Greenhouse Temperature Categories

Ventilation is primarily a method of controlling the temperature of the house. The ventilators are increasingly opened or the extractor fan is used more frequently as the temperature rises above the most favourable point. This will differ at different times of the year and according to the kind of plant being grown. To simplify description, gardeners usually distinguish four main temperature ranges for the greenhouse and refer to them as cold, cool, intermediate and warm.

The Cold Greenhouse

A cold greenhouse is one that has no method of artificial heating. In winter the temperature will sometimes fall well below freezing point and so, at that season, no really tender plants can be grown in it.

On bright days in summer the temperature might run up to 38C.(100F.) with-out ventilation, but would quickly fall at night, which would be bad for most plants. So the best average temperature for a cold house in spring would be about 13 C (55°F.) rising to 16 or 18°C. (60 or 65°F.) in summer, but it may fall as low as 4°C. (40°F.) at night.

The Cool Greenhouse

In the cool house there must be enough artificial heat to exclude frost in winter and generally a minimum of 7°C (45°F) is recommended. In spring and summer optimum temperatures similar to those for the cold house will be aimed at but with less fluctuation -no more than 5°C. (io°F.) below the optimum – at night.

The Intermediate Greenhouse

In an inter-mediate house a winter minimum of I3°C. (55°F.) is maintained with an average from autumn to spring of 13 to 16°C. (55 to 60°F.) and never falls below about 7°C. (453F.). In summer the optimum will be 16 to 21C. (60 to 70F.), maintained even at night at about 55F. For a warm greenhouse a further 3 to 5°C (5 to 10°F.) must be added to all these temperatures.

Cost of Heating

Increasing the temperatures at which a greenhouse is to be operated considerably increases the cost of heating it. A rise of 5°C. (10°F.) in the average temperature of a greenhouse from autumn to spring can easily double the fuel bill.

On the other side of the reckoning must be taken into account the fact that a well-heated greenhouse is easier to manage than an under-heated greenhouse. Plants are more likely to grow well and losses from disease will be reduced. All the same, the cool or intermediate range of temperatures are usually the best with which to start one’s greenhouse experience.


There remains the question of shading. In winter most greenhouse plants can do with all the light they can get but in summer it is very different. Cacti and other succulents may take all the sunshine that is going and so may some bulbs that are resting and ripening at that time. But most plants that are in active growth will need some protection from direct sunshine.

There are many ways of shading a green-house, one of the simplest being to spray or paint limewash on the glass. This is effective but it cannot be adjusted to suit changing conditions. As a rule the limewash is applied in late spring and washed off in early autumn and the plants have to make the best ofit in between. In practice it works quite well.

Another way is to pin butter muslin to the rafters inside the greenhouse. This cannot be removed very quickly but it is easier to get rid of than limewash.

A further refinement is to have blinds, either inside or outside the house, which can be raised or lowered at will. The blinds may be of plastic or thin material if inside, or of hessian, wood laths or split bamboo canes if outside. Whatever kind of blind is used, it will be possible to adjust the shading whenever the gardener can attend to it.

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