Choosing a Greenhouse
Greenhouses come in all shapes and sizes — and today no garden need be without one, however small, for there are many simple and easy-to-erect models as well as more elaborate and decorative ones.
The acquisition of a greenhouse opens up an exciting new area of gardening, where you don’t have to worry about extremes of weather or damage from cold, wind or excessive rain. You can regulate the amount of water your plants receive and, because fertilizers are not washed away by rain, you can feed them correctly.
Temperature and ventilation can be adjusted and, by applying shading, even the light may be altered to suit plant preferences. Pests and diseases are easier to combat and the plants are protected from damage by birds and small animals.
Most greenhouse styles offer the choice of a glass-to-ground or a base-wall design. The glass-to-ground type allows very efficient use of space for growing. When staging is fitted, there is enough light beneath for the accommodation of a varied selection of plants. The all-glass house is also ideal for tall subjects like tomatoes and chrysanthemums. By arranging your space carefully you can grow many different kinds of plants together. For example, you can grow tomatoes along the south side, followed by chrysanthemums — putting the pots on the floor. Your staging can then run along the north side, providing a surface on which to grow a variety of pot plants, with some space below for plants that like a certain amount of shade (as do many house plants). On the end wall you can train climbers bearing either beautiful flowers, or edible crops like cucumbers.
But for some purposes a greenhouse with a base wall of brick, concrete or timber boards (usually called a ‘plant house’) is preferable. Plants liking deep shade can be grown below the staging. This design is more economical to heat artificially and therefore is preferable when the greenhouse does not get much sun. It is also a good choice where high temperatures have to be maintained for propagation or for tropical plants (many of which do not demand much light). Greenhouses with a base wall at one side and glass-to-ground at the other are also available. These should be oriented east-to-west with the base to the north.
Before buying your greenhouse, make sure that you are not infringing any rules or regulations by erecting it in your garden. If you are a tenant you should seek your landlord’s permission; should you move, you can take the greenhouse with you. If you are a freeholder and wish to have a permanent structure (like a lean-to against the house wall) you will almost certainly requirefrom your local council.
You must also do some advance thinking about the foundations of the greenhouse. You can lay old railway sleepers or concrete footings, or build a low, cemented brick wall on which the greenhouse can be free-standing or screwed into position. Do have a path of concrete, brick or paving stones down the centre of the house, for dry feet and clean working.
The best positions
Freestanding, rectangular greenhouses get most benefit from the sun if you orientate them east-to-west, with the long sides facing north and south. The ‘high south wall’ greenhouse must, in fact, lie this way to catch as much winter light and sun as possible. Staging, if used, should run along the north side.
With a lean-to, you may have no choice in the siting, but what you grow in it will depend on which way it faces. An east-or west-facing lean-to is usually fairly versatile since it gets some sun and some shade. If north-facing it will be very shady and is best devoted to pot plants (such as cinerareas, cyclamen, primulas and calceolarias) and house plants for permanent decoration. A south-facing lean-to can become very hot in summer and, unless you want to install shading, you should choose sun-loving plants like cacti and succulents.
Glass or plastic?
Glass has the unique property of capturing and retaining solar warmth. It also holds artificial warmth better. Polythene has a limited life and for long-term use a rigid plastic (like Novolux) is the wisest choice. Plastic surfaces are easily scratched by windblown dust and grit, causing dirt to become ingrained and a loss of transparency over a period of years. Plastic also becomes brittle with weathering and may disintegrate.
However, plastic is advisable if the site is likely to be the target for children’s games or hooligans out to break glass, or where quick erection or portability is desirable, or where temporary weather protection is all that’s necessary. Moulded fibreglass greenhouses are also available, but tend to be expensive and the fittings have to be free-standing. Aluminium frames Aluminium alloy (often white-coated) is now tending to replace timber for the frame as it has many advantages. It is lightweight yet very strong, and prefabricated structures are easily bolted together. Sophisticated glazing, using plastic cushioning strips and clips instead of messy putty, means that the greenhouse can easily be taken apart for moving. There is no fear of rot, warp or trouble from wood-boring in sects, or need for maintenance, like painting or treating timber.
These may look better in a period-style garden. Select one of the more weather-resistant timbers such as cedar, teak or oak, but remember that all timber needs painting or treating with a wood restorer or preservative from time to time.
If your greenhouse receives plenty of sun you will need shading in the summer.
Slatted blinds, run on rails over the exterior of the roof, are efficient but costly. They also have to be made to fit. Interior blinds are far less efficient in reducing temperature, since the sun’s heat-producing rays have already entered the greenhouse, though they do help to prevent direct scorch.
The simplest and cheapest effective method of shading here is with an electrostatic shading paint that is not washed off by rain but can easily be removed by wiping with a dry duster. You apply and remove it like a blind.
All year round ventilation
Ventilators are usually fitted in the roof and sometimes in the sides as well, reducing excess heat in summer and controlling humidity at all times.
Types of heating
It makes sense to heat your greenhouse, if only to keep out frost, since this greatly widens its usefulness. Your heating need not be costly if you don’t waste it.
Both oil and paraffin are easy to store and portable heaters are particularly valuable as supplementary heating in periods of extreme cold or during power cuts. Use paraffin heaters that are specially designed for greenhouses; the blue-flame design is best.
Electricity is trouble-free and gives accurate temperature control, but it can be expensive if used wastefully. Fan heating is very effective providing the fan and heat output are controlled together (preferably with a separate, rod-type thermostat); avoid a heater with a continuously-running fan. Ventilation becomes almost unnecessary with this system, and if you line your greenhouse with polythene to form a kind of double glazing you can cut fuel consumption by up to half.
Convector heaters and electrically-heated pipes are also efficient when used together with an accurate thermostat. NaturalThere are special greenhouse heaters using natural gas, with good thermostatic control. They can also be adapted to work from bottle gas, though this makes them more expensive to run.
Natural gas, oil and paraffin, when burned, all produce carbon dioxide and water vapour. The carbon dioxide is beneficial to plants, but the water vapour can be a nuisance in winter when you will do better to keep the greenhouse dry. You will need some ventilation to keep down humidity and supply air for the fuel to burn, but this cold air does mean that some of the heat is lost.
Other types of heating Solid fuel (with the heat distributed by hot water pipes) and oil-fired boilers are still relatively cheap methods of heating. Hot water pipes, linked to a boiler, maintain high temperatures but are costly to install. Heated propagators You will need some form of heated propagator so that you can germinate seeds without heating the whole greenhouse to a high temperature. Electrically-heated models are simple and cheap to run.
There is a wide choice of equipment here. The water can be fed to the plants by overhead sprays, trickle-feed pipelines, by capillary sand benches or capillary matting. In the case of capillary watering, the sand or matting under the pots is kept constantly moist by whatever automatic system is installed.
A paraffin lamp will give you enough light in the evenings for most jobs, but if the greenhouse has ansupply you can install either a lamp-holder and lamp bulb or a fluorescent tube.