Greenhouse Types are classified according to the temperatures maintained, as follows:
- Unheated greenhouse No heating at any time.
- Cold house Not allowed to fall below freezing.
- Cool house Minimum about 40 / 45 °F (4/7°C)
- Warm house Minimum about 55`°F (13°C) Stove house (sometimes Minimum about 65 / 75°F called hothouse) (18/24°C).
These classifications are not too rigid. Sometimes the 40 / 45°F house is described as a cold greenhouse. The 55°F house called a warm house above may be described as an intermediate house.
The most popular type of greenhouse is the type we describe here as the cool greenhouse. With a winter minimum of about 40°F (4°C) you can grow a vast range of plants from all over the world. Most of the plants from temperate zones will thrive, and many sub-tropical and tropical plants can be kept alive over winter, although they may not actually grow or prove decorative or useful then. The cool house is certainly the one to aim for if you are planning a general purpose greenhouse for growing all the favourite pot plants, and as an aid to the garden in raising bedding plants and for garden propagation. For this reason this site is mainly concerned with cool house work, but plants needing only the protection of glass, and no artificial heat at all, are also dealt with, together with plants demanding only frost protection. At this point it is worth drawing some extra attention to the possibilities of the unheated and the cold greenhouse.
THE UNHEATED GREENHOUSE
The purpose of this greenhouse is to give protection from the excesses of nature only wind, cold, and rain. It can be used to grow better anything that normally grows outdoors in this country. However, it should be realised that hardy plants often object to being coddled with extra warmth and humidity and perhaps poor ventilation. The function of the unheated greenhouse must be to give weather protection only. It is ideal for all those plants that flower early and may have blooms prone to weather damage. Some examples are camellias, alpines, some of the more tender small shrubs, innumerable spring flowering bulbs, lilies, some succulents and cacti, ferns, fuchsias, hydrangeas, carnations, chrysanthemums, and many garden annuals and biennials grown in pots. In the last-mentioned case, a remarkable standard of perfection can be achieved often so high that common annuals become something quite exotic! It should be realised that an unheated greenhouse can always be provided with warmth at any time if convenient or necessary -say in early spring for starting plants into growth or for seed sowing and so forth. It can also be employed for giving winter protection from frost or excessive cold to roots of the more tender garden plants grown in terrace pots, small tubs, and other ornamental containers. Sometimes roots can also be taken up from garden borders and stored in an unheated greenhouse whilst they are dormant over winter.
THE COLD GREENHOUSE (not allowed to fall below freezing)
This type can be used for most of the purposes described for the unheated greenhouse, but often it is best to make use of the warmth for getting earlier flowers or crops and for protecting plants that are invariably killed or severely damaged by frost in the open. In those parts of the country where the winter is severe, all those plants that can be seen thriving outdoors only in the west and south will be very happy. The cold greenhouse is the best for growing many bulbs to perfection, and most of the spring flowering kinds will benefit by being gently forced, and flower much earlier. A very great range of plants that are mistakenly thought to need the cool house can also be grown quite easily in the lower temperature of the cold greenhouse.
These include such popular subjects as cinerarias, salpiglossis, calceolarias, and believe it or not the exotic bird of paradise flower, Strelitzia regina (usually classed as a warm house plant!). There is much scope for further experiment in this respect.
YOU CAN HAVE MORE THAN ONE TYPE OF GREENHOUSE
Although it is usually best to begin with one type of greenhouse, usually unheated, cold, or cool, there is no reason why you should not indulge in, say, all three forms of greenhouse gardening at the same time. The best way to do this is to have a greenhouse divided into compartments, each being maintained to give the right conditions for the plants. Of course, you can always add further greenhouses, separately sited, to your garden if this is preferred. Those plants preferring a fair warmth can be economically grown in warmed frames inside a cold or cool greenhouse, provided they are reasonably low growing and compact.
There are very many ways in which you can use a greenhouse. Before choosing a structure it is absolutely vital to be clear in your mind about what you want it for and what you propose to do. If you are taking up greenhouse gardening for the first time, or are still vague about the scope of the subject, it would be advisable to glance through the rest of the site and return later to the matter of choosing a greenhouse. The reason for this is that the market now abounds with different designs, shapes, sizes, and constructional materials, and one might suit your purpose better than another.
CHOOSING PLANTS TO SUIT A GREENHOUSE AND VICE VERSA
A common mistake made by beginners in greenhouse gardening is that they try to grow too many different kinds of plant under the same conditions. Thought must be given to how much warmth, light or shade, or humidity and ventilation, the plants require, and every attempt made to provide the best environment. Too often, for example, cacti (which like light and a dry atmosphere) are put with shade-loving plants preferring moist air; or you may see fairly hardy plants, needing only frost protection, in a cool greenhouse where the temperature is unnecessarily high for them. Even more absurd examples are plants that may eventually reach a considerable height given a house with a very low roof, and large, high greenhouses used for nothing but low salad crops or the like, which could be more economically grown in frames.
