Choosing a Greenhouse

For enthusiastic gardeners, a greenhouse is an extension of the garden, enabling you to enlarge your range of plants. Within its protected environment the length of the growing season is longer and, if heated, tender plants and others at sensitive stages of their life cycle can be nurtured.

Size and Style

Greenhouses vary considerably, from modest lean-to and freestanding models to fully customized and palatial Victorian styles. Your choice will be limited only by price and by the size of your garden, but it is wise to go for the biggest you can afford and fit in. A useful minimum size worth considering is 1.8 m (6 ft) wide x 2.5 m (8 ft) long, but smaller models can still be a boon in restricted gardens. Bear in mind, however, that a central pathway takes up at least 60 cm (2 ft), whatever the size of greenhouse.

Greenhouses come with vertical walls or with sloping or curved sides, allowing in more light, which is of critical importance for winter crops. A lean-to model is a popular option for a sunny house or garage wall, especially in small gardens. It has the advantage of being warmer than a free-standing model as residual heat is stored in the wall, which should ideally be painted white to improve light levels inside. There are also hexagonal and other multi-sided greenhouses designed for small spaces.

Materials

A greenhouse should let in as much natural light as possible to benefit the plants inside. However, the glass or glazing panels must be supported by a strong structure, which can vary in thickness, depending on the material.

Metal Frames

Aluminum greenhouses are virtually maintenance free as they are almost totally resistant to corrosion. The surface does become pitted and covered by a whitish deposit in the early years, but thereafter it remains almost totally unaffected by climate. Painted aluminum is an attractive, though slightly more expensive option. The glazing bars are slim so there is less obstruction to light than in wooden models. Various systems have been developed to hold the glass, including a continuous glazing-bar cap which traps the glass in position, and narrow rubber strips and spring clips that secure it. All are designed to give a complete seal against drafts and drips. Most aluminum greenhouses come with a galvanized steel base for extra strength, wooden frames The longest-lasting greenhouses are constructed from hardwoods, which are highly rot-resistant and very strong, so glazing bars can be quite thin. They are, however, very expensive, and their use has declined in recent years.

Softwoods are widely used, but although they have great strength they do need constant maintenance, without which they quickly rot. Not only must they come treated with preservative, but this must be re-applied regularly for extra protection. Western red cedar is popular because it is somewhat more rot-resistant, but as it lacks the strength of other woods, models have thicker glazing bars and a more solid overall construction. Many modern wooden greenhouses have grooves cut in the glazing bars so you can slide and clip the glass panes in position, rather than having to fix them with nails and putty.

Glazing

For centuries glass has been the traditional material for cladding greenhouses. It admits more light and retains heat for longer than any other material. Horticultural glass is 3 mm (1/8 in) thick and quite heavy; each 60 x 60 cm (2 x 2 ft) sheet of glass weighs about 3.5 kg (8 lb). Toughened glass is available at extra cost. You can check the quality and light transmission of a sheet of glass by looking at it edge on: the greener the color, the poorer the light transmission.

In recent years rigid plastics have increased in popularity as an alternative to glass. They do more or less the same job, but can be curved. Corrugated or flat glass-reinforced polyesters are the best substitute for glass, with 90 per cent or more light transmission. However, they are more expensive than glass and attract atmospheric dirt because of the static electricity they build up. Polycarbonate is cheaper, with lower light transmission but improved heat retention if twin or triple-walled.