Tim purpose ot a greenhouse is to provide plants with a more sympathetic growing environment than would Impossible in the open air. While providing plants with all the light they need, a greenhouse offers both heat and protection against wind.
The earliest ancestor of the greenhouse was the high stone or brick wall built around a cultivated area. Such walls offered not only wind protection but, by retaining heat from the sun's rays, conditions warm enough to grow relatively tender crops in the borders along south-facing walls. Even tin-weak spring sunshine heated the walls facing the noon sun sufficiently to protect the blossoms ot fruit trees against a few degreesofnight-air frost. More ambitious gardeners supplemented the sun's heat with stoves built into the walls and by extending the w.ili capping. Today, gardeners lucky enough to have a south-facing stone or brick wall often grow tender plants against it, and truit growers use nets to deflect spring frosts from the blossoms.
The term 'greenhouse* originated, according to one tradition, 111 the method used by gardeners to keep the leaves ot imported orange trees green until well into the autumn. Many wealthy English travellers brought back orange trees from the Grand Tour of Mediterranean countries. The trees, growing in barrels, would normally shed their leaves in the late summer, but gardeners discovered that they would retain their leaves for up to a couple of months lunger if they were protected against the autumn frosts. As a result, in many of the grander estates in England, orangeries began to appear in the late 16th century at first open-sided buildings with thatched roofs. Later the thatch was replaced by glass, and eventually the sides and ends of the buildings were also glazed.
The orangery was, of course, a special type ot building lliar evolved in response to the needs of particular types
of plant (exotic shrubs as well as oranges) growing out of their normal environment. But it serves to emphasize the first decision that the gardener needs to make when he is planning to buy (or build) a greenhouse: what exactly is it to be used for? The question is especially important il you are plan ning to grow not one type ot plant but a range of crops throughout the year. Cropping programmes can vary greatly, of course, and by the same token they demand a great variety of growing conditions.
Some crops, such as tomatoes, mid-season chrysanthemums, and vegetables, will be grown in the border soil (the soil around the base of the interior wall). Since these plants need a lot of direct sunlight to promote growth, it follows that the greenhouse must be glazed virtually to ground level on all sides. Moreover, the greenhouse must be high enough to allow the tomatoes and chrysanthemums, in particular, to attain their lull growing height. In general, a greenhouse with vertical rather than sloping sides would seem preferable tor tall-growing plants in the border soil. But the question is not quite as simple as that. The angle of slope (which can vary considerably) is an important factor. Slightly sloping sides will do little to impede growth -and they will admit more light than vertical sides. But it the angle is great, you will not be able to raise tall plants in the part of the border soil next to the greenhouse wall.
In the case of pot plants, and where an area is being set aside for plant propagation, the plants and young stock, will usually be grown on benching. This will need to be of sufficient height to allow one to tend the plants without having to bend over them;
Above left Greenhouses became popular in Victorian times Right A modern conservatory based on a 19th century design
and the light beneath tin.1 benching will he oflinlc, if any, value. In such a case the lower sides of the greenhouse may be made of brick or board. These materials have the advantage of allowing less heat to escape rhan does glass.
As I will be explaining later, heat Conservation is an important factor in greenhouse gardening, and tfthe greenhouse is to be used both tor growing crops in ilie border and on benching it is not unusual to have a solid i 111 (3 It) high side wall on one side and glazing to the ground on the other side. On the face of it this is ,ni attractive solution, but in practice it prevents any simple system of crop rotation.
To make a final choice the prospective owner will have to take his considerations a stage further. Is the site an exposed one? Do the crops to be grown need fairly high temperatures? It" the
Above Drawings show lypical forms ¡¡nd materials of modem greenhouses 1 Straight-
dpd iilloy framed, 2 Sloping-sided alloy■ framed. 3 Alloy-framed lean-to. 4 Polygonal alloy-framed. 5 Partly boarded cedar-tramed 6 Polythene-coveted steel-framed Far lefi Permanent benching needs to be Sturdily constructed to take the considerable weight of a pot-plant display Near left Cucumbers (cordon-trained) growing in raised beds Of well-rolled compos! The partly boarded sides ot the greenhouse reduce heat loss answers to such questions arc 'yes', then the advantages of one solid wall might well outweigh the advantages of being able to alternate the border in which the crops are being grown.
The choice of a greenhouse does not rest simply between those with glass to the gronnd and those with side wails made partly of some solid material. Perhaps a simpler and cheaper structure will suffice, in which case it is necessary to consider the materials other than glass that can be used.
Gardeners today realize that the best results are obtained by providing optimum growing conditions for their plants, and this usually means protecting them from the extremes of the elements. Consider for .1 moment what is meant by 'optimum conditions'. In the shoot zone - that part of a plant above the level of the soil - light, warmth, and moisture have to be available in balanced proportions, and we now know that the level of carbon dioxide (CO.) in the air is also critical. In the root zone the growing medium has to furnish the plant with nutrients and moisture in addition to providing an anchorage. In later chapters I will be giving some guidance about how to obtain the optimum conditions for some major crops.
Light is one of the most important factors in growth, and consequently designers of greenhouses aim to let in as much as possible. In theory the hemispherical shape is best, for the sun's rays are always at 90 to the panes facing towards it, and consequently iess light is deflected, especially if the glass i>. clean (a point often forgotten by greenhouse owners, too few of whom clean their glass on a regular basis).
If the hemisphere is the ideal shape, why are all greenhouses not so built? The answer is that it would he impractical. Tor instance, it is difficult to fit doors into a curving surface, and the materials the designer has at his disposal - glass, timber, steel, and light alloy - are most readily and cheaply available in straight forms. The same difficulties arise with ventilators, which need to be incorporated into the structure, and with benching and shelving,
The slender frames of alloy greenhouses allow the maximum amount of light to reach the plants 1 n this example however. faulty siting — near a large tree and with the back wall against a fence - considerably reduces the amount of direct sunlight reaching the greenhouse Interior throughout the day which would have to follow the curve of the hemispherical wall. Yet another problem is how to make the best use of the growing area. Tending the plants and spacing them correctly is much more difficult in hemispherical greenhouses than in rectangular ones, For these reasons the conventional greenhouse shapes continue to predominate, although there are plenty of interesting variations on the basic themes.
The slender frames of alloy greenhouses allow the maximum amount of light to reach the plants 1 n this example however. faulty siting — near a large tree and with the back wall against a fence - considerably reduces the amount of direct sunlight reaching the greenhouse Interior throughout the day
Some basic elements of a well-equipped greenhouse Not all the equipment. 0/ course, would be in use at the same time
Some basic elements of a well-equipped greenhouse Not all the equipment. 0/ course, would be in use at the same time thermostat, 12 Internal blind 13 Externat blind 14 Automatic vent controller; 1 & Fluorescent light; 16 Electric insecticide/ fungicide evaporator. 1 7 Electric fan 18 Polythene growing bag; 19 Paved pathway of concrete slabs
Key to numbers 1 Headei t^nk 2 L ompressor (raises water pressure). 3 Misi propagator;
4 Bottom-irrigated growing container;
5 Waterproof electrical sockets. 6 Benching. 7 Shelving 8 Tubular electric heaters.
9 Border soil. 10 Humidifier 11 Rod type
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