Special qualities in modern polythene covers make polytunnels a more viable option for growers

Not that long ago, polythene tunnels were seen as a second-class option, opted for only by growers who did not want to incur the expense of a glasshouse. These days, however, polythene-clad tunnels and structures are often chosen ahead of glasshouses thanks to the special qualities than modern polythene can provide.

Polythene for use in horticulture has evolved rapidly and thanks to spectral filters, additives and polymer technology, growers no longer have to put up with intense, stifling heat on sunny days and chilly temperatures at night. Nursery workers and plants are also no longer soaked from above as condensation drips from the roof.

No grower wants to spend money re-covering tunnels every few years, but today you can expect a lot longer lifespan from polythene. Visqueen’s Lumisol polytunnel covers are backed by a seven-season guarantee against premature ultraviolet degradation but, depending on location and climate, covers can last for up to 15 years.

Thermic Efficiency 

But perhaps the greatest area of interest to nurseries today is the in the thermic efficiency of film. Thermic films work by allowing short-wave infrared radiation to enter the tunnel. This then heats up all the items inside the structure, including the floor, pots and plants. The heat is radiated back as long-wave infrared radiation but filters in the film reflect much of this back into the tunnel or house.

Working with the University of Lancaster and the University of Reading, Visqueen has developed a new generation of ultraviolet-open polytunnel covers. Previously Visqueen offered different films for different purposes and users had to prioritise needs but now the choice is as simple as clear or diffused.

Combining all the benefits of previous films — thermic and light-transmission properties — this film allows short wavelength ultraviolet light to enter the structure. This can benefit both edible and ornamental crops, enhancing fragrance, flavour and colour as well as potential earlier harvesting and longer shelf life. Ultraviolet-open films are also thought to increase cell wall thickness, leading to stronger plants.

Lumisol Diffused, Visqueen’s light-diffusing polythene film scatters the light as it passes through and enables it to penetrate deep into the plant canopy, hitting the plants from all directions and thus preventing higher foliage from shading lower plant parts. Lumisol Clear, due to its clarity, is said to allow around two per cent more light to pass through compared to light-diffusing film and this extra light may be beneficial in some crops.

Both films boast anti-fog condensation reduction to reduce the risk of water dripping on the plants and prevent the loss of light caused by droplet formation. As the effectiveness of this reduces over time, Visqueen offers anti-condensation solution Sunclear for re-application through a sprayer. Both films are also 180 microns thick (720g) and have a five-season manufacturer’s guarantee against premature ultraviolet degradation. Standard rolls are available in widths of 7.3m, 9.2m, 11.1m and 14m, all cut to the required length.

What of the future? 

Today, polytunnels offer commercial growers an environment for growing fruit, flowers, vegetables, nursery stock and bedding plants with the advantage of covering large areas of ground relatively inexpensively. They also offer growers a chance to keep up with advances in technology.

Nigel Carr, our Commercial Director explains: "A grower is not going to change the glass on a glasshouse every seven or eight years, yet diffused glass is now the 'in thing' in Holland and one day we are sure to see photo-reactive glass become available. With a polytunnel, you plan to recover it every seven or eight years or so, and that gives you the chance to improve the technology you are covering your crops with."

UV Absorption

This new generation film combines all the benefits of "Luminal" (great thermic & light transmission properties), but unlike normal greenhouse films that prevent short wavelength UV light (UVB) from reaching plants, this UV transparent film allows UVB to enter the greenhouse thus enhancing the fragrance, taste & colour of the crops. In addition, UV transparent plastic films promote plant anti-oxidants which are good for health & encourage stronger plants. This greatly benefits plant raisers & young growers as stronger plants can be easier to plant out.

Anti-Fog (Condensation Reduction)

Thanks to its unique additive, Lumisol is able to control the condensation of water on the film. Providing the film is properly installed, water will condense uniformly on the plastic sheet rather than form as droplets. This reduces the risk of water dripping on to the plants below and prevents the loss of light caused by these droplets, which can account for as much as a 25% light reduction.