Heating by means of oil heaters is comparatively widely practised, and in a small greenhouse there is a lot to be said in favour of this method. There are however a number of disadvantages. If a high temperature is maintained the cost, although much less than electrical heating, is higher per cubic foot of space than the boiler system. Then too this form of heating, as it relies upon a supply of oxygen for combustion, has a very serious drying effect on the air; this can be minimised by using water trays, but it is not easy to maintain a buoyant atmosphere even when these are used.

On the other hand, the temperature is automatically controlled by a thermostat, so that very little attention is required, and certainly no stoking, which can be a somewhat unpleasant task on a cold night. Of all the various electrical systems available probably the most useful is that based on a water circulatory system, as there is some small reserve of heat should the current fail and, in addition, the drying effect on the air in the greenhouse is not so marked as in the normal radiant type.

Admittedly some fumes are given off but these, being heavier than air in the main, tend to remain low and are, in any case, soon dispersed by the normal circulation of air. The most efficient stoves are undoubtedly the blue flame burner types, and these are also the most economical to run-on the other hand if badly adjusted they can kill every plant in the house; the flame must be blue and the stove free from spilt oil.

Although a compromise, there is no doubt that it is very effective; growth will not he quite as fast as in a better heated structure, but this will not make much difference to the average amateur. There are two forms of soil heating cable available; one is the high voltage low resistance type which can be operated directly from the mains; this is the cheaper of the two: the other, a low voltage cable system, requires a transformer to step down the voltage.

Build the breeze blocks stood on edge into a rectangle the same size as the frame, leaving one or more blocks out on one side to allow the stove to be put into position, and place the sheet of metal or asbestos into position on this. The frame is then placed on top of this structure; part of the area inside the frame should be of good potting soil to take the tubers and part with peat to accommodate the pots of cuttings.

The greatest disadvantage of the frame lies in the fact that to tend the plants within it has to be opened, perhaps at a time when weather conditions are far from ideal. Nevertheless if opened on the side opposite to that from which the wind is blowing and sacks or other coverings are draped to cover as much of the open sides as possible, there is little chance of a serious check to growth.