Most people heat a greenhouse to keep it frost free, not warm. Pelargoniums, citrus, young plants and cuttings get through a UK winter if the temperature stays just above freezing. That takes far less energy than heating it properly, as long as you insulate first and use a thermostat.
Step 1: insulate
Every bit of heat you keep in is heat you do not pay for.
- Bubble insulation: line the inside of the greenhouse in autumn. Fix it to the frame with clips and keep the roof vent free.
- Twinwall polycarbonate: keeps more heat in than single glass, all year round. See twinwall polycarbonate.
- Close the gaps: check the door seals and missing panes before winter.
- Heat only part of it: a curtain of bubble wrap across the middle lets you heat half the greenhouse.
Step 2: choose a heater
Electric fan heaters
The most convenient choice if you have an outdoor socket. A fan heater with a thermostat switches on only when the temperature drops and moves air around, which also helps against mould. Use only a heater made for greenhouses, plugged into an outdoor socket protected by an RCD.
Tube heaters
Low-wattage tubes along the bottom of the wall give gentle background heat. They suit small greenhouses.
Paraffin heaters
They work without electricity, which helps on an allotment. But they give off water vapour and fumes, so you need to open a vent a little, and you have to fill them and trim the wick.
What does it cost to run?
An electric heater uses its rated power for every hour it runs. A 2 kW heater running for one hour uses 2 kWh. Multiply that by the price per kWh on your electricity bill. With a thermostat set just above freezing, the heater only runs on cold nights, so the real cost depends on how cold your winter is and how well you insulated. Every extra degree on the thermostat costs you more.
Measure first
A max-min thermometer shows how cold your greenhouse really gets overnight. Check it in October and November before you decide how much heat you need.
Tips
- Water less in winter and only in the morning, so plants go into the night dry.
- Open the vent on sunny winter days to let damp air out.
- Raise pots off the cold floor onto staging.
Our picks for heaters and insulation are below.
