Nick Morgan, of the Royal Horticultural Society, Wisley, talks to Iris Jardine about greenhouses.
Buying a greenhouse should never be an impulse buy - there are too many elements that need to be considered - size, location, ventilation and heating.
The range of different greenhouses varies from as little as £180 for a small six by eight foot at a DIY store to over £18,000 for something custom-built. The adage that the more you pay the better the product is true in this case, and even a step-up in size can have significant implications. The rule of thumb is to buy the largest one you can afford, providing you have the space.
When buying a small one, it is important to be sure that there is enough room for access, bearing in mind that you will be carrying trays of plants. Another consideration is head room, particularly if anyone using it is tall.
The main purpose of having a greenhouse is to protect your plants from the elements, namely the cold, wind and wet. It is therefore important that it is well-made so that it does not lose any heat.
Allowing light in is the ultimate priority, so the larger the pane of glass the more light can be let in. Positioning is also a major factor, although there might only be one place to put it. While maximising on the light, you need to make sure it is sheltered from prevailing winds. Also, shadow takes away light, so look at trees, buildings, hedges and fences, as without it plants fail to grow, particularly in winter. Maintenance is important, and when autumn sets in, it is necessary to make sure you clean all the surfaces - in particular the glazing bars - with hot, soapy water. Pressure washers are a good buy, as they can get into all the crevices.
Coupled with good lighting is the need for good ventilation, bearing in mind that light also brings heat. Understanding what you require, and installing good ventilation, makes management easier.
As a guide, one-fifth of the floor area should equal the minimum ventilation area required in the roof. On top of this, you also need aeration in the sides. There are lots of extras available, such as louver-vents to combat any problems you may have.
Summer months are particularly hazardous when there can be too much heat, which can stress the plants. This can be resolved with blinds which can be adjusted as required.
A cheaper, but more time-consuming way is to paint your glasshouse with special shading, which has to be taken off at the end of the summer or before, depending on the climate.
On bright summers days, raising the humidity by spraying water over the internal surfaces will help reduce the glasshouse temperature and cut water loss from the plants.
Think about the services that you will need, such as water and electricity. The further away from the source - presumably the home - you put the construction, the more expensive it is to install.
Having a greenhouse can give you the benefit of freshly grown vegetables all the year round as well as flowers and plants to keep your garden colourful.
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