In athe gardener can create his own climate to suit the needs of almost any plant. He can heat the air when it is too cold and add moisture to it when it is too dry. He can provide varying degrees of shade or exposure to sunshine and he can even provide extra “sunshine” where it is required in the form of artificial light. The water supply can be adjusted to meet the needs of every individual plant throughout its changing periods of growth. It is not surprising that the possession of a greenhouse enormously increases the range of plants that can be grown. It can be used as a place in which to raise or strike and so the outdoor garden with new plants; it can be a permanent home for plants which could not thrive in the normal climate of an unaccustomed country: or it can be used for the cultivation of special or out-of-season food crops. These uses are not mutually exclusive but the greater the variety of uses the more ingenuity must be exercised to provide, within the same structure, for the differing needs of various types of plants. For this reason it is wise to commence with a limited range of plants with similar requirements, and to diversify as experience is gained.
Types of Greenhouse and their Fitments
There are many different types of greenhouse and much has been written about ideal shapes and materials. Yet, in practice. Gardeners have proved conservative and since the demand is mainly for a few conventional types, these are the ones made by-most manufacturers and most readily obtained.
These types are the span-roof and the lean-to and they may be glazed to ground level or to within about 2 1/2 ft (75cm) of the ground with solid walls of wood, brick or concrete from the glass to ground level.
The span-roof greenhouse is intended to stand in the open with light reaching it from every side, whereas the lean-to greenhouse must stand against a wall or other solid background which deprives it of light from that side. A development of this is the kind of structure often referred to as a home-extension, sun room or loggia which is intended to be built on to the house to give extra living space as well as to provide a place in which to grow plants.
The advantage of the span-roof house is that a wider variety of climatic conditions can be provided within it. The lean-to scores, first in cost, and also probably in running costs as it may require less artificial heat to maintain the required temperature. Sometimes it is possible to heat the lean-to, and even more the house extension, from the normal heating system of the dwelling house, and this can effect further economies in cost.
Full or Partial Glazing
Something rather similar applies to glazing to ground level as compared with the house that stands on low solid walls. The fully glazed house provides better lighting but may be more difficult to heat. Aesthetics play a part here too, for though a fully glazed house can look attractive from outside if plants are growing from ground level, it is less attractive if the plants within are on staging with empty space or storage beneath.
Wood or Metal Frames?
Much has been written about the rival merits of wood and metal frames. Wood is, in general, cheaper and it is easy to fix fitments, such as shelves or blinds, to wood-framed houses. Deal needs fairly frequent painting if it is to be maintained in good condition, but western red cedar resists decay and can be used without painting or other treatment.
Metal is durable, and since metal glazing bars are usually narrower than wooden ones, they cut off rather less light, though this seldom appears to be a critical factor. Iron and steel rust so readily that they require a considerable amount of maintenance. This does not apply to aluminium alloys but these are comparatively expensive. However, they may be regarded as the ideal materials for greenhouse framing.
Ventilation is exceedingly important, not so much because plants require fresh air as because it is only by changing the air in a house rapidly that its temperature can be kept down in sunny weather. It should be possible to get 30 complete atmospheric changes per hour in a greenhouse even though this may only be required on a few days each year.
There are two principal ways of ventilating small greenhouses: one by hinged ventilators, the other by extractor fans. The most important ventilators are those at the apex of the house, usually hinged close to the ridge bar, because hot air rises and so escapes most readily at the top of the house. Usually there are also some hinged ventilators in the sides to allow cool air to enter but these need not be so large, or so numerous as those at the top of the house. Hinged ventilators can be automatically operated by a simple piston and lever device which obtains its power from a heat-sensitive liquid inside a sealed cylinder.
In a small greenhouse, fan extractors are usually fitted in the end panels, as high up as convenient for the same reason that this is where the hottest air will be. It takes quite a powerful fan to give an equivalent rate of atmospheric change to hinged ventilators of good size. As a rule, electric fans are operated by a thermostat which automatically switches them on and off at a predetermined temperature.
Artificial Heating Artificial heating greatly increases the utility of a greenhouse, provided that sufficient heat is available to exclude frost at all times. The method by which the heat is applied is not of prime importance so long as no harmful fumes get into the greenhouse.
Broadly, the possibilities are paraffin, solid fuel, electricity and gas, and many different types of apparatus are manufactured to consume each. In some, the air is heated direct, in others via pipes circulating hot water from a boiler. Water systems tend to be more costly to install and may require more maintenance, but good installations of this type give an excellent distribution of heat, a quality most appreciated in greenhouses of fair size. In small houses there is much to be said for the cheapness and simplicity of direct air heating provided that it can be done without the introduction of harmful fumes.
Some forms of heating, notably electricity and gas, lend themselves to thermostatic control, by which the heat is automatically turned off or on as the temperature of the air in the greenhouse rises above or falls below a predetermined level. This greatly increases the accuracy of control which the gardener has over his artificially imposed climate and also usually results in a con-siderable fuel economy. But thermostats should be carefully sited in the greenhouse, screened from direct sunlight, which may falsify the true air temperature, and placed where the mean rather than the extreme temperatures of the house are registered.
Siting the Greenhouse
Unless there is some overriding reason to the contrary, such as that a greenhouse is to be used exclusively for the cultivation of shade-loving plants, it should be sited in the open where it gets the benefit of sunshine for as many hours of the day as possible. It is easy enough to provide shade when and where it is needed: not so easy to provide extra light when it is lacking.
Fuel and Water Supplies If the greenhouse is to be heated by electricity or mains gas it will be wise to bear in mind the distance these supplies must be brought. If they are coming from the domestic supply it may be a good reason for siting the greenhouse near the dwelling house even though this involves some loss of light.
Water will be needed and it is convenient, though not essential, to have it laid on in the house. If it is so connected it will be possible to use it for automatic systems ofsuch as capillary bench watering and mist but these are labour-saving refinements, not essentials to good cultivation.