Polytunnel cladding/plastics

Advances in films mean growers can now further reduce summer temperatures under plastic.

Isn’t science great? Just look at the range of plastic films introduced in the past 15 years. Not only can we use films to protect crops from the harsh realities of weather, we can use many of the new films to adjust the internal environment of a house or tunnel to the benefit of the plants and, hopefully, to profit margins.
There are several criteria to consider when investing in the construction of a new tunnel or house. The type of cladding is a major consideration but first you should think about the structure itself.
Choices may be limited by the available space on the nursery or by the need to link it to existing facilities, but in all cases it is important to consider the height of the proposed structure both in terms of the side walls and the overall size. Also think about the bay widths, the points of access, door sizes and the need for ventilation. All dimensions should reflect the types of crops grown, the environment they need and also the production methods and transportation used on the nursery.
Other things to consider when looking at the structure include your local climate. The likelihood of strong gales in exposed areas and the possibility of heavy snow falls make additional  strength a structural necessity.
When it comes to the cladding, whether for a new structure or as material to re-clad an existing house or tunnel, options continue to proliferate. Technological changes are pushing chemists to develop new polymers and additives — and horticultural scientists are finding new uses for them. Some of the new films can bring enormous benefits in terms of improved crop quality, reduced production times and savings in agrochemicals. But, of course, these will cost more than standard polythene claddings.
Although some growers continue to look for the best buy they can find, and therefore opt for standard polythene, interest in “smart” films is at an all-time high. Many nurseries now use Thermal Heat Block (THB) films to maximise the available light by increasing the amount of diffused light entering the structure. The result is lower temperatures by day (nicer to work in), increased levels of light — especially photosynthetically active light — and improved night-time temperatures. Other bonuses of THB films include good growth rates and reduced scorching.
The next generation of films has further benefits. This year XL Horticulture is introducing two new films, and is incorporating a thermo-block additive in others, to give even lower summer temperatures than those experienced under existing THB films.
“Outside UV-light transmission levels are normally down to 280nms, but glass or polythene block all UV below 350nms. HDC 108 Nursery Stock trials show significant improvements in plant quality where total UV transmission films are used. Therefore, all XL films, other than the disease-control film SteriLite Premium, will be what are called UV open,” explains the manager of XL Horticulture, Les Lane.

The latest products
The first new film added to the range and incorporating this technology is SunMaster TCF/AF (temperature control film with anti-fog properties) in a five-year grade. In addition to possessing “UV open” technology, SunMaster also uses a new aluminium-based thermal filter. “Research in the hot climates has shown it to reduce summer temperatures by approximately 15 per cent more than existing THB calcium carbonate filters,” says Lane. In addition, SunMaster uses XL’s SuperStrength technology to give a lightweight, longer life and more gale-proof film than is achievable with ordinary UVI/EVA formulations.
New stock of SteriLite Premium will use this latest aluminium thermo additive, but will retain its total UV block for pest and disease control.
The second new film is SuperStrength Clear 5, a five-year film suitable for any crops where a clear film is already used. In addition to being UV open, this film uses XL Horticulture’s SuperStrength technology for lightweight, strong films so is recommended for windy sites.
Other additives provide spectral modifications. For example, there are those that screen out light in the far-red end of the spectrum and are used to affect plant morphology and thus save on the use of growth regulators.
When you look at the different types of cladding available, it is important to study the light transmission and relate that to the crop and your objectives. The film that best suits your needs will depend on your crop.

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