giving roughly twice the insulation value of glass while still diffusing light evenly across your plants. That insulating gap is exactly why twin-wall is the standard for serious cold-climate growing.
A frame that holds its shape under load matters too. The Drivhus is built around 6mm twin-wall polycarbonate on a powder-coated aluminum frame rated for 30 psf of snow and 115 km/h winds, so it stays sealed and square through a winter that would rack a flimsier kit. A tight, well-insulated shell is the cheapest heating upgrade you can buy, because every degree the structure holds on its own is a degree you don't pay to replace.
Sizing and safety: put it together
A workable cold-climate setup for a 162 sq ft greenhouse at a frost-free target looks like this:
- A thermostatically controlled 1500W to 2000W electric fan heater on a dedicated GFCI circuit.
- A north wall of 10 water barrels for thermal mass.
- Bubble wrap on the north and west walls, vents weatherstripped and sealed.
- A min/max thermometer so you actually know your overnight low, plus a backup alert (a Wi-Fi temperature sensor that texts you below a set point is worth every penny when a cold snap hits).
If you're chasing a warm 18°C target in a region that hits -25°C, step up to propane and double down on insulation. And whatever you run, plan for a power failure: a string of cold nights with a dead heater can wipe out a season, so a frost-tolerant plant selection is its own form of insurance.
Final thoughts
Heating a greenhouse through a Canadian winter is less about brute force and more about matching the heat to the goal. Set a realistic target temperature, knock the heating load down with thermal mass and insulation, then size a thermostatically controlled heater to cover the gap. Do it in that order and you'll keep plants alive through February without dreading the hydro bill. Start by deciding what you actually need to grow this winter, measure your real overnight lows with a min/max thermometer for a week, and build your setup from there.