How to buy - Polytunnels

The protection of plastics cladding can give your business a competitive edge, Sally Drury suggests.

Polytunnels - image: HW
Polytunnels - image: HW

Any plant or produce that is less than perfect will be difficult to sell and will not attract the price needed to sustain profit margins. Whether you are growing shrubs or strawberries, protection may be the solution. A plastics-clad tunnel or house can give protection - and more.

Away from the damaging effects of the weather, crops have a better chance of producing the size and quality required by the demanding markets. Additionally, the internal environment of the structure can be modified to encourage better colour, stronger plants or reduce the incidence of pests and disease.

Key considerations

Choice of structure may be limited by space, but in all cases it is important to consider heights - side walls and overall height, together with bay widths and access points - in relation to the crops grown and production and transport systems. Think about the need for ventilation and, if a multi-span structure, adding guttering to collect rainfall.

An assessment of the site is vital if the best structure is to be supplied. Shelter and typical wind speeds, the direction of the prevailing wind and the likelihood of gales are important factors when it comes to the specification. You should also look back at snow records for your area. Snow is another loading that must be taken into account when working out the strength required.

The speed of technological change in terms of cladding has been rapid over the past 10-15 years. The use of polymers in cladding materials means that, whether you are building a new structure or recladding an existing one, you need to consider all the options. Some can bring you enormous benefits in crop quality, reduced production time and savings in agrochemicals and heating.

Standard polythene cladding materials are still available and many growers will simply look to buy the best they can afford. But interest in "smart" films is at an all-time high. If you are looking to go down this route, you need to think about the plants and the conditions they need, along with the desired lifespan of the film. A longer lifespan will reduce the frequency and expense of recladding.

There are disadvantages to using ordinary polythene film, the main one being condensation build-up. This will result in reduced light levels and drips that fall onto crops and workers. Disease is encouraged in such conditions. Fortunately, anti-condensation films have put an end to that misery. Such films also have useful heat-retention properties.

Research and development work looking at polymer additives has brought further benefits. Films are now available that incorporative chemicals to selectively block or enhance certain light wavelengths, improve growth rates and inhibit certain pests and diseases.

When looking at different types of cladding, it is important to study light transmission and relate that to the crop. Thermal heat-block films are commonly used on nurseries. They maximise the available light by increasing the amount of diffused light entering the house or tunnel. The result is lower temperatures by day, increased light levels and improved night-time temperatures. Other advantages of these films are seen in enhanced growth rates and reduced scorching.

Spectral modifications

Different polymers and other additives can provide spectral modifications. Those that screen out light at the far-red end of the spectrum can be beneficial in their effects on plant morphology, while those screening out ultraviolet light can help reduce fungal disease and aphid attacks.

Such smart films may cost that bit more, but savings can be made in growth regulation chemicals, pesticides and energy. But remember, the film that best suits your needs will depend on your crop.

You should also give thought to how the plastics film will be disposed of once it has come to the end of its useful life. Some firms will take back old cladding, but charges vary.

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