How to heat a greenhouse

Heating a greenhouse is only economically viable if the structure is well-insulated. Even if your garden greenhouse is made of an insulating material – such as double walled polycarbonate sheets or double pane glass – it is still possible to increase the temperature by a few degrees by lining the interior walls and roof with bubble wrap. This extra layer can be attached to the frames using special clips. Triple layer bubble insulation has significantly higher insulating power than low-end alternatives, and is also much more resistant.

Bear in mind that not all plants have the same temperature requirements. For this reason, some gardeners choose to set up two small greenhouses instead: an unheated greenhouse, with little or no heating for cold-hardy plants, and a heated greenhouse kept at a much higher temperature (around 10°C or even 15°C).

Paraffin heaters

These smaller heaters are primarily used to heat small greenhouses of up to 5 square metres and so-called cold greenhouses.

This type of heating works to keep frost at bay and maintain a minimum temperature overnight. Paraffin heaters can also be brought out in extremely cold conditions.

Be sure to only use low-sulphur paraffin (also known as kerosene) as other fuels may produce toxic emissions to plants.

Electric heaters

Various types of specially designed electric heaters can be used in greenhouses including infrared radiators, fan heaters, tubular heaters, and so on.

If you have the option, prioritise systems that promote air circulation. These heaters produce a more even temperature throughout the greenhouse and will heat up the smallest nook and cranny reducing the risk of disease. Some models can even operate on a fan-only mode to ventilate the space during the summer months.

For a small greenhouse measuring just a few square metres, a small electric heater may suffice.

Comparing greenhouse heaters

Whatever option you choose, make sure to only use equipment specifically designed for greenhouses. Don't forget to air out your greenhouse on a regular basis. It is a good idea to do so when the weather is warm. Correct ventilation will help to reduce the risk of diseases that thrive in closely confined spaces.

A greenhouse gas heater can be a cost-effective option. However, it is difficult to regulate the temperature with this type of appliance so the greenhouse must be carefully monitored. In addition, gas can emit fumes that are toxic to plants and humans – especially if not regulated correctly. As with paraffin heaters, gas heaters emit water vapour.

Propane gas cylinders are more commonly used as butane freezes as soon as temperatures drop to 0°C.

An old-fashioned alternative heating method involves placing basins of hot water in the greenhouse. This method can still be used as a temporary solution (to deal with excessively low nighttime temperatures, for example) as those extra few degrees can make the world of difference.

A heated propagator for early planting

Early seedlings or delicate cuttings don't require a lot of space and a heated propagator will provide more than enough room. This system consists of a tray with a plastic cover with a heating element in the base.

Alternatively, you can easily make your own propagator using a polystyrene fish box. Simply layer sand at the bottom of the box and install a terrarium heating cable over the top before covering the cable with another layer of sand.