Polytunnel films are advancing on several technical fronts, offering growers longer seasons, improved yields and better crop quality. Being more durable, they are suited to more permanent structures, meaning lower replacement costs over time.
One notable recent arrival in this market is XL Horticulture’s SunMaster Cool, an entrant in next week’s UK Grower Awards at London’s Grand Connaught Rooms. The result of eight years of research and originally developed for the nursery stock industry, SunMaster Cool reflects back infrared radiation thanks to the incorporation of micro-fine aluminium particles. This reduces summer temperatures by up to 25% while letting more light through than "white" reflective films.
"There has been a vast amount of technological development in plastics over the last 20 years, not due to horticulture but to all the other things like car parts, many of which used to be made of glass or metal," explains XL Horticulture managing director Les Lane. "These new polymers and copolymers give new possibilities. It’s technology-driven."
Horticultural polythenes "used to be three layers, then five-layered films began appearing", he adds. "But we have developed a seven-layered film with the manufacturers as part of a €12m investment there, and can vary the thickness of each layer." XL’s Greek partner Plastika Kritis "is big enough to do this because they have a €134m turnover and are active in 62 countries", he points out.
New film in use
The new film is already in use at "half-a-dozen" UK blueberry growers including one 5ha site in Scotland, says Lane. "A lot of fruit growers were asking us for a cool film. Now they are using it to schedule their cropping. It allows them to have a fresh crop when the crop under conventional film alongside has already been picked. You can hold blueberries in a cold store but they won’t be as fresh."
It also offers thermic properties, moderating temperature differences throughout the year. Lane explains:
"Polythene doesn’t insulate and normally it’s transparent to short-wave infrared radiation while letting long-wave infrared through. But thermic films additionally reflect the long-wave infrared back into the tunnel. So as well as being cooler in summer, it keeps in more heat in winter, which for the grower means it brings spring forward."
SunMaster Cool has a total of 45 worldwide patents including for its anti-condensation additives, he says. "Condensation droplets can spread disease in a tunnel, especially with strawberries." Such coatings have so far taken the form of surfactants, which prevent droplets forming on the film, he adds. "But this doesn’t allow condensation to settle at all, so it’s not there to start with." This "super-hydrophobic" coating "is not affected by age and should last for the life of the film".
No cleaning in future?
On the outside "we are trying to make cleaning unnecessary, though we aren’t there yet", says Lane. "Normal polymers are quite rough but the ones we use are unique to us and give it a super-smooth surface, so dust and fungal spores, which would block the light and degrade the film over time, just wash off."
XL says SunMaster Cool costs no more than an ordinary thermic film and has a higher puncture resistance than any other on the market. Its 160mu thickness has a design life of 10 years and a six-year warranty.
Another film up XL’s sleeve is the SunMaster Crystal Thermic, which has 93% light transmission, compared with 94% for glass. "It’s virtually as clear as greenhouse glass as well as being ultra-smooth and UV-stable", and "would benefit pretty much any crop", says Lane. "We know that early strawberries, for example, need light as much as heat." It is already being fitted as standard by French glasshouse and polytunnel supplier Richal Serre.
At last month’s International Soft Fruit Conference in the Netherlands, horticultural engineering expert and retired senior researcher at Leibniz University of Hannover Dr Burkhard von Elsner’s recommendations on protected growing under film appeared to support XL’s approach.
"Avoid high air, leaf and fruit surface temperatures to prevent heat stress and sunburn," he advised soft-fruit growers. "Because of restricted ventilation, the inside air temperature of a polytunnel exceeds the outside temperature by up to 10° under full sunlight, while the inside air velocity is reduced to about one-third of outside.
"Coloured screens may influence physiological and morphological development of plants but more research is needed because the transfer of results from other locations may be inappropriate." Growers should "prefer plastic film with high UV-B transmission".
On this last, point Lane comments: "All except our Sterilite films are UV-open. Glass blocks wavelengths up to 350nm, but outside you have wavelengths down to 280nm." Strawberries grown under such light "will root better, be ready quicker, have higher Brix and antioxidant content, and will have thicker cell walls better able to resist disease", he says. "We only do smart films. It’s the way to go."
Structure: protects against wind, rain, frost, sun and birds while allowing opening for ventilation purposes - image: Fruit Security Holland
Wisel and Powerflex: Foil systems offer alternative to conventional polytunnel structures
Growers attending last October’s National Fruit Show were presented with an alternative to conventional polytunnel structures for fruit growers and tree nurseries that its supplier says allows easier ventilation while still affording the same protection from the elements.
Fruit Security Holland is the regional supplier of systems designed and manufactured by Fruit Security of Austria, which distributes its Wiesel and Powerflex Foil systems throughout Europe.
Its owner Gerben van Veldhuizen, who is also president of the industry research body Dutch Fruit Group, says: "Depending on the combination of net and film, it’s protection against wind, rain, frost, sun and birds. Polytunnels will do this too, but the problem you have with polytunnels is how do you open them to ventilate?"
The format was developed in Austria originally to provide frost protection, van Veldhuizen explains. "They have a lot of problems with frost so they do a lot of testing and innovation for frost protection on a dedicated 2ha site. From that we change little details each year. We are always researching things like clips. In Britain and in the Netherlands you have a lot of wind so you need movement. We even have an automated system that you can open from your mobile. Especially for blueberries, you want to be able to open and close the roof very fast."
Fruit Security now also offers fully enclosed systems that are able to keep out fruit pests such as Drosophila suzukii, which have devastated fruit production in some areas across Europe. "Especially for cherries, the whole system has had to change," he points out. Van Veldhuizen puts the cost of materials at €35,000-€40,000 per hectare — around €4 per square metre, though this drops to around €1.50 for hail netting only.
"In the beginning this was about hail, but there are wider economic reasons. You can have insurance for when you have damage by the weather, but then you are out of the market for that season and it can be hard to get back in, so it’s better to protect."