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Greenhouse Growing : Protective Structures

Greenhouses | Frames | Cloches | Walk-in tunnels | Low tunnels


Low polythene tunnel with early potatoes

Low polythene tunnel with early potatoes

Glass was once the only available glazing material, and it is still the best, letting in light and retaining heat better than any alternative. It is more expensive and not as safe as plastics, but it lasts much longer. Safety glass is available and, though it is more expensive than ordinary glass, it should be considered for a conservatory.

Rigid plastics are not as expensive as glass, and they are safer, but they do not let in as much light; nor do they look as well, except for perspex, but this is expensive, too.

A new greenhouse, well stocked

A new greenhouse, well stocked

Polythene is cheap and safe, and allows in light well but does not last so well as the other materials. PVC and U-V inhibited films are more expensive than polythene but last longer.


A garden frame is just a large, low bottomless box with a translucent lid. The sides of the box can be made of wood, concrete, galvanised iron or any other building material – very often salvaged scrap materials will do.

The frame can be any length but should not be wider than 120 centimetres for comfort and safety. It should be 25 centimetres high in front, and about 50 centimetres high at the back. The ‘lid’ is a number of ‘lights’, each consisting of a wooden frame with glass or plastic – on it. Each ‘light’ matches the width of the garden frame in its own length and should be about 75 centimetres wide, and can be made of suitable timber lengths.


A ‘cloche’ originally was a bell-shaped glass jar placed individually over tender plants. The term was extended to include continuous structures of glass sheets supported by iron brackets. These continuous cloches were normally about 45 centimetres high but are now unusual, because low plastic tunnels and cloches have taken over.

Walk-in tunnels

A ‘walk-in’ tunnel, or polytunnel, can substitute for a glasshouse. Being relatively cheap to put up, it is possible to cover a larger area economically. A polytunnel can be of any length – using a greater number of the tubular steel supporting hoops.

Tunnels are generally sold as kits and the standard widths commercially available are 4.2 metres and 5.1 metres. Walk-in tunnels can be difficult to ventilate properly, and the plastic will have to be replaced. It usually lasts two or three years, although there are more durable kinds of plastic film that last for five years.

Low tunnels

A low tunnel is a row of wire hoops – 45 centimetres high, 75 centimetres wide – supporting a 1.8 metre wide polythene film. Length of row is variable. At each end of the low tunnel the film is tied firmly to a short stake. The wire hoops are 1.8 metre lengths of strong wire with an ‘eye’ twisted into them about 20 centimetres from each end.

Strings tied into each ‘eye’ and stretched across the polythene hold the tunnel film in place. Although both garden frames and low tunnels are limited in use by their size, they are very successful for low crops such as early and late vegetables, early strawberries, and cuttings.


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