Some modern greenhouses have been designed for convenience, plus aesthetic appeal, notably the circular styles that can be used in the garden as pleasing focal points. In general they cannot be said to enhance the garden scene as works of architectural beauty. For this and other reasons, greenhouses are all too often relegated to the end or far corner of the garden to a site which is much less than ideal. If one is going to the expense of buying a greenhouse and perhaps heating it to grow favorite flowers, fruits or vegetables, the best possible place must be chosen. This means a site away from overhanging trees and from tall buildings and if possible some sort of protection from strong, cold winds.
In temperature latitudes, the sun sinks back towards the equator from late summer onwards and hangs low in the southern sky (northern in the southern hemisphere) during the late autumn to early spring period. If it is intended to grow food crops or flowers in winter, maximum light is required at this time, so it is very important that it is open to the south. To get the very best of the low winter sun, the long axis of the greenhouse should be aligned east-west or as near as possible. This avoids too much shading from the roof supports and glazing bars and allows maximum light to enter. Glass to ground sides can also be an advantage, particularly if they lean inwards as in the Dutch light structures and some other designs, for the maximum amount of the sun’s rays penetrate when they strike the glass at or near to a 90 degree angle.
For ease of maintenance, and to get the greatest amount of enjoyment from using it during the winter months when weather outside can be bad, the greenhouse should be as near to the house as possible. Indeed, there is much to be said for the lean-to greenhouse, conservatory or sunroom attached to a south-facing wall, with direct access from the house if this is feasible. It can also have the advantage of being easier to heat and can sometimes be linked up with the heating system of the house, which can save a good deal of money in installation and running costs. Where summer sun is reliable, a lean-to greenhouse with solar collectors can also contribute to heating a house.
Every greenhouse manufacturer, and there are many, has their own patented designs. However, there is little that can be done to diversify the appearance of a small greenhouse to any great extent without greatly increasing the costs, so there is a remarkable overall similarity between designs. In outline they usually resemble the basic dwelling house, an oblong box with a simple span roof. Such variations as there are involve hipped or double hipped roofs and inward-sloping sides plus many variations in the length to width ratio. There are also the lean-to structures, in effect a greenhouse split in half length ways, made to fit snugly against the side of the house or a free standing wall. Larger lean-to greenhouses are often called conservatories, especially when there is direct access from the house, but strictly speaking the term refers to any greenhouse maintained as a collection of plants for yearlong display and including floor level beds for permanent shrubs and climbers. The modern sun-room often functions as part living quarters, part conservatory, and when heated as part of the house and can be an excellent place in which to grow some more tropical plants. If it is intended to grow a lot of plants in a sun-room type extension, it is worthwhile having the roof glazed or surfaced with clear plastic corrugated fiberglass, with the addition of blinds for hot summer days.
There are also circular structures, already mentioned, and multi-faceted or geodesic domes. One very successful design is the Dutch light greenhouse. This is made of a chosen number of wood-framed single-paned lights, bolted together so that the sides slope inwards. Although such a simple structure, it is remarkably strong and the large areas of glass allow plenty of light to penetrate.
The mention of wood brings us to a consideration of the available building materials for greenhouses. These are wood, aluminum alloy, steel and concrete clad with glass. A further alternative is also available, structures of tubular steel or a metal alloy clad with polythene sheeting (plastic film). Polythene or plastic film houses of this sort may be the traditional shape, or rather like a Nissen or Quonset hut, and known as tunnel house. There are also fancy-shaped- designs, rather like small marques in outline but with triangular-faceted sides and roof supports. Polythene houses which are much cheaper than glass can be useful in sheltered sites. However in windy sites they can be split or wrecked in a gale and even in sheltered sites the sheeting will probably need replacing after two or three years. Moreover they can be difficult to ventilate in summer.
