How do I heat a greenhouse cheaply in winter?

Almost all the money wasted on greenhouse heating goes on heating air that did not need to be warm, in a structure with no insulation, aiming at a temperature higher than the plants required.

Safety

What you'll need

• Horticultural bubble insulation
• Thermostatic electric greenhouse heater (optional)
• Max-min thermometer
• Horticultural fleece (optional)

Step by step

1. Decide what you actually need.Frost-free, around 3 to 5C, keeps most overwintering tender plants alive and costs a fraction of a warm greenhouse. 10C or more for propagation is a completely different order of expense.
2. Insulate before you heat.Horticultural bubble insulation clipped to the inside of the frame cuts heat loss substantially. Large-bubble horticultural grade lets more light through than packaging bubble wrap.
3. Shrink the heated space.Partition off a section with bubble wrap or a plastic curtain and heat only that. Heating a quarter of the volume costs roughly a quarter as much.
4. Seal the draughts.Check the door fit, any broken panes, gaps at the base and loose glazing clips. Draughts undo insulation faster than anything else.
5. Use a thermostatic heater.A heater without a thermostat runs constantly and costs several times as much. A thermostat set to the minimum acceptable temperature is the single biggest saving.
6. Put the heater where the air moves.Low down, blowing along the length rather than into a corner. Warm air needs distributing or you heat the roof and freeze the bench.
7. Use a heated propagator for the demanding things.If only a few trays need warmth, a small heated propagator uses a tiny fraction of the electricity of heating the whole structure.
8. Use fleece for the coldest nights.Draping fleece over plants on the bench on a hard frost night adds a couple of degrees at the plant, which is where it matters, for nothing.
9. Group plants and raise them off the floor.Clustered pots on benching stay warmer than isolated pots on cold concrete.
10. Measure what you are getting.A max-min thermometer at bench height tells you whether the heater is doing anything useful. Most people find they are heating to well above what they need.
11. Avoid paraffin heaters unless well ventilated.They are cheap to buy and produce water vapour and combustion gases, which drives the damp and mould problems that kill overwintering plants.

Tips

• Frost-free costs a fraction of warm. Decide which you need before buying anything.
• Heating a partitioned quarter of the greenhouse costs roughly a quarter as much.
• A thermostat is the cheapest thing you can buy and saves the most.

Common mistakes

• Running an extension lead from the house to a greenhouse heater — It is a serious electrocution risk in a wet environment and most extension leads are not rated for continuous outdoor load. A greenhouse supply must be a proper installed circuit.
• Heating the whole greenhouse to keep six pots alive — The six pots could go in a partitioned corner, a cold frame or a spare room. Heating a whole structure for a handful of plants is where most of the cost goes.

If it doesn't work

Questions people ask