Controlling Greenhouse Ventilation and Humidity

Ventilators should be fitted in the ridge of every greenhouse — on both sides in the case of spantroofed houses — about 2 ft. in depth and in total length equivalent to at least half the total length of the house. Houses with part glass and part brick or wood sides should have ventilators of large size in the glass and it is an advantage if they can have small box ventilators with hinged or sliding wooden doors in the brick walls also. Greenhouses standing on brick or wood sides only should certainly have such box ventilators every 3 ft. along these walls.

The purpose of ventilation is to allow as free a current of air as possible, without severe draughts, to assist in the control of temperature and humidity. As a rule, top ventilators alone are used in winter, and then only by day when the weather is reasonably fine. They may be opened an inch or so for three or four hours around midday, according to outside conditions. In summer, when the weather is really hot, top and side ventilators may be opened widely to let currents of air pass through the house from bottom to top. Automatic ventilator openers are available which can be adjusted to operate at required temperatures. An alternative is to use extractor fans, which can be thermostatically controlled, usually fitted high up in the end walls of small greenhouses.greenhouse-ventilation

In unheated or moderately heated houses it is often possible to trap enough sun heat to keep the air warm throughout the night if all ventilators are closed an hour or so before sundown. Note that in the spring a clearing sky and falling wind towards evening is usually a warning sign of a sharp frost.

Atmospheric Moisture. This is required in varying degrees by different plants. Cacti like a comparatively dry atmosphere, whereas most tropical plants need one that is saturated with moisture. The majority of popular greenhouse plants stand midway between these extremes. Excessive humidity will encourage mildew, damping off, and other fungal diseases. Excessive dryness will result in scorching of the leaves and stems, often mistaken for disease. Scorched leaves develop brown patches which are dry and parchment-like without any sign of mould. Ventilation usually increases the dryness of the air especially if combined with some artificial heat.

The necessary moisture is maintained in three ways: by syringing the plants themselves, by damping down the paths, walls, and stages, and by placing water in shallow evaporating trays over the hot water pipes.

SYRINGING must be practised only with plants known to like moisture and should be discontinued while plants are in flower or are ripening fruits. As a rule it is done in the morning when the temperature has begun to rise, but before the sun is shining fiercely. In some cases several syringings may be required each day. Avoid syringing late in the evening, as this lowers the temperature and encourages condensed moisture on the foliage early in the morning which, in turn, causes scorching. Tepid water is to be preferred to cold.

Damping down can be done with any plants if moisture is required. Again it is advisable not to work late in the evening nor to use very cold water.

Evaporating trays are used mainly in hothouses and vineries, and have much the same effect as damping down, except that the moisture is given off night and day.

Watering. Plants require varying quantities of water according to their nature and period of growth. In a general way plants with very small leaves, e.g. heaths, require less water than those with large leaves, e.g. caladiums, while cacti and succulents require less water than those of ordinary character. The maximum quantity of water is required while plants are in full growth and producing flowers and fruit. Most plants have a resting season when little or no water is required. It is most marked in plants with bulbous or tuberous roots, many of which can be kept completely dry for several months each year after their foliage has died down.

No hard-and-fast rules can be given regarding the quantity of water required. This will depend upon many changing factors, such as the weather, time of year, state of growth, and type of soil. It is never wise to wet the surface soil and leave lower layers dry. When water is given it must be in sufficient quantity to moisten the soil right through. If top watering is being practised with watering-can or hose, application usually should not be repeated until there are definite signs of dryness. To tell this the pot may be lifted and its weight judged, since damp soil weighs more than dry, but usually a little experience will enable the gardener to judge requirements by sight alone.

Automatic watering systems are also available of which one of the easiest to install is the capillary bench. This is a 1tin. deep layer of sand and pea gravel on a perfectly level, water-retentive surface such as polythene film, kept constantly wet so that pot plants placed on it draw water upwards by capillary attraction as they require it. Various devices are available for feeding water to such beds.

Water may be applied from a watering-pot either with or without a rose which should be used only when watering seed pans, seedlings, or freshly potted plants. The drawback is that water flows slowly, the surface becomes deceptively wet and foliage is unnecessarily splashed. Established greenhouse plants are almost invariably watered direct from the spout, which should be held close to the soil to prevent splashing.greenhouse-watering-system

Watering by complete immersion of the pot in a tub of water is useful for established plants which require abundant moisture, e.g. hydrangeas. Watering by partial immersion, i.e. by holding the pot or pan almost to its rim in a tub of water so that water rises from below through the drainage hole, but does not flow over the surface, is useful for seed receptacles, especially if they contain very small seedlings liable to be disturbed by top watering. Partial immersion should be continued until the surface of the soil is darkened by the rising moisture.

Shading, Only ferns and a few foliage plants require permanent shading, but many greenhouse plants, even normally sun-loving varieties, may require temporary shading from strong sunlight in the summer. This is particularly so in small houses which are heated rapidly by the sun and may become excessively hot in summer even with maximum ventilation.

Shading is of two types, semi-permanent and temporary.

SEMI-PERMANENT SHADING is obtained by painting or spraying the glass with lime wash or one of the proprietary shading compounds. If lime wash is applied outside (the usual method), it should be rendered adhesive by adding a little size to it. Lime wash is made by slaking quick lime or fresh hydrated lime in sufficient water to give it the consistency of milk.

TEMPORARY SHADING is obtained with blinds of hessian or chain laths. These are attached to rollers fixed to the outside ridge of the house so that they can be pulled down over the glass as required. As a rule such shading is needed on the south side only. Alternatively plastic blinds may be fitted inside the house or muslin may be attached to the roof rafters to provide a screen.

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