Larger and taller polytunnel structures needed

The demand for larger and taller polytunnel structures is increasing as the search continues to create optimum conditions under polystructures.

Advancements in both the structures themselves and the film that goes on them have made buying a polytunnel an even more effective and efficient way of growing, displaying and covering crops.

Films especially have developed at a fast pace with clear, white and diffused films being just some of the options that have been produced to allow different light and heat levels inside a polytunnel. Other changes have seen massive improvements in UV levels and anti-drip coating as well.

Meanwhile structures themselves are being developed to meet the demand of growers wanting taller and bigger polytunnels, and work has also been done on ventilation and heat reduction to help create the optimum conditions inside.

“There is certainly a continuing trend towards growers asking for bigger areas of polytunnels,” Chris Roland of Fordingbridge, tells The Commercial Greenhouse Grower. “Lots of growers are asking us for bigger areas and also more access into the polytunnels as they look to introduce more automation. This is due in part, to the on-going rise in labour costs, and has meant closer attention needs to be paid to the end and side access points. We are also finding that there is a strong demand for multi-purpose tunnels where growers can grow, sell and keep their products, especially in the ornamental sector.”

Mr Roland says while some growers are demanding larger polytunnels, many are opting for smaller structures but more of them over a larger area. “This might mean slightly more expensive up-front costs but the maintenance cost for smaller structures will be less than larger ones over time,” he says.

Fordingbridge work with all the leading manufacturers of films including Visqueen and XL Horticulture and he agrees development has been ‘impressive’ over the last decade. “In actual fact, we have come full circle with films. There have been, and still are, many options available but growers are mainly choosing between clear, white and diffused because they offer so many benefits these days and have become universal.”

Elsewhere, Nigel Carr, Commercial Director at NP Structures which incorporates Northern Polytunnels says there is a clear demand for higher structures at the moment. “Many of our polytunnel enquiries are for higher structures. Our standard height to the underside of the gutters used to be 2.2m, but we are increasingly supplying polytunnels which are 2.5m and 2.7m to the gutters. We have also seen an increase in the size of the enquiries, with a significant number of projects between ½ acre and 5 acres,” he says.

This means closer attention needs to be paid to ventilation. “Large structures require good ventilation. The development of fully automated roll-up sides, along with polytunnel roof extraction fans have allowed Northern Polytunnels to remove the human element from temperature control, thus removing labour costs and creating a more harmonised environment,” says Mr Carr. “Heated polytunnels for soft fruit production to extend picking dates from March through to December, are another notable development, and by working closely with growers we’ve been able to develop a cost-effective structure which allows just this,” he adds.

But how far can polytunnel design really go in the future? “I suppose the holy grail for polythene manufacturers would be to produce a photochromic film which reacts to light. So, on bright sunny days in summer it diffuses light to reduce greenhouse temperatures and spreads the light more evenly, yet becomes clear on dull cloudy days (especially during the colder months) to take full advantage of the limited light which is available. I’m not sure where the boffins are with this, or even if it’s economically possible, but this kind of technology has been used in sun glasses for years,” Mr Carr tells the Commercial Greenhouse Grower.

So, what research is still needed in the sector? “If you gave me a significant amount of money I would look at developing photochromic films, but with less, I’d probably look at entering into a partnership with a grower to design a polytunnel suitable for the production of high value salad crops (tomato, pepper, cucumber etc.) which are currently grown in glasshouses,” he adds.

XL Horticulture has worked hard on developing spectral filters, being the first with UV open, UV closed and Pr/Pfr manipulation films. Their latest examples which are creating a lot of interest are their other new 7 layer films.

XL Horticulture’s SunMaster CrystalTherm, for example, was developed as the first horticultural film in the world to be made from seven layers. It also has a permanent anti-drip system for the life of the film which offers ‘excellent’ protection against condensation and an ultra smooth external face to repel dirt and delay the onset of algae. It is immensely strong, but the clarity is similar to glass and is suitable for most horticultural crops. Even though the film is only 160 mu it is claimed to be stronger than competitors’ 180 or 200 mu film and has been tested to a puncture resistance of 1200 gms, a strength at break test of 30 mpa and has a stretch before break result of 640%, according to the company.

The SunMaster SuperThermic gives extra frost protection also. Normal polythene and glass block the UV below a certain level (350nms), but SunMaster film are claimed to give you the advantages of growing indoors without losing the benefit of the outdoor sunlight.

According to XL Horticulture, plants are slightly more compact, colours are more pronounced, particularly reds and blues and you get stronger rooting and higher ani-oxidants in fruit grown under SunMaster films. The outdoor levels of UV give thicker cell walls and therefore greater disease resistance.

In the Netherlands, VDH Foliekassen reports a continued high demand for greenhouse structures with plastic. The advances in film technology mean that if you chose the right structure and film, then polythene can be just as effective as glass, says the company.

VDH poly greenhouses are claimed to be ideal for most type of crops, as well as young plants and flowering and bedding plants. The company says that crops grown within its greenhouses are proven to be generally more hardy with ‘more compact habit and more intensive colouring.’ VDH use thermal anti-condensation plastic films from 0.18mm to 0.22mm thickness for all its greenhouses. The light transmission ranges from 88% to 92%, depending on the type of film. The films also have UV stabilisers while low thermicity means the heat is retained inside the greenhouses, especially important so not to allow too much cooling down at night.

Developing ‘smart films’ has been a priority for BPI which produce the Visqueen range of horticultural films. For almost two decades Visqueen has championed the development of SMART films through ongoing research sponsorship at the UK’s leading plant science universities.

The resultant project outcomes have delivered a pioneering Visqueen range that includes temperature control films; films that manage a crop’s response to red and far red light, and films that actively manage UV light transmission.

Visqueen has, in parallel, continued to invest in its manufacturing infrastructure to ensure that the characteristics of its SMART film range are optimised for the benefit of growers on a continuous basis.