Market report - Polytunnels

Growers who want to re-clad their tunnels should do so sooner rather than later, Sally Drury advises.

Plastic films are now four or five times as strong as before - image: Haygrove
Plastic films are now four or five times as strong as before - image: Haygrove

Had a quote for polythene recently? Too much? In truth, it is going up and it is likely never to be this cheap again. Growers looking to re-clad tunnels in the near future may benefit from ordering sooner rather than later.

For new builds, the price of the tunnel cover or cladding is low compared to the cost of construction. But, with the price of steel also rising, it could be worthwhile bringing projects forward. And, perhaps, it is worthwhile upping the specification you have in mind. Last winter showed the benefit of crop bars in adding strength to tunnels.

"The price of polythene has gone up and up and up, and a lot of manufacturers are saying it will go up more. Growers planning a polytunnel should think about it soon," suggests XL Horticulture managing director Les Lane. And with regard to snow loading, he adds: "Last winter I saw tunnels in Scotland with crop bars standing next to flattened tunnels without crop bars."

It is not that long ago that a polythene tunnel was one of the cheapest, most basic forms of protection a nurseryman could provide for plant production. They had low-tech two or three season covers. Twenty years on, we have plastics films that are four or five times as strong and we have smart films that manipulate plant growth, making plants grow taller, shorter or with better colouration, just by filtering the wavelengths of light that enter the structure.

Other films manipulate spectral transmission to reduce fungal disease and aphid attacks, while heat-retention films speed production for earlier harvests and a lower carbon footprint. In terms of structures we have seen an increasing number of new builds taking the form of multi-span tunnels and incorporating a rising level of sophistication that matches greenhouse installations. Field-scale operations are also moving more towards a glasshouse mentality to maximise productivity. And development continues.

At its open day earlier this year, Herefordshire-based Haygrove showed off a new electric roof venting. With manual venting of traditional tunnels typically costing between £300 and £500 an acre to operate, automation is seen as the way forward in saving labour costs and at the same time providing a more responsive system for controlling internal environments.

Haygrove's new roof vent system uses mains electric to power the vents and tests to date show that the system works over 200m of tunnel with the motor operable at one end. A 7x30mm steel purlin is attached to the hoop to provide a 1.5-2m vent size, depending on the hoop shape.

Where no mains electric is available, the ventilation system can be operated mechanically, over a length of 120m, using a Makita power drill. The drive mechanism incorporates a gearbox, the roof vent being operated via a telescopic drive shaft that works through the arc as the roller tube opens. Add electric or mechanical doors, and tunnel growers can further reduce labour costs and improve tunnel environments.

Designed for permanent sites, the new Haygrove tunnel doors can be fitted and also lifted together in lengths up to 140m from a single gearbox. These can be selected either as a mechanical gearbox driven with a manual handle or Makita battery drill to operate lengths up to 50m (six 8m doors), or alternatively feature electric operation via either mains electricity or a battery cell linked to a solar panel to drive the full 150m of 18 8m doors.

Lifting time is typically 30 seconds for the mechanical system or around 12 seconds if using mains electricity. Both systems are suitable for use with standard, windy or SMART end kits and use 170g per square metre woven laminated material offering 88 per cent light transmission and supplied with a five-year plastics warranty.

Plastic and steel systems

Further development of Haygrove guttering has resulted in 4-Series plastic and 4-Series steel systems. The plastic gutter system uses ultraviolet-treated 170g woven laminated plastic supplied in 100m or 200m rolls. It is 1m wide and incorporates a rope stitched into the perimeter.

A smaller lower-cost bracket has been designed to provide the anchoring point for the base of the gutter into the Y piece. Gutters are secured using the gutter saddle bolted into the base of the top Y piece of the tunnel leg. The material is then secured onto every hoop using a gutter clamp.

Termination is crucial with the plastic gutter system. To ensure strength, a 4-Series Gutter Termination leg is used. It is longer and made with 50x2.5mm extra-strong steel. Two drainage options are possible - at the termination leg with a drainage hole to secure the downpipe or through a down bracket secured inside the tunnel.

The 4-Series steel gutter system is formed into 2.35m long sections using 0.8mm and 750mm pre-galvanised steel. The steel gutter is attached to the hoops every 2.2m leg spacing and a steel clamp is used, which has bolts through the gutter. The clamping brackets are also used to secure the rope lugs on the inside of the gutter for roping the polythene. Drainage is managed by specifying a 2.35m steel section with a down pipe. These can be used at the ends of the tunnels or with the tunnel leg row itself if there is a low point or the need to shed water.

