Advanced cladding, better ventilation and more durable frameworks are among the improvements offered by the latest polytunnels, says Sally Drury.

Cladding: spectral filters used to modify the light spectra coming into polytunnels - image: Northern Polytunnels
Cladding: spectral filters used to modify the light spectra coming into polytunnels - image: Northern Polytunnels

Only 15 or 20 years ago, nurserymen would consider polytunnels as a cheap alternative to glasshouses. Clad in polythene, the hooped structures were ideal where a crop needed protection from the elements but where the close environmental control afforded by a greenhouse was not imperative.

How things have changed. With the development of plastics films, polytunnels are now viewed by many nurserymen and growers to no longer be a compromise on glasshouses but actually to be an improvement on them.

In the early days polytunnels were single-span and low - "little more than walk-in cloches", as XL Horticulture managing director Les Lane prefers to put it - and the thick polythene cladding would only last for a couple of years or so.

Today, polytunnels are wider and higher, often multi-span constructions. Furthermore, with the use of co-polymers, cladding films are thinner, have better light transmission and last much longer as well as offering a host of filter-induced benefits. From thermal protection to altering plant morphology and reducing pest and disease attacks, plastics technology and the use of spectral filters offer growers many options.

"When we started our polythene business over 20 years ago, all films were either 180mu or 200mu thick and only lasted two-to-three seasons," says Lane. "Technology has come a long way in 20 years and you definitely don't need a thick film to make it strong now that we are using modern co-polymers."

Making the cladding thinner means that a greater area can be covered with every one tonne of polymer used, but the addition of spectral filters was a real game-changer. "Very often nurserymen are trying to grow plants which are not native to the mid/northern European light spectra," Lane explains. "We thought, why compromise? Let's modify the light spectrum coming through the polythene to make it match, as far as we can."

Discussions with growers, then trials with universities and research stations followed. Cladding was used on Lane's own commercial nursery tunnels and trialled by other growers. Today, XL Horticulture includes a wide range of claddings - clear, diffused, thermic, ultraviolet-open for stronger plants with improved colour, ultraviolet-blocking to reduce fungal diseases and aphid spread, green for shade-loving plants, blue for height reduction and the production of compact plants along with a range of films for soft-fruit production. There is also film for propagation purposes.

One of the latest films to come from the XL stable is SunMaster ChrystalTherm. In the Evolution 2015 series, this film is for all crops where maximum light is essential. It is the first horticultural film to be made from seven layers and has a new patented Anti-Drip system that outlasts conventional alternatives. It also has an ultra-smooth external face to repel dirt and delay the onset of algae, is immensely strong, yet boasts clarity similar to glass.

Also in the Evolution 2015 series is SunMaster Cool, designed for growing plants that like to be kept cool in summer but that still appreciate lots of light. It uses the latest microfine aluminium diffuser, resulting in a tunnel that is 25 per cent cooler in bright sunlight than a clear-film clad tunnel. Just added to XL's catalogue are Evolution 2016 Repair & Anti Hot Spot Tapes made from polythene foam rather than PVC foam, so they do not give off chlorine that could damage ultraviolet stabilisers in the film. Silver grey in colour, the tape blends in with the steel and reflects light.

Design challenge: aluminium end frames and automatic louvres improve strength - image: Northern Polytunnels

Added benefits

It is easy to see that science and technology have taken polytunnel cladding into a new sphere, beyond the capabilities of glass. But growers also want structures with added benefits, particularly when it comes to ventilation. Northern Polytunnels commercial director Nigel Carr has noted increased interest in thermostatic operation of the company's roll-up side vents, roof vents, end louvres and polytunnel roof fans.

"By controlling ventilation automatically you not only control the temperature precisely but also remove the human element," he says. "Temperature is controlled 24/7 based on actual temperatures and not what a person thinks is the temperature. This extra level of sophistication is more widely required for multi-span polytunnels but is also used in single spans. This advancement further closes the technological gap between modern glasshouses and polytunnels."

