A greenhouse can be one of the most valuable additions to a garden and offers the opportunity to grow a much wider range of plants, often making these available earlier or later in the year. Despite the array of designs available, the simplest constructions are usually adequate for many gardens. A greenhouse measuring 2.5 x 5m (8 x 16ft) will provide sufficient space for most needs.
Aluminium is usually the best material for a frame, as it needs little or no upkeep and casts little shade. Wood, whilst attractive, needs periodic upkeep and the bulkier frames can cast excessive shade inside the greenhouse.
The best glazing material is glass, as it allows the passage of 90 per cent of external light through, and does not degrade in sunlight. It also helps to reflect heat radiated from within the glasshouse back into the structure. Polycarbonate sheeting is resistant to breakage, lightweight, durable in adverse weather, and good at retaining heat, yet only transmits 83 per cent of the external light.
To maximise the success of greenhouse growing, the conditions within it should not fluctuate widely, so do invest in a maximum/minimum thermometer to help you to control the temperature inside. Ensure the design you choose has provision for ventilation to avoid it overheating in summer months, and can be insulated easily for winter. Roof vents are the most useful, and ideally should be on both sides of the roof ridge and equivalent to 15-20 per cent of the floor area.You can even get thermostatically controlled vents that open as the temperature inside rises.
It is wise to provide some shading, too, such as a white shading paint applied to the inside surface of the glass that reflects some of the light. Shade netting can prove effective, although this allows all of the light through the glass and tends to absorb the heat from it. It can also reduce the quality of the light that does reach the crop.
Carefully plan the layout of growing areas inside to use every available bit of space. Many greenhouse plants will grow well in open soil borders, in ground prepared in a similar way to borders outside. Grow bags are an alternative if the greenhouse has to be placed on a hard standing or paved area. A shelving system might be better if you want to grow specimens in pots, and has the advantage of enabling you to work at a comfortable height. It is particularly useful for raising seedlings and propagating plants.