A cold greenhouse is not warmed by a heating device. The environment is warmer than the outside – this is due to the heat of the sun’s rays being trapped inside the covering material – be it glass or plastic – during the warm season, and heat accumulated in the soil being released back into the greenhouse during the fall.
In summer, cool air enters the greenhouse through side ventilators near the base of the wall, keeping a steady stream of fresh air flowing around the plants. The roof might contain larger ventilators to let out air heated by the sun.
During the hot season it’s quite common to keep all ventilators and maybe also the doors open at all times, day and night. In spring and fall, if the day promises to be sunny, they may be opened in the morning and closed in the evening.
However, on hot days there is a danger of the plants being damaged by the fierceness of the sun, and ventilation doesn’t provide sufficient protection. To counteract this, the glass can be shaded by an electrostatic shading, which also offers protection against rain. This shading can be painted on at the onset of warm weather, and easily wiped off when the climate turns cooler again.
An electric fan placed on the floor of the greenhouse, or attached to an end wall, will also aid cooling during very hot spells, ensuring good circulation of cool air to keep your plants happy.
Wooden slat staging along the sides of the greenhouse might support a selection of flowering plants in pots – standing these in trays of wet sand, fed by a cistern attached to the wall, should ensure a constant supply of water.
Or, along one side, there might be an open border, the soil dug over and fortified with leaf-mould or peat. Coarse sand might be added to heavy clay soil.
This area would be suitable for growing tomatoes, or maybe a grape vine. A hose fed from a cistern would keep the soil well watered.
Outside – why not grow late-flowering chrysanthemums in pots? These can be brought back inside the greenhouse in September when the tomatoes are ready – and will flower beautifully in October and November to brighten up the approach of winter.
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