Shading when necessary is of the utmost importance, so greenhouse blinds play a very important role in maintaining optimal temperatures. Many beginners in greenhouse gardening fail dismally merely because they do not appreciate this fact. In winter the greenhouse benefits from all the sun it can get, but in summer the sun-trap effect can be too much of a good thing. An unshaded greenhouse in a sunny position in summer can become so hot that everything inside is ruined in a few hours -even tropical plants! Nearly all greenhouses will require shading at some time or other.
The purpose of shading is to protect the plants from intense light, especially the shade lovers, and to keep the greenhouse cool. To keep down temperature, the suns rays must be stopped before they get inside the greenhouse. This means some form of exterior blind or a preparation applied to the glass that will reflect the heat rays away before they get through. For many years either blinds were fitted to the greenhouse roof or a shading paint was applied to the glass when necessary. The former is somewhat expensive, the blinds usually having to be made specially to fit any particular greenhouse. However, blinds have the advantage that the shade can be removed easily with changes in weather. Blinds are available made from timber slats or bamboo, and possibly aluminium will soon appear on the market.
Types of Greenhouse Blinds
Blinds made from solid materials should of course let some light through hence the slats. Owing to the change in the suns position, the light passing through on a sunny day will gradually move over the plants so that exposure to the sun is not prolonged. Blinds made from fabric or plastic and in the sheet form should be semi-transparent, and so should shading paints applied to the glass.
Blinds are best fitted on rails so that they do not come into close contact with the roof. This is so that heat absorbed from the sun is not directly transferred to the roof. A few inches of air space between the blinds and the roof is desirable to allow dissipation of the heat absorbed to the air.
It should be appreciated that blinds fitted to the interior of the greenhouse may protect the plants from injury caused by intense sunlight but will not keep down temperature so well.
This will be understood by reference to the section on How the greenhouse works, on. Roller blinds made from various materials are available for interior fitting where this method is thought more convenient.
For many years green shading materials were applied to glass. This is in fact quite wrong and due to complete misunderstanding of the principles involved in shading. It is of no advantage to have any shading material, including blinds, coloured green.
The green pigment in plants actually absorbs energy for photosynthesis. It is cool in a woodland glade because this energy is being taken up by the foliage and what is not used is dissipated to the air. When a green substance is applied to glass the situation is absolutely different. Energy will be absorbed and transferred directly to the glass. A green-shaded greenhouse can actually get hotter than an unshaded one.
Research in this country and in Holland has proved that the best colour for greenhouse blinds is white. In fact this has been known to people in tropical countries for centuries who avoid dark-coloured clothing and often whitewash their homes. In sunlight a dark surface becomes much hotter than a light-coloured one. Shading paints applied to the glass should therefore be white. In the past shading paints had to be applied and left for most of the summer owing to the difficulty of removal. They were also often troublesome to prepare and apply.