Anything less than the best will not do for the biting climate of Arbroath in Scotland. Second best, for example, blew down almost immediately when the weather-blasted growers at LM Porter tried to put up polytunnels to protect their soft fruit from the winds barrelling in from the North Sea.
The grower of strawberries, blackberries, raspberries and blueberries must have its best polytunnels. It covers 70ha with structures, so innovative polytunnel design is crucial for keeping Angus Soft Fruits - the supplier group of which the farm is a part - in ready supply.
LM Porter needed a more robust, well-sealed tunnel that would more than double the production period for soft fruit and it took trial and error. The first effort, an off-the-shelf multi-span contraption of 32mm-diameter tubes in 1.5mm-thick metal, was not up to all those winds smacking into the 130-micron, three-season polythene. So farm manager David Warden and his team perfected their own system to withstand the rigours of the Scottish climate.
They came up with a Spanish tunnel-type structure, built like a tank. Spacing of hoops went from 2.2m to 1.2m to give super strength. Meanwhile, the 180-micron polythene was like armour plate to that wind and much thicker than the flimsy old stuff used previously, now lying somewhere in a crumpled heap.
"A five-acre block that produces 10 tonnes of fruit becomes one tonne if you get an inch of rain," Warden explains. "Good polytunnels trebled our business. The old season started at the end of June and finished late August. Now it starts in the first week of May and runs to the first week of November. You can pick large volumes of fruit over a longer period, utilise your workforce better and consistently supply supermarkets over a longer period."
LM Porter also wanted a model for mid-season crops, so Warden's team went back to a multi-span tunnel but with 40mm-diameter, 2mm-thick steel legs spaced at 2.2m. This Haygrove-designed telescopic system can be raised and lowered to regulate airflow and comes with so-called "smart ends" - strengthened front and backs for added sturdiness. It does the job.
Warden says polytunnel technology is bolder and more exciting, with increasingly snazzy automated doors and venting. But just as important as front-end technology are the small "tweaks and on-farm development" needed to tailor a system to your personal needs. LM Porter's needs are great - it grows 1,500 tonnes of strawberries, 250 tonnes of raspberries and 100 tonnes of blackberries.
The big test came this winter, and LM Porter was fortunate. Most of the polythene was off the hoops when snow and frost were at their worst. But other more inland sites within the group were not so lucky and 30cm of snow and -8 degsC temperatures caused damage.
Growers can overwinter crops away from polytunnels, says Warden, but this is expensive. Tunnels also throw up their own problems and are no guarantee to crop success, he adds, referring to the hotter temperatures and higher humidity within poly enclosures. William Aveling, an asparagus grower in Cambridgeshire, does without polytunnels because his land is stodgier than the free-draining sandy soil needed in colder weather when the tunnels go up.
But Asparagus Growers' Association chair Andy Allen says the £3,000 cost per acre of covering his crop is worth it so he can meet the needs of his farm shop early in the season. He uses 400mm-high by 800mm-wide compact Spanish tunnels for 8ha of land. A short seven-week season calls for a flexible, low-impact tunnel that is easy to put up - and take down.
Other growers need more sophistication, says National Polytunnels technical illustrator and 3D artist Kevin Bambury, who is surprised at how many people fail to understand the importance of the right fabric. Some growers go for less sturdy 500- or 600-gauge fabric to keep costs down, but the Visqueen films his firm supplies are between 720 and 800 gauge.
Luminal 720 gauge, for example, is an anti-fog cover that comes with a five-year guarantee to help salad crops and strawberries fight against condensation problems and Botrytis. The polymers used to make these fabrics ensure that they are more than thick and strong. Films today can block out ultraviolet radiation, while ultraviolet-open materials can affect the colour of crops and early growth habit.
National Polytunnels recently launched the Giant Cloche, a 21m-long tunnel up to 2.6m high and 6m wide for young crops. Polythene sides that roll up to the roof apex totally expose plants to the outdoors. Bambury explains that this saves growers "ferrying" plants in and out of the tunnel to maximise weather conditions.
"It has been designed to solve one of the biggest shortcomings - lack of a flexible, responsive ventilation system," he says. "Mesh skirts provide only limited air circulation and there's the problem of moving plants in and out as the weather changes. The secret with this cloche is in its twin steel arches, which act as a track for the side curtains to go up and down on their rollers."
Ventilation is a big issue for Stephen and David McGuffie, brothers who grow strawberries and raspberries under 30ha of polythene in Lichfield, Staffordshire. Venting at the right time is critical but a huge labour problem with a climate as fickle as ours "throwing three seasons at you in one day", says Stephen McGuffie. He may look at upgrading to auto vents.
The need may be even more pressing in years to come because this year the McGuffies go from trial to commercial planting of melons. Experienced in the use of tunnels for soft fruit and asparagus, they adapted polyframes to produce melons up to 4kg in weight. The fruit is grown in 7.5m-bay tunnels with 2m-thick steel and 150-micron plastic.
"You can't grow melons without polythene - we tried and it didn't work," say the McGuffies, who are more than doubling their growing area to 4ha this year. "Melons need high temperatures and just enough ventilation to reduce humidity, but less throughput of air than that needed for strawberries. We haven't got round to trialling different types of polythene yet."
So he may like to visit the Haygrove site in Ledbury in early March. The grower and polytunnel specialist is expecting 200 visitors to its three-day open house, where ventilation and fabric are at the top of the agenda. Businesses like the McGuffies' can spend between £300 and £500 an acre manually ventilating crops in traditional polytunnels, says Haygrove managing director John Berry.
"The ranges we are launching are all retrofittable and as well as venting the sides and end you can buy electric doors and roller slides. Lots of this has been done on a smaller, trial-type scale, but when you have several hectares under polythene you want a system that regulates opening and closing based on times of day or temperatures."
Berry is also looking forward to a few surprises in fabrics that will add novelty to an industry that has relied for too long on bog-standard polythene. He is coy about product details before the launch, but the woven laminated fabrics will have a life of around twice that of standard polythene, which needs replacing every three or four years.
It will also share the kind of hi-spec technical qualities of innovations such as Luminance THB, which can scatter light and moderate temperature. Customers want longevity and permanence, says Berry. Growers, keen to maximise productivity, are moving away from a "field scale to a glasshouse mentality", focusing on input per square metre.
Loss of soil sterilisation products, meanwhile, means there is less clean soil for growers, who are choosing instead to grow on artificial substrates under cover. Planning rules have also pushed growers, who once moved temporary polytunnels around the farm, towards more permanent structures. More rigid rules mean structures stay in their designated place.
BPI Visqueen, which part-funds PhD students to lead on research and development, is about to launch Lumisol, a tougher version of its existing Luminance fabric. The new version screens out heat but lets through the type of ultraviolet light that enhances crop taste, colour and fragrance, says UK sales manager Andy Barber. The "UV Window" is cooling and light-diffusing, he adds.
"Growers are becoming more educated in the potential of polytunnels and now realise they do more than protect plants from rain and are scientifically oriented with agronomic benefits," he adds. "Changing climate demands that we continue to look at R&D and evolve new additives. It has ensured most of the new products on the market have more credibility.
"But it's still often a case of 'suck it and see' in terms of the effect of polytunnels. We can make claims about the possibility of yield increases and enhancing flavour of crops, but the growers need to try it themselves."