An unheated greenhouse will extend your growing season - a heated one will extend it even further.

A heated greenhouse is normally designed to maintain a temperature of 4-7 degrees C (40-45 degrees F) over winter.

It will also ideally contain an automatic ventilation system.

As the temperature inside the hothouse rises above 21 degrees C (70 degrees F), a ventilator in the roof is partially opened by a special temperature-sensitive device. If the temperature continues to rise, the vent will be fully opened.

Shading can be provided by draw-down blinds on the inside of the glass or plastic, pulled down manually when there is strong sunshine. If the sun becomes hidden by clouds the blinds can be rolled back up. It's possible to get automatic blinds for the outside of the greenhouse, but these are expensive as they have to be custom-built to fit your greenhouse's dimensions.

If the internal temperature falls below 4-7 degrees C (40-45 degrees F), an automatic heating system can kick in - a thermostat will switch on an electric fan heater of, say, 2.5 kilowatts.

The greenhouse could contain, in one corner, a small mist propagator for rooting cuttings. A cheaper alternative might be a heated propagating case without a mist unit - but this would be less effective for cuttings which are difficult to root. However, a heated propagating case could also come in handy for germinating seeds in February or March (later in the year most seeds can be germinated in a pot covered with glass or plastic.)

There might also be staging along the greenhouse side supporting rows of cuttings which have already rooted. These can be put into a cold frame for hardening later in the year, before being transplanted to the garden the following spring.

More staging can hold potted plants in trays of wet sand to provide for automatic watering.

It's quite common to have frames outside the greenhouse. Frames heated by electric cables make good environments for young plants such as calceolaria, cineria, and solarum capsicastrum. Frames without artificial heating might be used to grow fruits such as melons.

A heated greenhouse may be more costly to set up and maintain than its unheated cousin, but it opens up many new gardening possibilities.