Crop enhancement

Demand for better energy efficiency, yields and quality is driving improvements in polytunnel covers, says Gavin McEwan.

Keder: modular tunnels can be expanded widthways or lengthways as need for growing space increases - image: Keder Greenhouses
Keder: modular tunnels can be expanded widthways or lengthways as need for growing space increases - image: Keder Greenhouses

There is now a variety of polytunnels available to growers ranging from simple temporary structures of polythene sheeting tied over hoops to more solid structures whose design sophistication can approach that of glasshouses. "People are investing in better-quality structures with vertical sides," says Polybuild product designer Jonty Swales.

"A twin-skinned tunnel also keeps in the heat and reduces condensation. Most people these days want to be able to use energy more efficiently — and if you have a warm day followed by a sharp drop in temperature at night, your plants get a shock. But by having an extra layer, you can lessen that by allowing the temperature to drop more slowly. Also, if you have any damage, an inner sheet gives you a backup."

Meanwhile, the use of Lumisol films (see box, p33) overcomes one of the perceived limitations of growing under plastic — the effect that it has on the light spectrum passing through.

Northern Polytunnels is among the installers ­recommending Lumisol films. "The polythene is a small part of the overall expense so you are as well to go for a premium product," says commercial director Nigel Carr.

The benefits of UV-transparent film "have been slow to get out, particularly among more low-tech growers", he adds. "But a strawberry grower for the supermarkets can compare his tonnage from one season to the next, so can see the improvement."

As to the merits of diffuse polythene in particular, Carr says: "The more foliage a crop has, the more it will benefit, as it gets round the problem of upper leaves shading lower ones, although you lose a little — two per cent — in light transmission."

As to whether growers are prepared to meet the expense of a more elaborate structure, Swales says: "Growers have had a hard time over the last couple of years and have been holding back but they are wanting to make up for that now. This year is looking very busy for us. We have also done well out of the winter storms."

Hoops are mounted on triple sockets, which are strong and allow the tension of the polythene to be adjusted, he explains, adding that hoop spacing can be reduced for extra strength on exposed sites." It needn’t be an all-or-nothing decision, he points out. "Often with a new business we will supply a single-span initially but design in the option to expand in year two so the tunnel can grow with the business."

The Government’s ostensibly more relaxed attitude to planning is not always reflected on the ground though, Swales adds. "Permission is still quite subjective and localised — what one authority will definitely allow another may take a quite different view on.

"A lot of our competitors are working with higher eves to keep the hot air up high, but planning requirements may mean you have to keep within a certain height. We advise customers that they may need planning permission, though it’s up to them to seek it. They may ask for a structure that meets regulations and seek permission for it retrospectively."

Even further along from the simple temporary polytunnel are the structures provided by Keder Greenhouses. Business director Electra Hatcher says: "People don’t understand how our products are constructed—– they see them as being in the same category as polytunnels."

Keder offers the unique option of cladding with Polydress LP-Keder polythene sheeting — a kind of bubble wrap sandwich that is claimed to be safer, lower-maintenance and more cost-effective than glass, while it is also stronger and more durable than standard polytunnel film. It also reduces summer daytime temperatures as well as retaining heat overnight, while reducing condensation, and is said to reduce heating costs by up to 50 per cent.

As might be expected, with 1,000 bubbles per square metre the sheeting scatters incoming light, by up to 83 per cent, so evening out temperature and avoiding areas of shading and burning, giving more even plant growth.

As with Polybuild structures, Keder uses pre-galvanised high-tensile structural steel, which is almost twice as strong when compared to standard steel tube. "Our steel tube is sourced from UK ­manufactures and every batch has full traceability," says Hatcher. "In addition, all of the brackets are manufactured and plated in the UK specifically for our structures."

Being modular, the tunnels can be expanded either widthways or lengthways as the need for growing space increases. "And if there’s any damage, you can repair a single panel," Hatcher adds. "For a commercial grower, that’s a real advantage."

For improved ventilation, Keder structures come with outer wall vents located high enough to avoid wind damage to the plants. These vents operated via a manual gearbox designed to be simple and maintenance-free.

For larger structures offering a greater span, a central roof-mounted vent system can also be incorporated. These can be upgraded with the addition of electric thermostatic control units, each opening and closing independently, with response and delay times as well as varying stages of opening and closing, each programmable.

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