Plasticcovered greenhouses

Undoubtedly the biggest rival nowadays to the conventional greenhouse is the polythene-covered structure, of which several designs arc available. Its popularity is due to its relative cheapness, resulting trom the simplicity of the framework and thesold-in-a-carton approach. The basic framework in most cases is of tubular alloy or galvanized steel, and 6oo-gaugc polythene sheeting is the usual covering material. Several factors affect the life of the sheeting so ti is not easy to be definite as to how often it will need replacing.

Polythene life will be extended by tile addition of an ultra-violet inhibitor (UV1) during the manufacturing process, and polythttne so treated is now widely available. The tautness of the cover and the care with which it is put over its frame can make all the difference between a long and a short lite. It is advisable first to wrap tape round those parts of the frame which might snag the cover and to put on the polythene sheet on a warm day. Polythene expands considerably when warm, and it is possible to get it drum tight by following the manufacturer's recommendations in this respect. Some greenhouses are supplied with polythene sheeting tailored to fit like a glove. As a result there are no unsightly folds in the cover and the effect is much neater. However, in the manufacturing process this requires welding the polythene, which is likely to deteriorate more quickly at the welds. With a measure of luck the polythene should need replacing only every two years,

Left above A lean-to greenhouse with roof and stde ventilators Narrow-section glazing bars cut out tb<" minimum of light Left below A lean-to growing case

A polythene covered greenhouse To compare costs with conventional greenhouses, first calculate the area that can be cropped although in very exposed situations annua) re-covering may he necessary.

Greenhouses of this sort provide growing conditions different from the conventional glasshouse and have to he used differently, particularly as regards watering and ventilation. Nevertheless, almost all plants thrive in them and the gardening scene is likely to see many more greenhouses of this type in the future.

ritiuj plastic clazinc; The 'cheap-and-nasty* image of rigid plastic is still firmly entrenched in the minds of many gardeners; but it is certainly worth considering carefully what advantages -not least in cost - rigid-plastic-covercd structures offer before making a final choice. Of the plastics available, extruded acrylic sheet 0.25 mm (0.01 111} thick is strong and flexible and has a life of 10 to 20 years when used in greenhouses. Clear i'VC is available in corrugated form to provide the necessary strength, and a 10-year life can be expected for this. These plasties arc satisfactory alternatives to glass, and while they are considerably more expensive, the framework to which they are fixed can he simplified, so that overall costs may differ only slightly.

For the newcomer to greenhouse gardening the apparently bewildering choice of structures can be narrowed down by following the steps 1 have suggested in tins chapter. It may seem at first sight to be rather a laborious process, but in the final outcome it will save you time, frustration and possibly a great deal of expense.

Siting

Many gardeners have little choice as to where to site their greenhouse. Letting m the maximum amount of light must be the overriding factor of the many that need to be considered, and the orientation of the greenhouse will have a bearing on this. Whether the ridge of the greenhouse is best run on a north-south or an east-west axis will depend to a great extent on the nature of the cropping programme. It cultivation of winter and early spring crops is important, then the east-west axis will provide better interior light conditions in winter than any other.

This is because the hours of winter daylight are relatively few and adequate sunlight is available only between about (o am and 4 pm. With the side of the greenhouse facing the available sun there will be less shadow from the framework of the structure and maximum penetration of the sun's rays through the transparent covering. A similar greenhouse on .< north-south axis would have its end facing the sun at mid-day and sunlight into the house would be broken up by many shadows.

tin the other hand, if the cropping emphasis is on summer and autumn crops the north-south axis is to be preferred. In mid-summer the side of the greenhouse faces the early morning sun and consequently it warms up quickly. At mid-day, when the sun is high in the sky and outdoor temperatures arc Soaring, the end of the greenhouse faces

A polythene covered greenhouse To compare costs with conventional greenhouses, first calculate the area that can be cropped the sun and the shading effect may prove beneficial. As the sun sets in the west, its rays again fall on the side ot the greenhouse, keeping up temperatures well into the evening.

