When a greenhouse is constructed, the space inside constitutes a special environment possessing its own miniature climate, known as the greenhouse microclimate. The properties of this microclimate are somewhat different from those of the general climate outside.
Temperature and Humidity
The first of these differences is that the temperature within the greenhouse is always higher than that of the air outside. When the sun is shining brightly the difference may be very great indeed, but on clear winter nights it can be as little as 3-5) F°, still a little warmer. The explanation for this is that heat enters the greenhouse by means of radiation from the sun and leaves it by means of radiation from the ground which it covers. The radiant heat from the sun is of short wavelengths and passes readily through glass, while that from the earth is of longer wavelengths and passes much less readily through glass.
Heating System
There is, therefore, a net gain due to the fact that glass behaves rather like a non return valve for radiant heat. This is called the ‘greenhouse effect’ and is a well-known phenomenon in buildings with large windows and in closed motor cars. The need to remove excess heat from greenhouses during periods of bright sunshine led, at a very early stage in their development, to the inclusion of ventilators in their construction.
An obvious difference between the greenhouse micro climate and the general climate is that no rain falls on the soil it covers. If this soil is used for growing plants serious consequences can arise if it is not irrigated by approximately the same amount of water it would have received naturally as rain.
The relative humidity of the air within the greenhouse is usually higher than outside it and this, coupled with its stillness when the vents are closed, provides conditions very favorable for the germination and rapid development of the spores of the fungi causing mildews and rots. Exerting some control over relative humidity (R.H.) is yet another task forced upon the gardener if he is to manage his greenhouse successfully.
Light Transmission
Another way in which the microclimate is different is in respect of light. By greenhouse, and so it is always darker within the house than outside it. In summer, provided it is not shaded by trees or buildings, there is light in abundance and sufficient enters the house to provide for all the needs of the plants.
Growing Lights
During the winter there is insufficient natural light for plants to grow in the open, let alone under glass, so it is obvious that everything possible must be done to allow the maximum amount of light to enter the greenhouse. It would be quite simple, albeit expensive, to provide sufficient heat within a greenhouse during the winter months to make it warm enough for tomatoes, but while they might survive they would certainly not grow satisfactorily, neither would they set and provide ripened fruit. This would be entirely due to insufficient light energy reaching their leaves to enable them to photosynthesise, the process by which plants manufacture sugars and starches which they use for growth and energy production.
If a greenhouse is to be used only from mid-April to mid-October the light problem is greatly reduced. But if it is heated and to be used in the winter months the problem is acute. There are five factors which control light transmission into the house: the shade cast by buildings and trees; the shade cast by opaque parts of the greenhouse such as glazing bars; the design of the house; its orientation; and last but by no means least, the cleanness of the glass.
Diffused and direct light
The ‘doubting Thomases may well say that their greenhouse is obstructed to the south and the glass is not all that clean, yet there is still plenty of light in it. This is perfectly true because they are talking about diffused or reflected light which comes in through the glass from all parts of the sky. It has never been possible to say precisely to what extent diffused light assists the plants to grow.
It certainly does not give the leaves anything like the same amount of light energy as direct light, as can readily be demonstrated by bringing a plant from where it can receive direct unimpeded light into a well-lit room in a house, and watching its deterioration. All the evidence we have confirms beyond doubt that it is direct sunshine which is all-important in making plants grow, and the greenhouse gardener who grows plants under glass must make this the first article of his faith.
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Greenhouse Growers 