Choosing a Greenhouse
Before embarking on the erection of a greenhouse the following highly specific questions should be considered:
1. Is the district concerned well served with natural light, sheltered or exposed, and free from industrial pollution? In the same vein, is the area a very dry one, raising maintenance problems with wooden houses?
2. Does the proposed site receive the maximum amount of sunlight?
3. Is the greenhouse intended for use the whole year round or only for spring/ summer?
4. What plants are to be grown and when — eg pot plants only, or general propagation followed byand late ?
5. To what level is the greenhouse to be heated — fully, partly or not at all? A minimum of 13-16°C all the year round, or merely to provide frost protection regardless of how cold it is out of doors?
6. What facilities are available for heat, with particular reference to the possibility of linking the greenhouse system with that of the dwelling house?
7. Is the greenhouse intended primarily as a room to relax in, or primarily for the growing of plants?
8. What size and shape of greenhouse is contemplated? Is money no object, or is there a strict budget involved? Might it in practice be better to consider a combined shed and greenhouse, or even a porch or conservatory?
9. What constructional materials are considered suitable — wood, treated metal or alloy?
10. Must allowance be made for occasional absences from home, with vents, heating and perhaps watering and feeding being carried out in a semi-automatic manner?
11. Has any thought been given as to whether the greenhouse is to be fixed or mobile to allow a system of?
12. Would a low cost, plastic greenhouse or frame achieve all that is required?
13. Type of glazing system, whether nailed, clip or sealed, the last named being highly desirable.
The level of natural illumination in any district varies not only according to latitudinal placement, but also with nearness to large bulks of water, the presence of industrial pollution and in relation to several other factors. A garden may be situated at the top of a hill and so receive good light, yet be exposed to strong winds, making the maintenance of a high temperature all the year round both difficult and costly. Poor winter light makes the growing of light-demanding early and winter lettuce, for example, extremely difficult in many northern districts, and in industrial areas atmospheric pollution detracts still further from the available light. While deficits in heat levels can be made good relatively easily, deficits in light intensity cannot so easily be remedied.
The gardener receiving poor winter light and often gardening under conditions of extreme exposure and winter cold may be better advised to have a permanently benched base-wall type of greenhouse instead of an all-glass type, which will not only reduce heat loss and keep fuel bills down, but serve better for spring propagation or year-round pot plant growing activities, followed by some later cropping. It is unfortunate that base wall typeare not now so readily available. On the other hand, in an area of good light with no exposure problem, an all-glass greenhouse could be used successfully for a completely unrestricted production programme over the full year.
Desirable temperature levels
The amount of heating required in any greenhouse is usually determined with regard to the minimum winter temperature to be maintained. The following are the four normally accepted levels for greenhouse heating.
This provides frost protection only, keeping the temperatures between 7-10°C in winter. Type of greenhouse is not critical apart from winter light issues, other than that a base wall reduces heat loss.
The use of a separately heated propagating frame in the cool greenhouse is becoming very popular as it allows an intensive propagation programme at very low cost within the cooler atmosphere of the greenhouse.
Intermediate, or moderately warm, greenhouse
In this heat range a minimum temperature of between 13-16°C (55-60°F) is maintained continuously throughout the year. Such a temperature range is suitable for the great majority of, and a wide variety of propagating activities can be carried on in the greenhouse itself (as opposed to within a separate propagating frame). It costs money to keep a greenhouse at these minimum temperatures, yet on the other hand there are great benefits in having a twelve-month gardening calendar. The culture of light-sensitive crops such as early or early lettuce should be avoided in poor light areas, irrespective of the heat level which can be achieved, unless one is prepared to invest in artificial lighting and the operating costs of this.
This rather frightening title refers to a greenhouse for the culture of extremely heat-demanding plants such as some orchids, ferns and other epiphytes, in which the minimum temperature range is 18°C (65°F) or above. Few amateur gardeners think in these terms for very obvious economic reasons. A more localized concentration of heat such as that provided by the larger types of propagating case may be an acceptable compromise on grounds of cost. The well-heated home is now being used more and more for heat-demanding growing activities.
No artificial heat is involved at all, all heat being derived from the sun. The gardener in a very mild situation can derive great value from a completely cold greenhouse; in colder areas it is normally useless over many of the winter months for anything other than hardy shrubs or plants.
The real limitation is, in fact, one of cost: it is perfectly possible to heat what started as a cold greenhouse up to stovehouse level, but it would be very expensive to do so, because, ideally, the design and materials used in a greenhouse intended to be run at stove temperatures would be different from those used in a greenhouse intended to remain unheated. Because of the higher temperature required in a stovehouse, you would tend to go in for double-glazing or polycarbonate sheeting and a house with brick half walls in order to minimize heat losses, whereas the greenhouse you might have purchased originally as a cold house, might well have been of glass-to-ground design which would have a very high heat loss. You would really do better to scrap the old greenhouse and start afresh with one better designed to do the job required.
A compromise may occasionally be reached, though this is really only practicable in fairly large greenhouses. This is a system that is sometimes adopted in the greenhouse of botanic gardens and other horticultural institutions, whereby the greenhouse is divided into two or more sections, each heated to a different temperature level, or bought as such. Alternatively heavy gauge translucent polythene can be used to create a curtain-type division, though this is not entirely satisfactory.
Value for money
The cost of a proposed greenhouse is perhaps the most pertinent question of all. Value for money should always be sought after, and there can be an immense variation between different greenhouses, not so much in price but in respect of serviceability, lack of maintenance and other related matters. More sophisticated types of wooden houses constructed of Western Red Cedar or pressure-treated wood are excellent buys for general greenhouse activities at low cost (particularly if glazing systems avoid the use of putty and nails). Alloy and treated metal houses in their more sophisticated forms are generally more expensive than wooden houses, although there are a lot of highly competitively priced alloy houses now available.
Intensity of use
What has so far been said gives fairly clear guidance concerning the year-round use of a greenhouse. To think in terms of natural illumination only and disregard temperature would be wrong. In the more southerly latitudes a completelycan be reasonably effective in winter, allowing overwintering of pelargoniums, storage of dahlias, and the housing of quite a range of pot plants that are fully hardy, like some hardy , primulas and many half-hardy bedding plants. The situation can be very different in more northerly latitudes where, unless the winter is exceptionally mild, a completely cold greenhouse is virtually useless over the coldest winter months, with the possible exception of a few very mild coastal areas exposed to warm ocean currents, and even then winter dampness can be a real problem. Lining with bubble or plain polythene can reduce heat losses considerably in a heated greenhouse.