With care and understanding a surprisingly wide range of different plants can be grown in the same greenhouse if proper consideration is given to where they are placed and how they are treated. However, some greenhouse designs may be a better choice for plant collections than others. Also, if you are only interested in growing, for example, grapes, orchids, carnations, or alpine plants, you can get structures specially designed for them.
BASIC TYPES OF GREENHOUSE STRUCTURE
Today there are three types likely to be encountered by the average home gardener. These are the partially glazed (having a dwarf wall of timber, brick, concrete, or some other material with the framework on top), the totally glazed or glass to ground, and the lean-to. The lean-to can be with a dwarf wall or it can be glass to ground. At one time a sunken type, called a pit by professional gardeners, was often seen.
This can be used for purposes similar to those suited to a dwarf wall type. However, the pit is a framework over an excavation in the ground, and this means there is much less warmth loss ` when it is artificially heated and temperatures tend to be more steady. For this reason it was popular for propagation and plants demanding a fair warmth. It is not ideal for the small home garden. It is not easy to install and requires a fair building proficiency, drainage can be a problem on some sites, the soil dug out has to be put somewhere, and it may take a good deal of time and effort to build and erect. Even so, provided the excavation is well constructed, it is still possible to get a framework made to lit over, assuming you are prepared to meet the extra expense of the special fabrication this will entail the door end will be below the ground level and entry will have to be made via a flight of steps.
The partially glazed greenhouse
This type is sometimes called a plant house. It is generally fitted with staging and used for growing collections of pot plants, thus most work is done at waist level. This may be important for the not-so-young or people with some infirmity.
The area under the staging usually receives less light. This can be taken advantage of in the growing of the many shade-loving plants suitable for the greenhouse. It should not be used for storing rubbish. It can also be used for blanching.
Partially glazed greenhouses may have brick, concrete, or timber bases. Sometimes bases of asbestos or similar compositions are encountered. These do not usually have the same degree of heat retention as a more substantial base wall, and one advantage of the partially glazed house is that heat losses are less than when the glass is taken to ground level. However, where warmth is concerned much depends on the site. On an open sunny site, any obstruction to the entry of the suns radiation means a loss of free heat. This could be a disadvantage in winter when the sun can shoot up the temperature under glass even when it is well below freezing outside on clear days. Greenhouses with base walls are not ideal for plants that have to be grown initially from ground level or for tall plants that will need the entire height. Again this is because the lower regions may be considerably shaded by the side walls. Plenty of light is always worth having–you can always shade if necessary.
The totally glazed or glass-to-ground greenhouse This type is probably the most versatile of all and the best choice if a general purpose structure is required. It can be fitted with staging or not, as preferred. If staging is fitted there is usually sufficient light underneath to keep many plants happy.
When shelving is fitted too, it is surprising how many plants can be well grown because there is usually good overall penetration of light. As already stated, it is easy enough to provide shade when necessary. A glass-to-ground house could. If needed, be used in the same way as a partially glazed house if the lower panes were either filled in with timber or plastic or the glass removed and replaced with similar material. However, such treatment might well spoil the structure or its appearance. A glass-to-ground house is excellent for all tall plants and plants like tomatoes and chrysanthemums. Climbers can be accommodated, and there is plenty of scope for baskets and plants in hanging containers. Sometimes it is convenient to have staging only on one side, reserving the other for plants grown from ground level 74). A possible disadvantage is that when there is glass to ground, and a path alongside the greenhouse, there is the risk of glass breakage through being accidentally kicked or being struck with wheelbarrows or the like.
The totally glazed greenhouse is also a good choice for many plants or crops to be grown on a semi-commercial scale or in quantity such as winter salad crops and cut flowers. Although some of these can be just as well grown in frames which are more economical to heat, where there is much personal attention and cultivation needed it may be more convenient and pleasant to work inside during the winter.
The lean-to greenhouse
The most popular form of the lean-to is the conservatory or garden room and home extension built against the dwelling and entered via a communicating door. A lean-to can also be built against a garden Wall or against some garden building or garage.
It is best for a lean-to to face south since there is then more scope because of the wider range of plants that can be grown with plenty of light available. However, even a north-facing lean-to can be useful for many shade-loving plants. In fact, numerous favourite greenhouse pot plants enjoy the shade in such circumstances, and a north-facing lean-to can become quite a successful conservatory if the right selection of plants is made.
Lean-to structures can of course have base walls or be totally glazed according to preference. Most of the garden room type lean-to buildings have base walls to give some privacy and reduce heat loss in winter. It may be of interest to know that an all-glass lean-to is of no use as a sun room if you want it for sun bathing: the rays that give a tan are absorbed by the glass and converted to warmth!
A lean-to is very economical to heat. Often it will derive sufficient warmth from a dwelling to keep it frost free. A leanto erected against a sunny garden wall may hold considerable warmth overnight. The wall stores the heat it receives during the day and radiates it at night.
For vines, climbers, wall shrubs, and many fruits, the lean-to makes a splendid home. The plants can be trained against the rear wall or up into the roof. It is also useful when rather high temperatures are to be maintained the year round.