It is estimated that there are more than 1500000 greenhouses in great Britain. Some may be used mainly as potting sheds and tool shelters, but there can be few that do not at sometime shelter plant ‘immigrants’ that are unable to withstand the British winter. Apart from conservatories, the main types of greenhouse are cold house, which is unheated except perhaps for soil warming equipment; the cold house, where are temperature of atleast 40°F is always maintained.
The average beginner will start with a simple cold frame or green house, with which a good deal can be done. Apart from protecting tender plants from frost and snow, one can raise in them, seeds early in the year and perhaps grow a few exotic plants that would not survive in open.
The more advanced gardener, however, will undoubtedly want some form of heating and this can always be added to a coldhouse if it is weatherproof. As a rule it is advisable to buy a greenhouse in sections. Not only is this type quick and easy to erect but in the long run it is likely to prove more satisfactory than an amateur built structure. Manufacturers provide detailed literature, and there is a variety of shapes, sizes and materials.
The traditional style is the span roof house, fitted with staging along two sides and a path in the middle. This design is very suitable for a large, open plot with full sunshine.
For a more restricted site, the three quarter span house is useful. One can utilize a house wall for the rear side, as in the diagram, or buy a self contained house with a wooden rear wall. It can have a single line of staging on one side and tiered staging against the wall.
The cheapest form of greenhouse is the lean-to, which also fixes against a wall. This is a good buy for a small garden; it is neat and space-saving, and the large roof area attracts plenty of light, while a brick rear wall tends to retain warmth.
Modern greenhouses are made in materials including wood, aluminum, steel, brick, and concrete. Most amateurs will chose either an all-metal type, which needs no painting, or an all-wood type, which is cheaper and also tends to retain warmth longer.
Gardeners fortunate enough to be able to obtain farm or stable manure can provide their cold house with a valuable source of heat under the soil beds, as well as a rich plant diet (excellent for early salads). The cold house is useful in winter for semi dormant plants, and for such spring flowers as daffodils, hyacinths, tulips and scillas. Some food crops such -- e.g. rhubarb, seakale, chichory -- can be forced in it, and as spring arrives the early seed sowing can be done under glass weeks ahead of outdoor sowings.
A house used especially for propagating in late winter needs as much sunlight as possible. The design affects the amount of light admitted, and it has been found that if the roof slopes at 30° to the horizontal, this allows the maximum amount of light to enter the minimum loss by reflection from glass. A number of houses are constructed to this design.
In the larger garden, the key to success is the service corner. Here the tender plants can be protected during winter and the half hardy annuals and vegetables raised from seeds before being planted out in their permanent quarters. For convenience in working the potting shed with its supply of soils, the green house and the frames should all be located close together, and it will be a great advantage to have a water supply available on the site.
Galvanized steel and aluminum green houses need no maintenance of any kind. They are easy to assemble and being sectional can be extended when required. The wooden staging can be taken out when the house is wanted for tomatoes, etc.
Traditional wooden greenhouse with red cedar cladding. Only the framing needs painting, though the cedar wood benefits by treatment with a water repellent preservative. This house is made in various sizes, and the main sections are interchangeable to suit different sizes. Note the neat gutter and down pipe supplying a water butt.
A cold greenhouse rarely satisfied for long: even the beginner soon starts to consider the heating apparatus. There are still many greenhouses heated by coke stove with a hot water boiler feeding cast iron pipes -- many commercial growers prefer this system because it is never subject to power cuts!
When a hot water system is being installed, do not try to economize bye reducing the length of the piping to be fitted. Best results are obtained from large areas of low temperature pipes; excessive heat makes the atmosphere too dry. Also, stoking it easier with extensive piping as the larger quantity of water retains it heat much longer. Normally, stoking such as system will be necessary two or three times a day.
A small greenhouse can also be heated economically by a paraffin oil heater. Special models are made for greenhouse use which are fitted with water troughs to maintain atmospheric humidity, which will burn for upwards of 48 hours without attention. These need little attention, but wicks should be regularly inspected and trimmed, and cleanliness of both the appliance and fuel is important.
While such a heater is all that many gardeners need, others are turning increasingly to gas or electrical installations. Both gas and electricity supply authorities provide a wide range of greenhouse aids that are worth studying before an order is placed. Both, for example, offer heat controlled by thermostat so that the gardener is not tied so much to his greenhouse. Electric greenhouse heaters, usually take the form of aluminum tubular units which can be built up in tiers on the walls or floor; there should be same number of units on either side of the house to ensure even heat distribution. Another type of heater is water filled to permit gradual adjustment of temperature after the thermostat has operated.
