Plastic, one of the most common greenhouse construction materials nowadays is not a substitute for glass. Plastics are by nature very much softer. They can be scratched and abraded much more easily. In some circumstances they may weather badly, particularly on windy sites where there is much wind-blown grit, sand, or dust. Unfortunately, once this happens dirt seems to collect on the surface and become ingrained. It may be most difficult to clean off and attempts may lead to making the scratches and abrasion worse.
Some plastics also change chemically with time and on exposure to sunlight over long periods. They usually tend to become brittle and they may crack and disintegrate.
A further disadvantage is that water does not wet a plastic surface and form a film as it does on glass. This often causes condensation to collect in droplets. These may constantly drip, especially if the roof is insufficiently sloped. The drips can be annoying to anyone working below, particularly in the home extension type of lean-to, and harmful to plants by encouraging excessively wet conditions during the winter months when they should usually be kept carefully watered. The effect is worse when corrugated plastic sheeting is used for a roof and the slope is only slight. Condensation then drips from the entire length of the corrugations.
In their present state of development, plastics are not really a wise choice for a permanent greenhouse intended to be in operation for many years, and probably heated artificially.
Only a few plastics of the more expensive type can be compared with glass for aesthetic appearance and clarity-clean, clear, sparkling glass is extremely pleasing. Some of the more flimsy plastics sag and buckle, and move about with the wind.
They can look terrible and do anything but enhance the appearance of a garden when used in a structure.
In spite of these severe criticisms which could well make plastic manufacturers my lifelong enemies!-to be fair they have a number of distinct advantages. They are lightweight and unbreakable. This makes them excellent for temporary plant protection, emergency repair, and for use when glass is impractical owing to breakage risk. They have in fact innumerable uses in the garden and in the greenhouse, which will be brought to the readers attention later in this site. Used with common sense and understanding, they can be invaluable and may replace glass in many instances.
There are a number of different types of plastic classified according to their chemical composition. Polythene is perhaps the best known and most frequently used. Ordinary polythene is not recommended for outdoor garden work. It soon deteriorates on exposure to sunlight and disintegrates. For making temporary greenhouse structures, use a special grade resistant to ultra-violet light. Even then, don’t expect it to last more than about two to three years (much depends on the site).
Flexible but stronger plastics are available, such as acetate sheeting and PVC, often sold under trade names. Some are reinforced with wire. These are of course longer lasting. For more permanent structures, rigid PVC sheets can be used, and these are usually corrugated for strength. One of the best grades for greenhouse construction is the ICI Novolux which is guaranteed for a number of years against weathering. ICI issue plans and designs for several greenhouse-type structures. These are attractive but may have practical disadvantages for serious greenhouse work.
All plastics used as a replacement or in place of glass should be as clear, colourless, and as transparent as possible. Great care should be taken in cleaning so as not to scratch the surface. Only one shading paint is suitable for application to plastic without fear of scratching on removal. This is Coolglass, the electrostatic type. Most plastics should not be put in close contact with creosoted timber, since they may become discoloured or damaged.
A recent plastic, perhaps the nearest to glass in appearance and hardness, is Transpex. This is an acrylic plastic similar to that from which dentures are made. At present it is expensive, but it is a good choice when something unbreakable but as near to glass in appearance as possible is required.
Greenhouse framework and glazing
Very recently there has been a great swing in favour of aluminium alloy framework. This is perfectly justified because it has valuable properties that make it ideal. It is rust-proof and rot-proof and it cannot warp. It is very resistant to oxidation and the effects of weather, and needs no painting, treatment, or maintenance whatsoever. A good aluminium framework will last more than a lifetime with practically no attention. It should be realised that this high praise applies to modern aluminium alloys. Some of the early alloys were far from weather resistant, and later development was greatly influenced by research into finding alloys of aluminium suitable for marine use and resistance to seawater. Aluminium framework is relatively cheap, and it is possible to make very sophisticated glazing bars permitting various simple forms of glazing with clips, and avoiding the application of putty. The various forms of patent glazing found among the manufacturers of aluminium houses all have the advantage of quick and easy glazing and simple removal of glass if necessary. Most aluminium houses are therefore easily taken down and can soon be put up again elsewhere if the need arises.
A new aluminium frame is shiny, but after a time the metallic sheen is lost due to a coating of protective aluminium oxide that forms through exposure to the air. More expensive frames are stove enamelled, and this finish retains its attractive appearance indefinitely. There is no objection to painting an aluminium framework with a gloss paint if desired, but it should be avoided if possible.
Aluminium frames can look out of place in some surroundings. They seem to fit modern gardens laid out in a formal style, and they go well with modern architecture; but in informal settings and when near period buildings, they can appear an intrusion. In such cases mellow timber might blend better, although white-painted frames can also look quite pleasant.
Galvanised steel is also used for framework, although on comparing price and properties it is difficult to see any advantage over aluminium. Galvanised steel does need painting. If the zinc galvanised layer is damaged to expose the steel below, rust will start corrosion. Steel is also much heavier and less easy to work, and it certainly will need regular painting or maintenance.
All metal frameworks are of course very strong (or should be).They will usually take the weight of hanging baskets or plant containers, and shelving, without fear of the roof collapsing.
They can usually be put together quickly and easily with a spanner and screwdriver, single-handed and without the requirement of strength, and even by very elderly or infirm people.
Glazing can be done with great ease either because of simple patent glazing with plastic strip and clips, or by the use of a special non-hardening plastic putty. Ordinary linseed oil putty must never be used on metal frames. There are special compositions that never set hard and so allow the frame and glass to expand and contract with temperature so that the glass does not crack or break.
It is sometimes said that metal greenhouses are cold. This is doubtful. It is true that metal conducts heat better than timber, but in a metal greenhouse there is less area of frame and consequently more light entry with greater benefit from the suns warmth. In any greenhouse the area of frame compared with the area of glass is so small that the difference is negligible in any case. However, in the dwarf wall-type greenhouse, or plant house, metal panelling for the base walls should be strictly avoided unless they are well lagged. A metal base will become icy cold and like refrigerator plates in winter, resulting in a fantastic heat loss.
All timber greenhouses will need maintenance from time to time, often every year. This constitutes painting or coating with one or other special timber dressings to restore appearance and ensure preservation. Perhaps the most popular wood is so-called western red cedar or similar cedar types. This has an attractive appearance and blends almost anywhere. It is remarkably weather-resistant and has little tendency to warp or attract wood destroying insects and fungi likely to cause rotting. Teak and oak are some other useful timbers. Cheap timbers and softwoods are best avoided unless you are prepared to spend considerable time each year in their restoration, and preservation.
The glass usually used in greenhouses is 24-ounce sheet. It must be clear, colourless, and of good quality. Opaque or coloured glass is not suitable.