With regard to roof ventilation, Carr has also found polytunnel roof fans - introduced by Northern Polytunnels in 2015 and winner of the best new product (professional equipment) award at the Four Oaks Trade Show - becoming a popular option. "The main reasons for this is their effectiveness, ease of maintenance, low running costs, and extremely good value at £175 each," Carr explains. "Since the roof fan won the best new product award at the Four Oaks Trade Show last year we have supplied hundreds of units, both for new builds and for retrospective installations in older existing tunnels."

Northern's roll-up side vents and rack-and-pinion roof vents now come with the option of thermostatically controlled 24V DC motors, giving fully automated operation 24/7. Several motors can be operated from a single controller, enabling simultaneous vent operation. Again, these are simple to install and represent good value at less than £200 per motor.

We have all seen gutters overflowing during heavy downpours and witnessed the inconvenience and destruction that can cause. In response to changing weather patterns and the trend towards the use of longer multi-span structures, Northern Polytunnels has taken a fresh look at guttering. So far the only company to do so, it has increased gutter size to cope with the much higher volumes of water experienced nowadays.

"Our gutters will hold two-to-three times the water volume of other gutters and have a larger spout, now measuring 160mm, to increase the discharge by approximately 250 per cent compared to a standard 110mm spout," says Carr. "Additionally, the spout is angled at 45 degs to aid the evacuation of water by a further 30 per cent."

Once a temporary or budget solution to cropping, polytunnels are now being built for the longer term. Cladding can be changed as production changes or when beneficial advancements are made, but the framework needs durability. Increasingly aluminium is being used in place of timber, both for the end frames and the side rails.

Carr points out the advantages. "Although the initial outlay is higher, it doesn't need replacing every 15-20 years like treated timber. Aluminium is also easily cleaned and keeps the polytunnel looking new. It is exceptionally strong and has none of the natural faults of timber."

Longer lifespan

Polyhouses.com, a division of Fordingbridge, has supplied film-clad structures for more than 50 years and uses high-tensile Z35 steel alongside precision aluminium extrusions to ensure that each tunnel or multi-span has a long life.

The company pioneered the use of a swaged joint connection throughout its entire range - again adding to durability - and was among the first to introduce aluminium guttering to help reduce ongoing maintenance costs. "Coupled with the use of quality components, this delivers a sturdy and robust structure designed to last for many years to come," says Fordingbridge marketing manager Christine Virginie.

A recent development from Polyhouses.com is the Hybrid Polytunnel, a structure that offers a maximum ventilation growing environment with standard 3m straight sides. Using 80x80mm box section stanchions and internal spacings of 16ft (4.9m) give greater growing or sales areas and roof vents can be added to work with roll-up side vents. The structure can be singleor twin-skinned and automation is optional.

For the soft-fruit industry Northern Polytunnels is now offering polythene films that have been specially formulated for each particular crop, climate/latitude and time of harvesting. This includes options such as the thermicity of the film, the level of diffusion, whether it is ultraviolet-open or ultraviolet-closed etc, so that field-scale fruit growers can increase yields at little or no extra cost.

Today's claddings and structures are now tailored to suit, and that is perhaps the key to the success of the polytunnel in recent years. There is no one solution for all purposes. Just like the glasshouse, polytunnel construction is now a turnkey project requiring many disciplines that include rainwater collection, ground cover fabric, pathways, irrigation, heaters, air circulation fans, benching and more.

Specially designed polytunnel pathways

Concrete pathways are expensive in terms of materials and labour, and they need to be sufficiently wide to ensure that trolleys do not veer over the edge. But that means extra width for pathways when you want the space to grow plants. With this in mind, Northern Polytunnels is now supplying rubber Danish-trolley trackway sections that simply interlock to give a path exactly the same width as a Danish trolley. The track is made from recycled plastic, can be moved if required, features an anti-slip surface and substantially reduces the noise of trolley wheels.

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