An advantage of using electricity, ofcourse, is that once a power supply has been brought into the greenhouse it becomes possible to install electric soil warmers, ventilators, soil sterilizers and so on. At the same time, one must remember that the cost of heating greenhouse by electricity rises rapidly as the temperature is raised. For instance, it costs more than twice as much to maintain a house at 50°F as it does at 45°F, and more than three times as much to maintain 55°F.
Thermostatic heating control not only ensures an even temperature but keeps down running costs. The type shown is made from steel and is extremely sturdy and very stable on the ground. It has a digital screen which eases the process to control temperature. The fan with this thermostat is really incredible giving smooth and gentle air circulation and the heat that emerges is a gentle heat to avoid scorching nearby plants.
Top ventilation is most essential of all types. Wherever possible the ventilators should be on both sides of greenhouse so that opening can be adjusted according to the wind. Always ventilate with a rising temperature and never allow the thermometer to stand too high before air is given, thus causing a sudden drop. Side ventilation is only used in summer weather when it cannot cause a draught. Always close the house early, particularly during the duller months, so as to shut some of the suns warmth into the house to maintain a genial atmosphere during the night. This applies to both types of ventilators.
The interior part of the greenhouse should be settled in conjunction with ventilating arrangement and type of heating apparatus. Shelves should be arranged so that water will not drip on to plant below. The path should be no more than the width of the door (usually 2 ft 6 in.) staging made of slatted wood to allow free circulation of air around the plants is most often used. Solid staging of sheet zinc, etc., supported on wooden framework, are often used in conservatories. These must be covered with coarse sand, washed pebbles or clean small clinkers. This last has the advantage of conserving moisture and during the annual cleaning can be washed in a sieve. When adopting this method, the edge of staging can be clothed with tradescantia or helxine rooted in ribbon of soil.
During the early spring the house is often crowded with annuals and vegetables for first crops being raised. An extra temporary shelf can be made from wooden planks slipped into supports fixed to the roof.
Easy Soil Heating
A popular method is to heat a soil bed in one corner of the greenhouse, or a bed in an outside frame, by means of soil warming cables. This allows tender seedlings to be raised, cuttings taken, etc., at low cost. A similar cable will provide bottom heat for a deep seed box, and the box can be fitted with a plastic dome or tent cover which gives headroom for larger plants. The mains attached by clips to the frame walls, or laid on 2 in. of washed sand and covered with a further 2-3 in. Pots and pans are stood on this sand bed, and granulated peat is packed around them to conserve the heat.
A recent development in greenhouse aids is this system of propagating cuttings. It is claimed that 95% success with cuttings become quite normal with the system, easy subjects taking root more quickly and difficult ones more certainly. The idea is that the cuttings are never allowed to flag; after planting in the rooting medium they receive a spray of fine water mist whenever they would otherwise begin to dry off. The spray nozzle can be seen mounted above the cuttings, which are in electrically warmed soil. In the corner is a humidity sensitive detector which turns on the water when the atmosphere becomes dry. Water from the fine mist spray then envelops the foliage of the cuttings until the humidity detector unit operates and automatically switches off the spray.
Working on the principle that greenhouse plants need water only in small amounts but often, manufacturers now market some ingenious automatic watering systems.
The drawing shows an automatic drip feed system which is connected to a convenient tap. Water is released through drip nozzles in a flexible irrigation line at intervals.
A variety of techniques of capillary watering is now used in equally in the greenhouse and nursery industry. One of these techniques entails using line-source tubes which would take water to a capillary mat. Water would then travel through the mat by capillary action pretty much like a sponge. Containers are positioned on top of a saturated mat, which permits the medium in the container to soak up water. Containers must be saturated before being placed on capillary mat systems to guarantee successful water consumption. Capillary mat irrigation systems can use 60% less water than overhead irrigation systems. Capillary mats have the limitation as other sub-irrigation types in that salts can build up in substrate over time without periodic leaching events.
Pests under glass are a special problem. With this device, small tablets are placed in a non illuminating glass bulb which is then plugged into a lampholder. When the lamp is switched on, it melts and vaporizes the tablets. This vapor spreads through the greenhouse, dealing with aphides, red spider and white flies.Automatic Pesticide sprayers ensures consistent, prompt and effective treatment against pests. These also allow biological control to be carried out without the use of pesticide, resulting in a significant financial saving