Concrete as a building material has never proved very suitable for small houses, mainly because the bulky supports keep out a lot of light. Wood was for a long time the traditional framework but unfortunately the amateur models were often made of the cheaper white woods which need regular painting. Today, western red cedar is the usual wood and this is rot-proof and does not need painting, although it is advisable to give it a coat of linseed oil or preservative every five years or so. The use of aluminum alloy also does away with maintenance painting. This is a primary reason why during the last 15-30 years, metal houses have gained popularity and now claim a major share of the market. In the view of a gardener this is a great pity. Wooden houses fit in so much better with the garden scene, particularly the attractive reddish color of cedar. In addition metal is a better conductor of both heat and cold and as a result heat losses are a little more from an alloy or steel house than a wooden one. A further point in favor of wood structures is the east of attaching extra shelving, wires for supporting climbers and hooks for hanging baskets. Although some metal houses are drilled for these things, all too often the holes are not where a grower wants or needs them. As the price of comparatively sized aluminum and red cedar greenhouses is roughly the same, why not choose wood?
Once the material has been chosen, the three categories of design can be studied. These are: those with a brick or boarded wall to bench height; those with a similar wall along one side only and one or both ends; those glazed completely to the ground. The first type is certainly a little warmer in winter, which can be an important consideration in colder regions, but the space beneath the benches is very unproductive unless forcing rhubarb, sea kale, chicory and lily-of-the-valley is to be a regular practice. A few shade tolerant plants can also be grown under benches, but only along the path side where there is illumination. Walling on the cold windward side only, especially in exposed places, marginally helps to keep down heat losses characteristics of such sites, though unwelcome shade is cast if this if this is on the west or east sides. Glass to ground structures allows full use to be made of all parts of the greenhouse. This is particularly important when one wants to grow as many and as wide range of plants as possible in a small space. In this sort of house, plants can be grown on the floor, the bench above the shelves that and if there is room, hanging in pots and baskets over all.
As there is so little variation in the standard designs from one manufacturer to another, one of the chief considerations in making a selection will be cost, and to find the best value it is necessary to shop around. The best plan is to contact as many manufactures as possible and to compare sizes, styles and costs. It will soon be noticed that costs vary much more widely than styles. It is important to look at those extras and accessories which are often essential to the basic structure. Most important is the base. Greenhouses are normally supplied without a base. This can either be made of concrete or brick by the prospective owner, or one made to measure can be purchased at additional cost with the greenhouse. Almost as important is the number of ventilators included in the price. Few if any of the smaller greenhouses have adequate ventilation for warm summer days; one to several extra vents will always be required. Ideally there should be at least two ridge and two side ventilators for every 6 feet of greenhouse length. Benching and shelving are always extra and whether this will be needed depends upon the use to which the house is to be put to. If you cannot build your own, order it together with the main structure; it will cost more if ordered separately.
Whatever the design and materials used, one important consideration is how big? For the beginner this is always a problem, and often cost dictates size. Basically it is a problem of growing space which should be considered in a three dimensional way. The square footage of the green house floor – easily arrived at by multiplying length by breath – provides the two dimensional factor and height the third dimension. The last can be very important if tomatoes, cucumbers and melons are to be the primary crops, or if a wish to grow tender shrubs or climbers is paramount. Bear in mind also that structures with inward sloping sides have less space above bench level. If possible, always go for the greenhouse which is one sixth larger than seems necessary, for the larger the house, the cheaper is each unit of growing space. If a larger house cannot be afforded at the time, then make sure that the one chosen can easily be added to by bolting on one or several more sections as needed.
In addition to the siting, some attention must be paid to the actual terrain where the greenhouse will sit. Ideally this should be level and well drained. If the ground slopes, it will be necessary to excavate a flat area and if it is very wet, some sort drainage system laid down. Wet sites are best prepared by excavating at least 12 inches of soil and putting down 6-8 inches of hardcore, topping it off with 4-6 inches of concrete. The platform made in this way should be 4-6 inches longer and wider than the base of the greenhouse and stand proud of the surrounding soil.
Alternatively, foundations similar to those of a dwelling house can be put in. Make sure that the greenhouse chosen can be securely anchored to the basses of this kind. Ground level soil beds, while an asset on most soils, are seldom satisfactory on wet soils and the bigger plants must be grown in large pots, tubs or similar containers.
If a concrete base is purchased with the greenhouse, it must be laid with care on accurately levelled ground. ideally the soil should have been uncultivated for a sufficient time for it to settle, for example under grass; if there is no such site, it must be well packed before finally raked level, to make sure that small variances do not cause unnecessary stress on the structure.