Greenhouses need to be ventilated to prevent temperatures rising above those best suited to the plants being grown; high temperatures lead to stress within the plants and may cause wilting. Ventilation is also required to control humidity levels within the greenhouse. Tomatoes, for example, require a humid atmosphere at least once a day for an hour or two to allow the flowers to set, but for the remainder of the time they like a drier atmosphere.
Setting the ventilation by hand involves guessing the weather conditions for the next few hours. If a person is out at work all day, he may have to open the vents early in the morning and close them late at night. In such a case the greenhouse may suffer from very wide temperature variations, to the detriment of the plants. For these reasons money spent on automatic ventilation equipment is a good investment. Several makes of automatic vent gear arc available that work on the principle of a special liquid expanding as it warms up and in so doing pushing a piston which in turn opens the ventilator. The temperature at which the vent opens and closes can be adjusted. If possible, arrange the system so that the ventilators open only on the side away from the prevailing wind.
Automatic ventilation systems help to keep the greenhouse at the required temperature
is used in the doorway. If additional ventilation is needed, an extractor fan, thermostatically controlled, may be used, or alternatively an electric blower-heater can be rigged up to serve the same purpose.
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