Constructing a Greenhouse Heater: Keep Your Plants Cosy Through Winter

🌱 GrowMore CookMore App

Know what to sow, when to harvest, and discover recipes for your garden produce.

Right, greenhouse heating. Now this is a topic that divides gardeners like almost nothing else. Some people say “never heat a greenhouse, it’s a waste of money and energy.” Other people have heated greenhouses full of tropical plants in January. I’m somewhere in the middle — I don’t heat mine to tropical levels, but I do keep it frost-free through winter, and the difference that makes to what I can grow and overwinter is absolutely worth the modest cost.

The thing is, an unheated greenhouse in a proper cold snap can drop below freezing. Which means the plants inside aren’t much better off than the ones outside. All that glass doesn’t insulate very well. So if you’ve got tender plants overwintering — pelargoniums, citrus, tender herbs, early seedlings — they need a bit of help on the coldest nights. Not tropical heat, just enough to keep the temperature above zero. Frost-free is the goal. And achieving that is easier and cheaper than you might think.

Now the title says “constructing a greenhouse heater” and I should be upfront — you don’t need to BUILD a heater from scratch (though I’ll talk about some DIY options). What you need is a heating strategy that keeps your greenhouse frost-free without costing a fortune to run. Because some people stick an electric fan heater in their greenhouse and then wonder why their electricity bill looks like the national debt. There are smarter ways.

Insulation First — The Free Option

Seriously, insulation before heating. It’s like putting a coat on before turning up the central heating. Makes sense, right? And yet loads of people skip this step and go straight to blasting heat into an uninsulated glass box. That’s like trying to fill a bath with the plug out.

Bubble wrap is the go-to insulation for greenhouses. The big-bubble stuff (horticultural grade) is best — lets more light through and insulates better than the small bubble packaging stuff. Pin it to the inside of the frame using greenhouse clips or drawing pins. Cover every glass panel. Yes, all of them. It reduces light a bit but in winter there’s not much light anyway, and the heat retention more than compensates.

I also partition my greenhouse in winter — I hang a curtain of bubble wrap across the middle and only heat one half. This halves the volume I need to keep warm, which halves the heating cost. All the overwintering plants go in the heated half. The unheated half is still useful for cold-hardy stuff that just needs rain protection.

Electric Heaters

If you’ve got power to your greenhouse (or can run an extension cable safely — and I do mean safely, with proper outdoor-rated cables and RCD protection), an electric heater is the most controllable option. The key is getting one with a thermostat so it only kicks in when the temperature drops to your set point. A heater running constantly will cost a fortune. A thermostat-controlled one that only fires up on the coldest nights is much more economical.

Tubular heaters are the classic greenhouse choice — those long, low-wattage heating tubes that sit along the base of the wall. They produce gentle, consistent heat and use surprisingly little electricity. A 60-watt tube costs about a penny an hour to run. Even if it’s on all night during a cold snap, that’s not going to break the bank. I’ve got two 120-watt tubes in my greenhouse on a thermostat set to 4°C. They keep the temperature just above freezing on the coldest nights and cost me about three to four pounds a month through winter. That’s it.

Fan heaters are more powerful but use more electricity. They’re useful for a quick blast to warm things up, but running one continuously gets expensive fast. If you use a fan heater, make sure it’s got a built-in thermostat and set it to the minimum temperature you need. Don’t set it to “comfort” level — you’re heating plants, not a living room.

Paraffin Heaters

If you haven’t got electricity in your greenhouse, a paraffin heater is the traditional alternative. They produce decent heat and the combustion creates CO2, which plants love (up to a point). They’re relatively cheap to buy and paraffin isn’t too expensive.

BUT — and this is important — paraffin heaters also produce water vapour and can create condensation problems, which encourages fungal diseases. You need to ventilate slightly even when heating, which feels counterintuitive but is essential. A slightly open vent at the top lets the moist air escape while the warm air, being lighter, stays in the greenhouse. Finding the right balance takes practice.

I used a paraffin heater for my first couple of winters. It worked fine but I got tired of the smell, the filling up, and the condensation. Switched to electric tubes when I ran power to the greenhouse and haven’t looked back. But paraffin is a perfectly valid option if electricity isn’t available.

Thermal Mass — Free Heat Storage

Here’s a clever trick that costs nothing. Thermal mass is any dense material that absorbs heat during the day and releases it slowly at night. Water is one of the best thermal mass materials. Place large containers of water (old milk bottles, black-painted barrels, even just buckets) inside your greenhouse. During the day, the sun heats the water. At night, the water slowly releases that heat, moderating the temperature and reducing the heating needed.

I’ve got a row of black-painted two-litre bottles along the back of my greenhouse. They absorb solar heat all day and release it through the evening and night. On their own they won’t prevent frost, but combined with insulation and a small heater, they reduce the amount of heating needed significantly. Free energy from the sun, stored in old milk bottles. You can’t get more budget gardening than that.

Soil Warming Cables

If you’re starting seeds early in the greenhouse, soil warming cables are brilliant. These are low-wattage electric cables that you bury in the soil of your propagation bench. They warm the soil to about 15-20°C, which is perfect for seed germination, without needing to heat the entire greenhouse to that temperature. Much more efficient and much cheaper to run.

You can buy purpose-made propagation mats too — basically the same thing in a convenient flat pad. Put your seed trays on top, plug it in, and your seeds germinate in the warm soil while the air around them can be much cooler. I start tomatoes, peppers, and aubergines on a warming mat in February, inside the bubble-wrapped greenhouse, with just the tube heaters keeping the air above freezing. Works perfectly every year.

Monitoring Temperature

Whatever heating system you use, a min/max thermometer is essential. It records the highest and lowest temperatures reached, so you can check each morning whether the heating is working properly and whether the greenhouse dropped below freezing overnight. I check mine every day through winter. It takes five seconds and gives complete peace of mind.

You can get wireless ones that transmit the temperature to a display unit in your house, so you don’t even need to go out to the greenhouse to check. Or smart thermometers that send alerts to your phone if the temperature drops below a certain level. I’ve got a basic wireless one and it’s been worth every penny. Had an alert last February at 3am when the heater tripped out during a cold snap. Got up, reset it, saved the plants. Without the alert I’d have lost the lot by morning.

Look after yourselves. Take good care. 🌱