Can you suggest a method to keep my greenhouse frost-free?
The temperature in the greenhouse should not be allowed to fall below 2°C (35°F).
This is most easily done by using an electric heater with a thermostat, which can be set to switch on automatically when the temperature falls below a preset level.
Gas or paraffin heaters could also be used.
If your heater is not thermostatically controlled, use a maximum/minimum thermometer to check that the correct temperatures are being maintained, particularly overnight.
A frost alarm can be a useful additional safeguard.
This can be set to sound an alarm (usually somewhere in your house) if the temperature unexpectedly falls to near freezing, for example if the heater breaks down or if there is a power failure.