Greenhouse gardening brings its own challenges for the hobby gardener. Ventilating the greenhouse to optimise the conditions is a key factor. There are many options available in terms of equipment, positioning and utilisation which should be carefully considered.
Few if any plants will survive the range of temperature and humidity swings that occur in an unventilated greenhouse.
Ventilation should not be considered entirely in isolation but rather treated as complementary to heating, watering, damping down and other related aspects of activity in the greenhouse. Nevertheless ventilation is important and should be planned early in the development of the greenhouse environment. The fact that the environment changes throughout the day needs to be considered.
Greenhouse Equipment for Ventilation
The glass of the greenhouse traps the heat from the sun. The air warms rapidly and rises and the standard equipment to both release warm air and circulate incoming cooler air are vents, often hinged like a window in the roof and sides of the greenhouse. Louvres, in the form of hinged slats, are used to provide more even air flow. Louvres also reduce draughts and are much more adjustable.
Automatic equipment can be very sophisticated and include reaction to changes in, for example, humidity and sunlight, but the popular type of automated vent opener contains a heat-sensitive compound which opens and closes the vent through use of levers. Aligned correctly to the shape of the greenhouse roof, these are perfectly suitable for most greenhouses. Automated louvres are also available but a conventional hobby greenhouse will usually use automated vents and manual louvres as an additional aid on very warm days.
Fans can be used as an additional component to provide ventilation in larger greenhouses and will also reduce humidity as they dry the air in the greenhouse. Fans for greenhouses are slow-running. They can be window-mounted extractor types, hung from the ridge of the greenhouse or placed on staging in the greenhouse.
Positioning of the fan relative to vents is important. An ideal scenario is one where, as air movement will be at its highest between one vent and another if there are no obstructions, the fan can be placed to provide circulation and cooling in the areas where the air may be more static and is changing less frequently.
Fans are usually thermostat-controlled and are often built into an electric greenhouse heater to run cold when appropriate. They can relatively prove expensive to run in some instances. Opening the door or doors of greenhouses is almost a last resort and should be viewed more as a convenience in suitable weather. By its very nature it is both less controlled in impact and inconvenient in practice.
Positioning of Ventilating Equipment
In order to find the optimum position for both providing ventilation and ultimately position plants appropriately, it is useful to measure and record the temperature in different positions within the greenhouse. Keeping a log of various aspects can be worthwhile as it then proves easier to note any pattern which emerges. Noting the position of the sun and even taking photographs to record the natural light and shade within the greenhouse due to the positions of equipment and plants can also prove useful. Prevailing winds will cool one side of a greenhouse more than the others, and wind direction may well be variable also.
In a traditional shape or the popular dutch light shape of greenhouse, which has vertical or sloping sides leading up to a roof with a ridge apex, placing vents high in the roof and vents or louvres low in the sides greatly aids circulation. One way to optimise their positions is by placing roof vents almost diagonally, i.e. one near the front end on one side and one near the rear on the other side, then placing louvres low in the sides of the greenhouse on the "opposite" diagonal. Ina normal cool greenhouse, the louvres could be manually operated when required, and dependent on relevant sun position, wind direction and the incumbent plants, either one or both louvres can be opened while auto vents are used for the roof.
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