Today gardeners are fortunate in being able to choose from a wide range of greenhouse shapes and materials of construction. It is wise to choose the largest you can afford or find a place for in the garden. Alternatively, select one that can be extended any time more space is required.

Both frames and greenhouses are usually now made from weather resistant timbers, such as cedar, galvanized steel, or aluminum alloy. They are easy to erect in a short time with screws or nuts and bolts, and usually without professional assistance. However, when a greenhouse is erected on a base wall of brick, it is better to obtain the services of a skilled bricklayer.

It is wise to take aluminum alloy first into consideration. It can also be obtained with white or green plastic coating. Aluminum is virtually everlasting and needs no painting. It will not warp, corrode, rot, or become attacked by wood-boring insects. It is lightweight, and most structures are easily taken down if it is desired at any time to change your site or home.

cold frame greenhouse

Galvanized steel is extremely strong, but it does need painting from time to time. If the zinc covering is damaged rust may set in. All metal frames lend themselves to simple glazing using clips instead of putty. In some cases metal frames may be thought too harsh for a garden.

The mellow appearance of timber may be thought to blend better. The most popular timber is red cedar which is attractive and relatively inexpensive as well as having a good weather resistance. Frames are usually square or rectangular, but types with a good slope to the ‘light’ (the lid that s raised) should be chosen. This is to ensure run-off of condensation.

Greenhouses are available with six or more sides to give a circular shape, but the square or rectangular and ‘barn’ type shapes are still the best for practical purposes. However, hexagonal or other roundish shapes often make pleasingly attractive greenhouses for ornamental plants and to use as garden features or on a patio.

Some greenhouses have the glass set at an angle so as to give sloping sides. This is to let the maximum sunlight in during the winter months when the sun is low in the sky. If the glass is angled there is less thickness to absorb the sun’s rays, since glass is not completely transparent.

Even so, very sloped sides can be a nuisance when working and make the fitting of staging awkward. For most home greenhouses a slight slope, or vertical sides, will be found more convenient.

Greenhouses with the glass to ground level, or almost so, are the most versatile. If fitted with staging there will be sufficient light underneath to allow some useful growing. Tall plants can be grown from ground level without suffering from lack of light in their early stages.

Greenhouses with a low base wall or boarded base (usually called plant houses’) may keep in warmth better, but the base obstructs sunlight and there may consequently be a loss of free solar warmth. Which is most suited is a matter of individual preference.

The plastics are now used extensively for both frames and greenhouses, but their advantages and disadvantages should be understood before choosing. Plastics have a limited life and being soft, they do not weather so well as glass.

Glass also has the unique property of trapping solar warmth and retaining it for some time, as well as holding artificial warmth better. On the other hand, the lightweight and unbreakable nature of plastic makes it ideal for where there are children or a frame or greenhouse needs to be moved about on a site, such as for vegetable growing. The most inexpensive plastic house uses polythene.

This must be a special grade called UVI, which is less affected by the ultra violet light in the sun’s rays causing ordinary polythene to quickly become brittle and disintegrate. UVI polythene will last for at least two years. Other more rigid plastics are longer lasting.

Novolux corrugated sheeting is said to last at least five years; and structures comparing favorably with glass can be constructed with it. It is excellent for making portable frames. With glass frames, the lights are often heavy and susceptible to breakage.

When choosing a greenhouse, see that facilities for ventilation are good. A sliding door is also useful as an extra vent. Some frames have sliding sections in the base which allows ventilation without raising the top.