What is the most cost-effective way to heat a small greenhouse in winter?

The cheapest heating is the heat you do not lose, so insulation comes before any heater. Bubble polythene lining the inside of the frame typically cuts heat loss by around forty per cent for very little money, and it is the single highest-return intervention. Seal gaps around the door and vents, and check the base for draughts — most of the loss in an old aluminium frame is through leaks rather than through the glass.

Then reduce the volume you are heating. Partitioning off a third of the greenhouse with polythene and heating only that section is far cheaper than heating the whole structure, and for most people the overwintering collection fits easily into it.

For actual heat, an electric fan heater with an accurate thermostat is the most controllable option for a small space. It only runs when needed, it moves air — which matters as much for disease prevention as the warmth does — and it has no combustion products. Paraffin heaters are cheaper to buy and useful where there is no power, but they release a great deal of water vapour, and high humidity with still air is exactly the condition botrytis needs.

Thermal mass smooths the swings usefully and costs almost nothing: black-painted water barrels or containers along the north wall absorb solar gain during the day and release it overnight. It will not keep a greenhouse frost-free on its own but it reduces the demand on the heater considerably.

Finally, set the thermostat honestly. Most overwintering plants need frost protection at three to five degrees, not fifteen. Every extra degree costs real money.