Avoid any shading of the site by overhanging branches, tall fences, or nearby shrubs. Overhanging branches can be particularly troublesome, for the drips from them dirty the greenhouse quickly and in gales quite small twigs banging against the greenhouse are Capable ot breaking glass. Even shrubs 2 m (6 ft) in height will throw long shadows in winter-time, and it may be necessary to rearrange some of the plants ueai the greenhouse in order not to deprive it of light.

Winter north south axis brings shadow trom frames.
Winter east west axis best for winter/early spring crops
Summer north-south axis best for summery autumn crops
Summer east-west axis may cause mid day overheating

Greenhouse sites to avoid: beneath a large tree beside tall-growing shrubs, against a north-facing wall, and in a positron exposed to winds.

Sifellota, fuchsias, arid pelargoniums provide colour ;it diUereni levels in this greenhouse constructed of western red cedar

The condition of the area of soil that will be used as borders is also important. Soil chat tends to become waterlogged in winter is unlikely to dry out simply because it is covered by a greenhouse: water has a great capacity tor moving sideways in many soils It plants are to be grown in the borders of a greenhouse on such a site, it wtil be necessary to raise the soil level i jo to 200 mm (6 to 8 in), preferably before erecting the greenhouse. Ensure the soil added is topsoil rather than subsoil, and take the opportunity to enrich it by adding peat or well-rotted garden compost.

Once the site has been chosen the conventional greenhouse is likely to remain there for many years and provide an important garden feature. This often leads to problems it crops are grown in borders because soil pests and diseases can build up over the years, and a faliing-off in the general health of successive crops is inevitable unless some form of soil renewal or sterilization is practised. On the other hand, the lightweight plastic greenhouse, the polythene-covered greenhouse, and growing cases and frames can be moved to a fresh site with ease. If you can do thisevery two years, you are unlikely to be worried by soil-sickness problems.

The site must be Hat and as nearly horizontal as possible. There are several reasons for this, quite apart from the need to make the structure stable. For instance, hot air rises, and greenhouses longer than about 5 111 (if) ft) that run tip a steepish slope will have a temperature differentia! ot several degrees from one end to the other. Another more obvious problem arises with watering, tor the lower parts of the borders in a sloping greenhouse will always be wetter than the higher parts. Finally, a sloping site makes staging difficult to erect and keep firm, and the plants growing on shelves and benching are more difficult to manage.

lie careful how you set about levelling a sttc: the top .200 mm in) of soil should first be removed and set to one side, and the underlying subsoil levelled: the topsoil is then replaced. This is quite a laboriousjobifthe greenhouse has a large floor area. An alternative method is to construct a base of bricks or concrete blocks in such a manner as to take up variations in soil level. Some makes of polythene-covered greenhouse are designed to allow erection on gently sloping sm-s without any need for moving soil. Such greenhouses have foundation tubes which arc driven into the soil, any uneveness ot the ground being taken up by varying the depth to which the tubes are sunk.

The greenhouse is often a prominent feature in the garden and children at play, especially with ball games, are likely to be the main cause of glass breakages, ^'protection becomes necessary, mm (l in) wire netting erected on a simple framework t m (3 ft) from the greenhouse is effective and cuts out the minimum of light, The alternative may he to tuck away the structure out of sight, but vandalism and theft of produce may be the price you pay for doing so.

On exposed sites protection from the wind is important, particularly winds from the north and east. Glass

Sifellota, fuchsias, arid pelargoniums provide colour ;it diUereni levels in this greenhouse constructed of western red cedar houses are not airtight, for air moves between the panes of glass as well as through closed ventilators and doors. Even in still conditions one to two air changes an hour can be expected, and more air changes will occur as wind speed rises over the glass area. The ideal screen is a filter rather than a windbreak and is about 60 per cent solid; hedges and shrubs are good tor this purpose. An alternative is fine wire netting - a double fence of netting some 75 to 150 mm (3 to 6 111) apart provides the almost perfect filter, while cutting out little light. A fence of this sort may be well worth constructing if the greenhouse is being heated and keeping the temperature up, or at least regular, is important.

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