The science behind polytunnels and cladding has advanced significantly and new developments continue to benefit growers, Sally Drury reports.

Polytunnels: multi-span plastic-clad structures give growers a construction more like a commercial glasshouse in terms of size
Polytunnels: multi-span plastic-clad structures give growers a construction more like a commercial glasshouse in terms of size

Polytunnels have come a long way over the decades since they were designed for use by growers. Then seen as a cheaper alternative to glasshouses, especially for those growers producing crops that did not require such a highly controlled environment, polytunnels were little more than an umbrella over the plants.

But they were umbrellas with problems. Early tunnels tended to be unbearably hot in the summertime and stiflingly airless, yet dripping with condensation that made working in them unpleasant as well as causing damage to plants and inviting disease infestation.

Light transmission was low and algae quickly built up on the outside of the structures, lowering light transmission even further.

Completely different

Today’s polytunnels are completely different. No longer are they just single-span hooped structures. The rise of the multi-span plastic-clad structures gives growers a construction more like a commercial glasshouse in terms of size. We have also seen improvements in engineering, leading to structures that are stronger and available with ventilation via roll-up sides, and today’s cladding gives growers a choice of environmental conditions to suit their crops and their budgets.

But it was the introduction of smart films, those impregnated with filters, that brought about the turning point in polytunnel technology. The science behind cladding materials continues to develop. Exciting times lie ahead for those growers with production under polythene.

A new range of plastics films — called The Evolution 2015 — is coming to market. It heralds further improvements in environmental conditions for plants, increasing plant quality and productivity levels as well as in some cases aiding the cladding process and lowering costs. The range is based around seven-layer technology.

Winning Best Professional Product or Service Award at the Four Oaks Trade Show in September, SunMaster DuraCool is the first of a new range of films from Devon-based XL Horticulture. A clear solution to reducing summer temperatures inside polytunnels with little loss of light, the film uses a microfine aluminium additive as its diffuser to lower interior temperatures by as much as 25-30 per cent. As few growers would wish to lower temperatures by that amount in winter, DuraCool is also thermic and has anti-drip properties. Light transmission is 78 per cent. Yet it is strong — 160 microns (640 gauge) — and, like all the films in the range, carries a six-year warranty.

Also in the new range is CrystalTherm, described as being "as clear as glass". Glass normally has light transmission of around 88 per cent. CrystalTherm has 91 per cent.

The films also have permanent anti-drip and anti-fog. In addition, they use another technology that makes the outside of the film super smooth so that it sheds dust and fungal spores, rainwater washing the dirt away and enabling them to stay clearer for longer.

Continental sizes

The whole range sees a move towards continental sizes — widths of 8m, 10m, 12m and 14m and cut to length to fit the tunnel. "In the past we have had a few greenhouses where 11.1m is a bit tight and they have had to go up to 14m, but now they can use the 12m, and for 9.2m they can use the 10m," says XL director Les Lane.

During production of the films, a continuous thickness control is used. This enables the films to be made to a thickness of +/- three per cent accuracy. The industry standard is +/- 15 per cent.

For growers needing insulation in the winter months, twin-skinned structures are proving increasingly popular. It is reckoned that twin-skin cladding can save up to 60 per cent on energy costs, but they tend to be difficult to install. Expected to be available from XL Horticulture early next year is a two-sheets-in-one combi-film intended to simplify the cladding of twin-skinned constructions. "It is really difficult to put twin-skins on a greenhouse or tunnel but this will make it easy," says Lane.

"The new film will have different films on the outside and inside — it will have a three-layer film on the top and a three-layer film on the bottom. It means you only need to put one cover on the greenhouse and then inflate it."

The manufacturer of The Evolution 2015 range is Plastika Kritis, based in Greece. The company has one of only three machines in the world capable of making wide-width films and the only one making agricultural films. Plastika Kritis makes films using virgin polymers. This means the resulting products are highly recyclable because polymers can be used up to seven times — the last time usually being in the manufacture of bin bags.

Life after polytunnels

With a turnover of around €230m a year, Plastika Kritis recycles about 5,000t of plastics films annually, turning the material into pipes for water and fibre-optic cables. The company describes itself as "vertically integrated" — it does not just make the films, it also makes all the ingredients that go into them. It has three factories making polythene and six making additives.

The new polythene film production line at Plastika Kritis cost €12m. The company has a 12MW wind farm and 340kW of photovoltaic stations, producing more energy than it uses. That is as green a production as you can get.

New product: Polytunnel Extraction Fan exhausts excessive heat

Designed to exhaust excessive heat from polythene-clad greenhouses, the Polytunnel Extraction Fan from Colne-based Northern Polytunnels & Horticultural Supplies won Best Professional Equipment Award at the Four Oaks Trade Show in September. Airflow is 4,600m/hr (2,700cfm).

Weighing just 7.5kg, the roof fan, which is IP55 protected and has a 200W motor, is straightforward to install. Users simply cut out a 30-35cm hole in the polytunnel cover, insert the fan from the inside in a rotating motion and clip it onto the support bars.

The unit requires a single-phase 220-240V power supply and comes with a mesh guard on the air intake. It is usually operated by an automatic controller and temperature switch — also available from Northern Polytunnels & Horticultural Supplies.

Priced at £195 +VAT for a single fan, discounts are available for volume orders and reduce the cost per unit to £165 when 20 or more are purchased.

The control panel for automatic operation costs an additional £179 + VAT and the temperature switch £139 + VAT. Support bar kits for 32mm, 50mm or 60mm diameter hoops can be purchased if needed and cost £39 + VAT.

Specialist help for recycling your old tunnel

It is illegal to burn or bury polythene so you must use a registered waste-removal company to properly dispose of polytunnel covers.

It can be tricky to comply, but most recycling services will require that the polythene is clean, dry and free of contaminants — particularly soil, stones, metal and timber. Ideally, films should be baled or tied.

As part of the BPI Group, polytunnel cover supplier bpi.visqueen works closely with sister company bpi.recycled products, Europe’s largest recycler of polythene. The company reprocesses more than 64,000 tonnes of plastics waste a year. BPI estimates that every tonne of plastics recycled prevents the release into the atmosphere of 1.4t of carbon dioxide, saves 1.8t of crude oil and reduces water usage by 90 per cent compared with using virgin material.

It is not just polytunnel cladding that gets the BPI treatment. Mushroom films and crop covers from growers and farms across the UK are also reprocessed in its £18m recycling facilities. The plastics films are turned into a variety of useful products such as Plaswood. An alternative to traditional materials, Plaswood is used in fencing and decking products, bollards, seating and other street furniture. It is low-maintenance, rot-proof, algae-resistant and can be fully recycled at the end of its working life.

With a carbon footprint at least 35 per cent lower than imported refuse sacks, The Green Sack range is also made from 100 per cent recycled British farm waste plastics.

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