Polytunnels - Undercover option

Better polythene and higher structures are making covered growing increasingly viable for open-field crops, Gavin McEwan reports.

Altus: format based on a high glasshouse framework but with an ultraviolet-open polythene roof rather than a glass apex
Altus: format based on a high glasshouse framework but with an ultraviolet-open polythene roof rather than a glass apex

Latest improvements to polythene films have made covered growing a more attractive option even for crops that have so far largely been grown in open fields. But according to suppliers, new higher structures are also key to creating an optimal environment for their growth.

"Ventilation is now the name of the game," says Graham Van der Hage, director at supplier Polybuild. "Plants want ambient outdoor conditions for 75 per cent of the time, so we work to achieve the most similar environment inside as they would have outside for that 75 per cent of the time."

In a covered structure that means going for height, he explains. "With greater air volume, changes in temperature are less sudden so the plants are less likely to suffer thermal shock."

Some of these new taller structures are actually replacing old glasshouses, he says. "We now have gutters up around 4m high — we have done one or two of these around the country, though we still also do them at 2.5m."

Strength issue vital

A stormy winter has put the issue of strength at the forefront of growers’ minds. "But with modern manufacturing methods, they are built to take it," says Van der Hage. "The roofs are shaped like aeroplane wings. All they need is enough concrete in the ground."

With improved materials and designs the structures are getting more durable, he says. "Product guarantees have gone from three to five years and the films are now pushing six years before you need to change them, as manufacturers have found better ways to avoid ultraviolet degradation. We always put anti-hotspot tape on the hoops though, which can get very hot and make the polythene brittle."

Alongside these are advances such as motorised wind-up sides, which can be fully automated or controlled by a simple rocker switch. "Twin skins on the roof and at the ends will save you 60 per cent of your energy, as well as further reducing the risk of thermal shock to the crop."

Support for higher formats comes from ongoing US government-funded research at Kansas State University, where researchers studying tomato and spinach cultivation claim not only to have verified the belief that growing produce in high tunnels boosts production but also to have shown that crops produced this way have a longer shelf life.

Cary Rivard, director of the university’s Horticultural Research & Extension Center, says: "People have known for a while that you can improve production by growing in high tunnels, but our findings showing that produce grown in high tunnels has a longer storage life and longer shelf life are something that have never been reported in the scientific literature."

A new project at the facility this year will investigate the role of light in crops’ phytochemical production and evaluate the role of new plastic films as well as LEDs in this. "Our goal is to increase the nutritional quality of crops grown in the high tunnel," says Rivard.

NP Structures is another supplier advocate of taller tunnels. Commercial director Nigel Carr says: "We are tending towards higher structures that are less susceptible to sharp fluctuations in temperature, since a bigger volume takes longer to heat and cool down."

New format developed

To this end, the Lancashire-based supplier has developed the Altus format (Latin for "high"), based on a high glasshouse framework but with an ultraviolet-open polythene roof rather than a glass apex, Carr explains. "We think it provides the benefits of both."

So far produced only as a scale-model prototype, it will initially be aimed at the fresh-produce sector but also of interest to ornamental growers. "This is viable for half an acre [2,000sq m] upwards — it wouldn’t work on a smaller scale. It will be available in a range of heights up to 4.5m to the gutters. Using the latest software we specify it to withstand UK-wide snow and wind loads — it meets EN glasshouse standards. We hope to have a saleable product this summer."

Such developments have been helped by the recent development of affordable ultraviolet-open films, he adds. "There are more salad growers going under polythene and there have been some big projects recently in the UK and Ireland. For a crop like lollo rosso it used to be hard to get them to colour up, but with ultraviolet-open film you get strong red leaves."

Carr continues: "We are working with a photobiologist who is developing film that will help to break down pesticides." But he points out that this is still "a number of seasons away" from being commercially available.

National Polytunnels is another supplier benefiting from the spread of covered salad growing, says sales manager Garry Summerfield. "We are doing large areas for leaf growers, especially around Evesham, where they are looking to increase their covered area including for crops like pak choi. We have been doing projects for 21ft- and 26ft-wide multi-span houses with side ventilation, covered with BPI Visqueen clear or diffuse film, which both installers and growers like."

These are often designed with large open ends to give access to larger field-type machinery, he adds. "For growers, glass is an alternative but it comes down to cost," he says. "Also with tunnels you can alter what you do with them by changing the polythene."

Programme develops cladding material

A "third generation" of polyethylene greenhouse cladding materials also suitable for polytunnels is being developed in an extensive three-part £750,000 project funded by the Government’s AgriTech Catalyst research programme.

The novel spectrally modified films are intended to give yield and quality gains, and even to reduce pest problems, principally by scattering ultraviolet and other light wavelengths while reflecting near infrared, within an affordable product.

The University of Reading will lead on developing materials, which will be manufactured by A Schulmans and British Polythene BPI, while the University of Lincoln will model the interaction between material scattering pattern and crop light interception, informing the material development as well as structure design.

Reading and East Malling Research (EMR) will determine the impact of ultraviolet, diffuse PAR and NIR on strawberry crops, with Reading focusing on crop responses and EMR on insect behaviour. Growers Berry Gardens, Haygrove and Finlays will then test the most promising materials under commercial conditions in the UK and overseas.

According to the project abstract: "The materials developed within the project will be exploitable within one year of its end," which falls in November 2018, after which an "incentivisation package" will support Berry Gardens and Finlays in purchasing the product.

It adds: "The improvements to food quality and yield that these films offer are expected to have considerable impact on the retail sector in terms of improving produce quality and through lower unit costs via increased yields. Reduced pesticide use will be attractive to consumers, who will also be beneficiaries in terms of improved food colour and flavour and reduced prices."

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