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Greenhouses

The ultimate goal of most gardeners is to have a greenhouse, for it extends the possibilities of the garden tremendously. Such a structure can be used for propagation, for growing tender or winter crops and for overwintering plants that cannot be safely left outside. There is a fourth, often unspoken, use and that is to keep the gardener dry and warm in winter.

Most of the horticultural operations that are done in a greenhouse can be achieved 
perfectly satisfactorily in cold frames. Most gardeners, however, prefer to carry them out in the warmth and comfort of the greenhouse, rather than bending over a cold frame in a cold or wet wind.

Electric fan heaters are very efficient.
When equipped with 
thermostats, they
only come on when extra heat is
required. They can also be used to
circulate the air on still, damp days.



Choosing a greenhouse

As with any equipment, the first thing to 
consider are your reasons for making the 
purchase. Why do you want a greenhouse? What are you going to do with it? This is an important stage, because answering these questions will help to determine the size. Cost and the available space will also obviously influence the size, but if possible, make use the prime consideration. Most gardeners, slightly tongue in cheek, will tell you to work out the size and then double it. There is some truth in this old saw, and many, if not most, gardeners wish that they had bought a larger greenhouse than the one they did. So buy larger rather than smaller if you possibly can.

Material

These days the choice is mainly between wood and aluminium, although it is still 
possible to find old0 wood. For most gardeners the choice is simply an aluminium frame, because it is the cheapest style available, but there are other factors to be taken into consideration. For example, wooden greenhouses are far more attractive than aluminium ones. However, although they usually fit more sympathetically into the garden, they are more expensive and the upkeep is more time-consuming. Wooden greenhouses are slightly warmer in winter. It is possible to make your own, working to your own design and dimensions.

Aluminium greenhouses are cheap and easy to maintain. The cheaper ones may, 
however, be rather flimsy, and in exposed positions the sides may flex and the glass fall out! They normally come in standard sizes, but because they are modular, there is a choice of the number of windows and their position. Some companies will build to your specifications, but this is obviously a more expensive option. It is now possible to buy aluminium greenhouses where the frame is painted, which partially disguises the aluminium.

Glass can now be replaced with plastic. Most gardeners prefer the traditional material, but if there are children around it is often more sensible to go for plastic on safety grounds.

This standard straight-sided greenhouse

is made of aluminium, but it has been
painted green, rather than being left
silver, so that it blends in better with the
colours of the garden.

Digging in

The old-fashioned idea of sinking the greenhouse into the ground is a good one as long as you can overcome any drainage problems. Steps lead down to the door, and on to a central aisle, dug out of the soil. The side benches are laid on the natural soil level and the roof springs from a low wall on the ground. The advantage of this system, apart from the fact that it is relatively cheap, is that the soil acts as a vast storage heater. Gardeners using such a greenhouse find that as long as they provide some form of insulation, no heat is required to overwinter tender plants. Traditionally, a wooden framework would have been used for the roof, but aluminium would do just as well.

Shape

The shape of the greenhouse is a matter of personal preference. Traditional styles have vertical sides, but some new ones have sloping sides, which allow in more light – especially useful during winter when the sun is low or if you have trays of seedlings on the floor.

Octagonal greenhouses are suitable for small sites, and many people find them more decorative than the traditional shapes. Because they are almost round, the “aisle” is just a central standing area, thus saving a lot of wasted space. However, the amount of useful space is still quite small.

Lean-to greenhouses can be built against walls, which not only saves space but also makes use of the warmth that is usually found in the wall, especially house walls. These are obviously much cheaper than a full greenhouse, but the amount of useful space within them is limited because the light does not come from all directions and plants can get drawn. Painting the wall white helps because more light is reflected back onto the plants.

The marigolds planted with a row of
tomatoes in this greenhouse will ward
off potential  pests, a technique known
as companion planting.

Ventilation

When you buy a greenhouse, make sure that it has as many opening windows as you can afford because the free passage of air through the structure is of the utmost importance. Stagnant air in a greenhouse is a killer, as all kinds of fungal diseases are likely to develop very quickly. Openings can either be covered with conventional windows or with louvres. If you are away during daylight hours in summer, the time when windows need to be opened on hot days, automatic openers can be used. The mech-anism opens the windows as soon as a specified pre-set temperature is reached. Having a door at each end helps on larger houses. In winter, windows should be left open as much as possible, and, when it is necessary to close them, use a fan to keep the air circulating.

Heating

There are various methods of heating a greenhouse, but one of the most versatile is with electricity. Although the cost per unit of heat may be greater, the control of its output through the use of thermostats is such that no heat (or money) is wasted, because the appliance comes on only when the temperature drops below a certain point. Thermostatically controlled gas heaters are also now becoming available. Paraffin heaters are cheap, but they need to be regularly filled and maintained and they produce large amounts of water vapour, which encourages disease unless the greenhouse is ventilated.

Heating bills can be reduced by insula-ting the greenhouse. Double glazing is the ultimate but is expensive. A cheaper alter-native is to line the house with sheets of clear polythene (plastic), preferably containing air bubbles. If you have only a few plants that need protecting, it is cheaper to close off one end of the greenhouse with polythene and heat just this area. If the number of plants is small enough, a heated propagator or a cloche over a heated bench may be sufficient.

An unusually shaped greenhouse with
a steeply pitched roof that not only
looks different from conventional
greenhouses, but has the added
advantage that the steep sides absorb
the low winter sun more easily. 

Shading

Greenhouses need to be as light as possible, especially during the winter, but at the same time bright sunshine should be kept out as this will raise the temperature too much. It is possible to buy shade netting, which can be draped over the outside or clipped to the inside of the glass. This is easy to remove in overcast periods. An opaque wash applied to the glass reduces the effect of the sun considerably, but it is time-consuming to keep removing it during dull weather, so it is usually left in place from early summer to mid-autumn. There is one form of wash that becomes transparent when it rains, thus letting in more light.

Fittings

The full height of the greenhouse is needed for tomatoes and cucumbers, which can be grown in growing bags on the floor. Benching or staging is a useful addition, at least down one side, and can be made of wood or longer-lasting aluminium.

If the staging has raised sides it can be filled with sand. This is useful for sinking pots in to help keep them warm and moist. Heating cables can also be used to keep the bench warm, and building a polythene (plastic) or glass cabinet or lid on top will turn it into an effective propagating bench.

Polytunnels

Polythene (plastic) tunnels are a cheap alternative to greenhouses. They are ideal for growing winter and early spring vegetables and for housing and propagating plants until they are ready to plant out. They are, however, rather ugly and can get very cold, and the polythene will need replacing every three years or so.