Greenhouses are available in all shapes and sizes, and take
forms that vary from the conservatory to an enclosed glass porch around the front door. All are very useful for the hobby
gardener and can accommodate a surprisingly large number of
plants of all kinds.

Modern greenhouses basically come in four forms; the lean-to, in
which one wall is formed by the home or other substantial
building; the span roof type, which is built on a dwarf wall and
has a pitched roof; the Dutch light kind, formerly constructed
of Dutch lights but in the modern form a glass-to-ground
greenhouse, and the hexagonal type that is, broadly speaking,
dome-shaped but with angular sides. All have their adherents,
but it is generally conceded that the span roof type of
greenhouse proves to be the most versatile when a wide range of
plants are to be grown.

Greenhouses can be constructed from many different materials. Polyethylene structures are cheap to purchase initially, but need re-covering every two years and in winter will often suffer wind or snow damage. Those made from corrugated acrylic sheet are little better, cracking and discoloring with the weather, although polycarbonate materials are more durable and can be recommended. 

Glass is obviously the most satisfactory option, but the best material for the framework is more difficult to decide. Cedar and teak that are kept well oiled are obviously ideal, but very expensive, while ordinary softwood painted white looks good but has a very limited life. 

Aluminum would seem to be the best answer as it does not rust, but with some greenhouses the bolts and screws holding the structure together are made from steel and are liable to corrode. So inspect any intended purchase of this kind very carefully, and select an all aluminum model.

Ideally staging should be provided at waist height down either side of the greenhouse and a shelf near the ridge is very useful if this can be kept above head height. Ventilation must be adequate and most gardeners agree that both side and ridge ventilators are essential for proper airing during the summer months. A good wide door is necessary for easy management. It should be wide enough to take a wheel-barrow without skinning your fingers and with no troublesome step or ramp.

Heating is desirable, but may be prohibitively expensive. If you do decide to heat your greenhouse then purchase an electrical fan heater. Not only does this provide warmth, but it also circulates the air on damp days in the fall. In the summer the fan can be switched to the cool air position to reduce the high temperatures and provide air circulation. Propane gas and kerosene heaters, although very reliable, are not so desirable as they produce a damp heat, and by virtue of their mode of operation do not readily circulate the air.

If you are able to afford to heat your greenhouse, it also makes sense to insulate it to minimize heat loss. There are many methods of doing this, but the clear plastic bubble insulation material now available is simple to fix and rarely causes any problems with condensation. Clear polyethylene has always been considered to be the most suitable insulation, causing minimal reduction in light and trapping a barrier of air. However, it has always been associated with condensation, a build up of light-reducing algae, and a fragility which is not so evident in modern clear bubble insulation.

Whatever insulation you choose, it is important when fastening it to the internal structure of the greenhouse that provision is made for the opening of ventilators. Insulation may well still be useful at night during spring, but on sunny days the temperature will soar and full ventilation may be necessary. If you cannot afford to heat your greenhouse, then consider investing in either a small propagator or a heated bench. Both are very useful and give the greenhouse much greater versatility.

Correct sitting of the greenhouse is vital if plants are to
develop properly. Choose an open part of the garden in full sun. It is a simple matter to shade a modern small greenhouse if the sun becomes too bright. The uncontrollable shade of a tree or building on the other hand can lead to very poor and disappointing plant growth.

About the author:
Philip Swindells has over 40 years gardening experience. A
former botanical garden curator and an international
horticultural consultant, he has worked extensively in the UK,
North America, the Middle East and Australia. The Author of more
than 50 gardening books, he has been awarded a Quill and Trowel
Award by the Garden Writers' Association of America. He is also
a former UK Garden Writer of the Year.