Tear-resistant laminated film

The latest film supplied by Haygrove is a tear-resistant woven laminated plastic. Weighing 140g per square metre, the film has a breaking strength of greater than 13kg per centimetre and provides 88 per cent light transmission. Diffused light is recorded at 45 per cent. The film can be sewn to create different sizes or shapes with ropes and eyelets, and can be sewn to insect nets.

For nearly a decade, bpi.visqueen's Luminance THB has proved popular with growers - the Smart film technology boasting temperature management, diffusion and light transmission capabilities. Technology now takes another leap forward with Visqueen Lumisol (clear and diffused). This new-generation film combines all the benefits of Luminance's thermic and light transmission properties, but unlike normal films that prevent short wavelength ultraviolet light from reaching plants, the UV-transparent film allows it to enter the greenhouse or tunnel, enhancing crop colour, taste and fragrance.

UV-transparent films promote plant antioxidants to encourage strong plants, which bpi.visqueen believes will benefit plant raisers and young-plant growers because stronger plants can be easier and quicker to handle and transplant.

Plant scientists from the University of Reading have shown that growing soft fruit under Lumisol (diffused) can lead to early picking and improve a wide range of fruit quality parameters, including the production of secondary compounds linked to human health, taste, fruit firmness and shelf life.

In commercial trails by Lancaster University and Stockbridge Technology Centre, Lumisol (diffused) has consistently produced the highest-quality propagation salad and vegetable crops, with increased leaf thickness, vegetative strength and root-to-shoot ratio. Light transmission is 87 per cent, light diffusion 90 per cent and thermic effect 90 per cent. The diffused version is offered in two thicknesses, with a guarantee for up to three or five seasons.

Thinner films, less waste

The trend for films in general is thinner, making if cheaper to transport and resulting in less waste for disposal. At the same time, films are increasing in strength.

"Throughout Europe manufacturers are producing thinner and thinner horticultural plastics," says Lane. "We already have 100-micron films that have been on trial for five years. The future is definitely thinner and it's down to Metallocene - a long chain molecule that gives it the strength. We have films that are half the thickness of old films but have greater strength."

Lane also reports that SunMaster Super Thermic covers are showing dramatic effect on inside temperature and heating-cost reduction. "For growers without expensive glasshouses and thermal screens, SunMaster Super Thermic has the potential to reduce their carbon footprint and save vast amounts on their heating bills," says Lane. It is calculated that energy reductions of as much as 30 per cent can be achieved.

The film is made by Plastika Kritis using technology developed by its master batch firm and is not available for general sale to other polythene manufacturers. The company has its own wind farm plus a factory covered with photovoltaic panels and, despite the power-hungry polythene production process, a surplus of electricity generated for feeding into the national grid. XL Horticulture is the only UK importer supplying films made by Plastika Kritis.

In addition to the heat gain in winter, crops can also benefit from the temperature-reducing properties of the diffused film in the summer months. "SunMaster Super Thermic is typically 10 per cent cooler on a hot summer's day than a clear film," confirms Lane.

The new film has anti-drip additive, giving it the same properties as AF films, but also has true anti-fog additive to reduce the number of days a fog occurs in the tunnel. Lane explains: "Thermic films keep the moisture in the atmosphere and that can spread disease. We've used an additive that halves the amount of time you would get a fog in the tunnel." This anti-fog additive is another formula exclusive to Plastics Kritis.

Impressed with the results of SunMaster Super Thermic film in trials at New Place Nursery, XL Horticulture has changed its Sterilite cladding to the super thermic formula. Sterilite cladding also benefits from a reduction in thickness - a factor that means the new formulation has not added to the price.

With tunnel covers thinner than ever before yet lasting longer, combined with ideal growing conditions for faster and better-quality crops, the green credentials of growing under plastics are improving. What is needed now is an economic means to recycle the old film.

Many manufacturers are happy to take back old polythene but spent tunnel covers are classified as waste material so require transport under a waste licence - a not inexpensive process. Talks are underway to have the legislation clarified or possibly changed. It is hoped that, possibly by next year, it will be feasible to recycle horticultural polythene.

MCGRANE ADDS SOPHISTICATION

Showing the trend towards more sophisticated multi-span poly-clad houses, Northern Ireland bedding and pot plant grower McGrane Nurseries put a Keder construction at the top of its list when investing in production facilities last year.

After visiting an existing Keder user, nursery director Stephen McGrane knew the Keder met with his requirements. "Keder has done a very good job and we are very pleased with the construction," he points out.

The new house, which features frost protection heating and overhead irrigation, covers one acre. It features the Keder roof vent system to ensure high levels of air flow and enable hardening-off of plants in situ.

This saves on labour costs for plant handling and movement while giving a high-quality crop.


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