Technology brings polytunnels up to date

More robust structures and latest venting techniques are helping to improve polytunnels, says Roger Chesher.

Polytunnels: a low-cost transportable field scale system, Image: BPI
Polytunnels: a low-cost transportable field scale system, Image: BPI

The end of the 20th century saw a significant change in the demand for fresh fruit and vegetables. An awareness of "food air miles" and the importance of traceability increased the focus on locally-grown fresh or chilled products.

Spurred on by campaigns such as "five a day", the demand continues despite the fact that historically the industry primarily spanned a mere eight-week season and the market relied on and was indeed dominated by imported fruit.

Admittedly there was always the option of growing under glass, but this was expensive. However, a low-cost flexible option was ultimately provided by the development of polytunnels. A significant leader in this field of technology has been Haygrove of Ledbury, which in the mid 1990s began to design polytunnels suitable for its own crops. The result was a low-cost transportable field scale system, following ground contours, scaled to fit field machinery and strongly flexible in order to carry polythene, shade, hail, bird or insect net.

This meant that the season was made earlier by around three weeks and extended well into the autumn, with guaranteed spray and harvest days and resulting in significant increases in yield and quality.

Today, the advantages of growing under plastic remain. But according to Tim Crossman, who looks after sales in western Europe and eastern Africa, there is a shift in Haygrove's business towards tunnels becoming more permanent. Over the past 10 years tunnels have become bigger, wider and stronger with better steel structures.

Although more expensive per unit, the cost per square metre is lower. "Labour is too expensive to move things around," says Crossman. "So we are making tunnels robust by putting more into them and fitting stronger polythene that can stay on for longer" Steel has moved from 1.5 to 2 or 2.5mm sections.

The phrase "glasshouse technology into the field" is currently the vogue at Haygrove. It means fitting tunnels with better irrigation, gutters and roadways. One particular innovation is the trellis tunnel, which incorporates cable technology to stress and tension the structure, giving strength without too much extra weight and the ability to withstand high wind forces.

Innovations from Haygrove include automatic venting to reduce labour and the introduction of the world's first telescopic tunnel, designed to provide adjustable air volume to enable winter covering for ever-more earliness. This glasshouse technology still comes at field scale prices, insists Crossman.

Weather resistance

Although based in Evesham, Pete Monahan of CLM Keder finds himself currently snowed under with enquiries from Scotland, where tunnels have suffered damage from heavy snow and the 100mph winds of the west coast region.

"The thing that differentiates our tunnels is their strength," he points out. "A robust framework designed to handle high winds, covered by Keder cladding." This is a very strong three-layer polythene sandwich that comes in panels - the layers have air trapped within offering high levels of insulation, trapping heat and saving energy.

Again said to combine the best of both glass and polytunnel, the Keder units stand in blocks up to five acres a time, with sixto eight-metre bays. With a 10-year guarantee against ultraviolet degradation, these galvanised steel structures are said to be unique and are manufactured in-house.


The trend towards strength and permanence was also echoed by Kevin Banbury at National Polytunnels of Fulwood, Preston. Although the company manufactures, builds, supplies, installs and advises on polytunnels for growers, nurseries and garden centres, the current area of greatest growth is in the domestic garden market.

That's not to say that development has stood still in the commercial market where ventilation, film technology and tunnel siting and orientation can have a drastic effect on both yield and growth patterns.

Conventional tunnels have changed little, explains Banbury. But National's focus has been on venting techniques, roll up curtains and, in particular, hybrid "super bay" tunnels giving increased workable area. These have straight sides utilising box section rather than tubular forms and allow for fork lifts, curtains, doors and irrigation.

This can be particularly useful if part of the unit has public access or where building regulations need to be observed. In this case the structure can accommodate stronger, flame-retardant polythene and the sides could be composed of semi-permanent polycarbonate, even constructing secure areas if needed. National Polytunnels definitely sees "super bay" as a rising trend because of its increased versatility.

Factors for change

Rob Tasker, sales director at Pro Tech Marketing (formerly Crop Pro Tec), is a great exponent and champion of polytunnel technology amongst UK growers. Specialising in Spanish tunnels, polytunnels, greenhouses and table-top growing systems, he explains some of the factors behind moving to more permanent structures.

"Growers are running out of clean land and with the loss of methyl bromide, soil sterilisation is less than satisfactory. It is costly to move labour around and workers now like to pick standing up, so the trend is for table-top growing on more permanent sites."

Polytunnel units are therefore heavier structures with more substantial steel and legs closer together. They are warmer, with stronger plastic extending to the floor, and the season has been extended from four to six or nine months.

Pickers don't want to be on their knees all day. Table-top picking costs are some 30 to 40 per cent lower than for bed-grown strawberry crops and help to reduce the impact of the labour shortage. Growing in a substrate avoids the problems of stale land and, with the tunnels no longer moved around, further labour savings are made.

Pro Tech's polythene films are manufactured to the company's tailor-made specifications in different thicknesses and formulations to meet the requirements of location and crop. With more permanent structures comes the need for tougher films. These are, of course, more expensive but on a seasonal basis they are more cost effective per square metre.

Polymer performance

An increased focus on the film requirements of "hybrid" tunnels has led to significant developments in the field of polymer research.

BPI is perhaps one of the best known producers of polymers and plastics across the whole agricultural, horticultural and packaging spectrum. Not surprisingly, it invests a great deal in research and development, sponsoring projects at universities across the world.

Polymers can give many more properties than mere strength, explains sales director John Phoenix. Obvious examples are clarity and softness. The company is currently in the midst of an overhaul that, with a new machine, will allow five-layer films to be made up to 22m wide and 120 microns thick - 18m film can be 200 microns and the film supplied to Haygrove is 20m at 150 microns.

While this represents significant technical development, it is the research into the optical characteristics of films that causes the greatest interest. It is possible, for example, to block out ultraviolet radiation or to allow it through fully. UV-open films, which are currently available, affect the colours of crops, their bushiness or early growth habit, and these films are of particular interest to seedling producers.

Research is about to start looking at the affect of UV light in degrading pesticide residues, something that, not surprisingly, is greatly interesting the supermarkets.

However, UV blockers are not quite ready yet but are known to enhance vegetative yield. This is useful for plants such as lettuce. More excitingly, insects are known to avoid UV-blocked environments, which could be ideal for pest control but not so for pollination.

Further work is underway on a specially formulated film that reduces infrared radiation and enhances useful light in the PAR range to promote healthy, bushy plants. This is a highly diffusing film that will also reduce greenhouse temperature.

The red ratio of the spectrum can also be controlled. "Solatrol" is a polymer that alters the ratio of red to far red light by absorbing light in the far red wavelength, removing the mechanism that can promote elongation and so reducing or eliminating the need for chemical growth regulators. There would therefore seem to be an exciting future for "green horticulture" utilising polymers.

Down gauging and diffused film

Equally excited by research opportunities is Les Lane, who owns XL Horticulture with his two sons. Better known as XLpoly, the company exclusively imports film from another family firm, Plastika Kritis of Crete.

XL's main thrust is towards thinner but stronger and longer-lasting films. "Down gauging" may cost more per tonne for film, but is cheaper per square metre. Lane is also interested in different spectral transmissions and supplies a diffused film which scatters light and gives no shadows. This also reflects near infra-red and is cooler, giving more even growth.

It also allows more winter light in - 95 per cent compared to glass at 65 per cent. Wavelengths as low as 280 nanometers are let through, whereas anything below 350 nanometers is blocked by glass. Horticultural Development Company research is showing that certain plants may benefit from this addition of lower-level UV light.

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