The provision of shade restricts solar radiation very considerably and this in turn reduces air temperature. It may be necessary for propagating activities or for certain groups of plants, or crops such assuffering from wilt diseases. The simplest method of shading is to apply proprietary shading material, usually green tinted, to the outside of the glass, or alternatively lime plus water which is less permanent. Much diluted emulsion paint is also useful but can be too persistent. All materials will invariably wash off by the end of the season, although some assistance with a long-handled brush may be necessary to ensure maximum light transmission in winter.
A shading material, Varishade, which is transparent when wet and white when dry, is available to apply to the outside of glass. Blinds can be fitted both outside and inside, and green polythene blinds, fitted at the ridge inside the greenhouse, are extremely useful. Slatted blinds can also be used for special plants such as orchids. It is important to ensure that the shading equipment purchased is suitable for the size of the greenhouse. As a temporary measure green polythene or plastic netting can be used, supported on wires, canes or any other rigid stiffeners.
Double glazing, which has become very popular in domestic circles, involves the use of two sheets of glass hermetically sealed so that a layer of air is trapped, the net effect of which is to reduce heat loss considerably. This is not always considered practical foron account of weight and cost, and such attempts as have been made by simply using two panes of glass on either side of wooden or metal glazing bars have not always been too successful, as it is difficult to ensure a tight enough seal to prevent the entry of dirt and moisture. Much more practical is the use of twin or treble walled polycarbonate sheeting, which is readily available and widely used for conservatory roofs, and is also highly efficient for reducing heat loss.
New designs of PVC sheeting are also more effective than glass for reducing heat loss. Both polycarbonate and PVC are, however, more expensive than glass to purchase. Polythene lining is frequently used as a simple form of double glazing, ideally using ‘bubble’ polythene, which can be tacked on to the inside of wooden glazing bars, using paper pads and drawing pins or staples. With metal houses a piece of wood inlaid into the groove in alloy glazing bars is possible, failing which a wooden framework can be used. Special clips are available for securing polythene.
Approximately a 25% reduction in heat loss can be expected when all but the vent areas are covered. Problems of excess humidity can develop for some crops, although this high humidity can be an advantage for propagating activities. Probably one of the main advantages of polythene lining is to prevent draughts and drips in older type greenhouses in poor repair. Light transmission is only slighthy reduced by the polythene lining. Thermal screens of various materials pulled across above head height or as rollups on outside of structure overnight are now much used commercially to reduce heat loss.
Carbon dioxide (CO2) enrichment
The natural complement of the gas carbon dioxide in the atmosphere is 300 parts per million, and this is normally a sufficient concentration for the photosynthetic needs of plants. Research has shown, however, that under conditions of good light intensity and compatible temperatures, the growth rate ofmay be restricted by lack of sufficient carbon dioxide. When the atmosphere is enriched up to approximately 600-900 parts per million (two to threefold) the velocity of photosynthesis increases.
The breakdown of organic matter in theresults in the production of CO2 and this takes place in the soil continuously. Where cultural methods involve the use of large quantities of organic matter, such as ‘hot’ beds or of or , a considerable quantity of carbon dioxide is produced which enriches the air in the vicinity of the growing plants. Plants vary in their response to additional CO2 and even certain varieties of the same species respond better than others.
The artificial enrichment of the atmosphere can be achieved in a variety of ways. The burning of paraffin heaters results in carbon dioxide in quantity, but unfortunately impurities such as sulphur are also produced and this can be harmful to plants if it reaches too high a concentration. The burning of propane gas in special burners is an efficient way of achieving carbon dioxide enrichment, propane gas being relatively pure. Natural gas greenhouse heaters would also produce CO2 without any of the complications of sulphur production. Liquid CO2 piped into the greenhouse from bulk supply tanks is a commercial technique still practised, as is the use of dry ice.
One of the problems of supplying extra carbon dioxide is checking the level of enrichment, this requiring specialized testing equipment. Ventilation should not proceed simultaneously with CO2 enrichment for obvious reasons.