Greenhouse Heating


Heating your greenhouse or conservatory is useful for plant propagation and allows a wider variety of plants to be grown. However, an unheated structure is still valuable for extending the growing season and bringing on both hardy and half-hardy plants. Our article on heated and unheated greenhouses discusses the uses of both in more detail.

The amount of heating you require will depend on the type of plants you want to grow. Choose a heater that is powerful enough to maintain the required minimum temperature efficiently. You will also need to consider the running costs, ease and cost of installation and how easy the system is to use.

The majority of gardeners usually only need a limited heating source; just enough to keep winter temperatures above frost level, although some require a system to keep the greenhouse warm and humid all year round.

If the greenhouse has electricity installed, heating may be carried out using a thermostatically controlled electric fan-heater; this is the most convenient, reliable and efficient way to heat a greenhouse. However, if your greenhouse is not connected to an electricity supply, then gas or paraffin heaters may be used.

Comparison of Fuel Types

Electric Fan Heaters

  • Reliable.
  • Easy to use.
  • Require little maintenance.
  • Efficient.
  • Thermostatically controlled.
  • May be moved around easily.
  • Fan element circulates heat and air, which maintains an even temperature.
  • Needs electricity supply.

Gas Heaters

  • Runs from either a mains gas supply or bottled gas.
  • Does not need electricity supply.
  • Bottles gas must be bought in and stored in a safe place.
  • Bottled gas cylinders need to be replaced regularly. It is advisable to have two bottles connected by automatic switch-over valve in case one runs out.
  • Propane gas releases fumes and water vapour - adequate ventilation is essential.
  • May be thermostatically controlled, although calibration may be difficult.
  • Not as convenient to use as electric heaters.

Paraffin Heaters

  • Inexpensive to buy.
  • Does not need electricity supply.
  • Not thermostatically controlled, so less efficient than electric or gas heaters.
  • Can be more expensive to run.
  • Plant-toxic fumes and water vapour released - adequate ventilation is essential.
  • Fuel must be bought in and stored in a safe place.
  • The wick and fuel level must be checked every day.

Greenhouse Heater Features

When using a heater that is not thermostatically controlled, check that the right temperatures are maintained by using a maximum/minimum thermometer. In this way, you can ensure that your greenhouse does not become too hot or too cold.

You might also consider investing in a frost alarm in case your heating fails for any reason. This remote system will alert you if the temperature drops below a certain temperature, enabling you to rescue and protect any tender plants.

Propagation Heaters

Many plants need higher propagation temperatures than can be realistically maintained in a greenhouse; propagation aids can provide the right amount of heat for seedlings without having to heat the whole greenhouse. Soil warming cables can be used in mist or conventional propagators - for safety, use a system where the cable has a wired in thermostat connected to an insulated, fused socket.