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23 November 2012

Choosing a Greenhouse

If you are a keen gardener you will probably want to raise your own plants from seed, and an essential requirement will be a greenhouse. They are invaluable for creating the right environment for raising tender plants and seedlings and for growing tomatoes etc. However, there are a few things to consider before selecting your greenhouse.

1) Position. Ideally, greenhouses should be sited where they can receive uninterrupted sun throughout the day. Protection from cold northerly or easterly winds, which can keep temperatures low in spring and slow the growth of seedlings and young plants, is useful.

2) Size. You may want the largest greenhouse you can fit in your garden or may be happy with a 6ft x 6ft or 6ft x 8ft. Mine is of the latter size and tends to get full to overflowing in early summer. Many people won't have the room for anything much bigger, or will have to allow for other garden requirements which will reduce the available space. Even a lean-to greenhouse against a brick wall, such as a garage, will be worth having.

3) Construction. Aluminium is the most common choice for greenhouses these days, and can be obtained in a coloured finish if required, though this will increase the cost. Aluminium doesn't need any maintenance, and the thin frame means that little light is lost. A timber-framed greenhouse can look more attractive and may be better suited to some garden styles. However the wooden frame will need periodic treatment unless you specify a lower maintenance cedar-wood timber frame, which will increase thew cost. As they are thicker than aluminium frames, Wooden frames tend to cast excessive shade inside the greenhouse. I bought my aluminium greenhouse from B & Q about nine years ago and it has served me well.

4) Orientation. This may be governed by the space you have available but an east-west orientation will slightly extend light levels during winter. A north-south orientation is better for summer crops such as tomatoes, both sides receiving several hours’ sun from the east and the west. Decide when you plan to use your greenhouse most, and orient it accordingly if you can, though most small greenhouses are relatively square so this isn't of major importance.

5) Height. The taller a glasshouse is at the eaves, the better the light transmission and therefore the wider the range of plants that may be grown. The eaves should be at least 1.5m (5ft) tall, with the ridge at least 60cm (2ft) higher to allow a door that gives easy access. Glazing to ground level gives the greatest flexibility, and most aluminium-framed greenhouses will be of this type, but half-walling, using masonry or timber will reduce heat loss. Dome-shaped greenhouses are also available but are relatively uncommon compared to the other two types.

6) Ventilation. A greenhouse needs sufficient ventilation, with roof vents being the most useful. They should ideally be on both sides of the ridge and equivalent to 15-20 percent of the floor area. Automatic openers that open or close in response to greenhouse temperatures are useful but you will also need to open the door during hot weather to aid air flow. Side vents are no substitute for roof ventilation, and while louvred vents allow regulation of air flow, they are hard to draught-proof in winter.

7) Type of Glazing. Glass is the best glazing material as it lets 90 percent of light through, does not degrade in sunlight and, unlike plastic materials, reflects heat radiated from within the glasshouse back into the structure instead of being lost. Also, if a pane gets broken, it can easily be replaced. Toughened glass should be used for doors and anywhere where people might fall onto the glass. The alternative is polycarbonate sheeting, which is a twin-walled plastic glazing that has the advantages of being resistant to breakage, lightweight, durable in adverse weather, and good at retaining heat. The disadvantage is that clear polycarbonate only transmits 83 percent of the light – much less than glass, and this makes it less attractive for growing seedlings, cuttings and plants that require high light levels.

I hope this article has helped in the selection of your greenhouse, and that you have many happy hours of plant-raising to look forward to in it.

An article by UK Gardening Directory
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