Ventilation in the Greenhouse
I never consider that the averagehas sufficient . No greenhouse, of whatever size, should be without top and bottom ventilation (on both sides in the case of a span-roof house) and there should be at least one top and bottom ventilator for every 10 ft. of greenhouse length. For obvious reasons, the number provided by the makers is commensurate with the cost of the product.
is required to change the air in the house and keep the temperature from rising too high. If hinged ventilators are used these should be mounted high up near the ridge to let hot air out and in the sides of the house, to let cool air in. especially when the weather is warm. The aim should always be to maintain a fairly steady temperature, but higher by day than by night.
The intelligent use of ventilators allows the experienced greenhouse owner to maintain the buoyant atmosphere which plants like so much. With ventilators fitted of the type I have described it should be possible to keep the air in gentle circulation at all times which is very much to the advantage of the plants. Night condensation can be much reduced for instance, by leaving the ridge ventilators open a little whenever weather conditions permit. As in all aspects of. One soon learns to master ventilation problems in the greenhouse and profit by experience.
Side ventilators are only used when the outside conditions are warm or relatively warm for a cold flow of air over plants can have a very adverse effect on their health. It goes without saying that draughts must be avoided for there are few plants which will put up with such conditions without protest.
In moderately heated or unheated greenhouses the conservation of sun heat to boost up house temperatures in the night hours is a number one priority during the cooler months of the year. Much can be achieved in this respect by closing all the ventilators well before the sun goes down and opening them up in the morning after the early morning chill has disappeared.
An electric extractor fan with thermostatic control can be fitted high up at the end of the greenhouse to give frequent changes of air which will be of great benefit to the plants. These fans have louvres which are kept open by a stream of air being sucked through the house. When the electric motor cuts out, the louvres automatically close by gravity.
An excellent way of circulating warm air in greenhouses is by electric fan heaters and if these are controlled by a thermostat they can be completely automatic in operation. Some will blow cool air when the thermostat cuts out the heating elements and so provide the gentle circulation of air which I have already said is desirable.
As far as I am concerned automatic ventilation is a ‘must’ for I am away from home so much during the daytime. The best automatic ventilation is provided by the non-electric type in which a cylinder of fluid expands and contracts to operate a rod attached to the ventilators, opening and closing them as the temperature rises and falls. This type, which does not have any source of external power, is absolutely trouble free and can be relied on.
Ventilation is necessary, of course, in garden frames, for the plants here require the same buoyant atmosphere as those in a greenhouse. In a propagating frame, too, ventilation is essential to gradually accustom seedlings and newly rootedto more airy conditions. Such ventilation can be provided by sliding the frame light up or down to allow the necessary amount of outside air in. Alternatively, it can be propped up at the end or on its side, whichever way avoids excessive breeze on the plants.