Polytunnels - tougher tunnels

With polytunnel use becoming standard, advances are focusing on sturdier designs to cope with a longer growing season, Brian Lovelidge reports.

An estimated 90% of UK strawberries are grown under protection to help ensure the reliability and continuity of fruit supply - image: Brian Lovelidge
An estimated 90% of UK strawberries are grown under protection to help ensure the reliability and continuity of fruit supply - image: Brian Lovelidge

Polytunnels have become an essential part of strawberry and raspberry production over the past 10 years or more. Without them there would be no soft fruit industry, despite local opposition in some areas.

A small number of growers, mostly those with pick-your-own units, still produce soft fruit without protection. But the depression of yield and quality due to adverse weather means that they can lose a large proportion of their income at times.

The weather-proofing of picking, whether the job is done by pick-you-own customers or employees - invariably seasonal agricultural workers - is an important factor in favour of protected cropping. Another is the extension of the harvesting season, for growers serving supermarkets in particular. For these outlets, the reliability and continuity of fruit supply are absolutely vital, preferably from UK sources wherever available.

So it is not surprising that an estimated 85-90 per cent of the UK's 6,770ha or so of strawberries and raspberries is grown in polytunnels, while for growers serving supermarkets the figure is more or less 100 per cent.

Probably some 75 per cent of the strawberry crop is still grown on raised beds, with tabletops accounting for the rest. This proportion is steadily increasing, despite the high investment involved, largely because they significantly reduce picking costs and eliminate the need for soil sterilisation.

Design sophistication

Although most polytunnels are the standard Spanish type, their design is becoming more sophisticated. For those adopting out-of-season cropping, for which the structures have to withstand high winds and heavy snow loads, sturdier metalwork is being used. This applies particularly to northern England and Scotland, where the harsh 2010-11 winter caused severe damage to polytunnels.

Their widespread use means that their assembly, dismantling and covering have become major operations. As many as 90 per cent of growers carry out these operations themselves. However, 10-15 per cent of the tunnels sold by Haygrove Tunnels, the largest of the UK's three or four major suppliers, are offered with a full construction service.

The basic 7.8m-wide Spanish tunnel covering five raised beds or six tabletop rows, generally considered the optimum size, takes 400-450 man-hours to assemble by an eight-strong team. The polythene comes in 10.5x800m rolls weighing 1.5 tonnes. It has a guaranteed life of three years but generally lasts for four or five and even longer in Scotland, thanks to lower ultraviolet light levels, or in the case of cherries because they are usually covered for only 12 weeks. Materials cost £20,000-£25,000 per hectare and the French type a bit more. Growers wanting winter protection pay a premium because the steel is of a higher specification.

Haygrove produces a range of equipment to speed up tunnel construction. It includes various sizes of hoop-carrying machine - the largest holding up to 400 hoops - a specialist drilling machine that makes the holes for as many as 1,000 legs a day, a polythene-handling machine and vertically adjustable working platforms.

The company's latest machine extrudes and fits gutters in situ to existing polytunnels for rainwater collection. Without gutters, heavy rain can result in considerable water splash that can lead to soil contamination of fruit along the outside of the leg row beds and the subsequent development of Botrytis and loss of quality.

Osprey Tunnels general manager Dave Leslie expects the expansion of the polytunnel area to continue but at a slower pace than a few years ago. The company, based in Perth, is probably in a healthier position than most because it has developed a strong export market across 14 countries. Further polytunnel expansion in the UK will come mainly from blueberry and cherry growers thanks to demand for their fruit being well ahead of supply, he says.

Osprey produces a range of tunnels in addition to the Spanish type, which is no longer its biggest seller. It has been overtaken by its Kosy tunnel, which has the advantage of achieving higher growing temperatures. This makes it particularly suitable for early- and late-season strawberry and raspberry production.

The Kosy tunnel's temperature is raised because the covers come down to soil level and are held in place by ropes stretched over the covers and anchored into the ground. If the temperature rises too far, the cover can be edged up between the rope and hoops for ventilation.

Growing demand

Demand also continues to grow for Elite Tunnels' range, claims Brian Watt, a director of the Brechin-based company. Units are sold throughout the UK. Its biggest seller is the Spanish tunnel but the Envirotunnel also meets a strong demand. The cover edge fits flush with the ground to improve heat generation and retention and a special door kit is available to seal the ends.

"Our market has been constantly expanding and we're very busy at the moment," says Watt. "We work closely with leading soft fruit growers to identify new trends and developments and this enables us to stay at the forefront of innovation."

Among Elite Tunnels' co-operators are Murray and Ross Mitchell of Fordoun, Aberdeenshire, winners of 2009's Soft Fruit Grower of the Year award. The company supplies devices to aid and speed up tunnel assembly including a leg-screwing machine than can be mounted on a mini-digger. In addition, it has a device that bends the hoops on site, for which it employs a specialist to help growers with the procedure.

The case for tabletops

Both Elite Tunnels and Osprey Tunnels produce tabletops. However, Osprey general manager Dave Leslie reckons that they are only suitable for growers who want to permanently site their tunnels near their packhouses or do not have sufficient good-quality, clean land for rotating their crops.

West Midlands ADAS soft fruit adviser Chris Creed says tabletops can stay on the same ground for 15-20 years. He finds that growers generally get higher yields, although they would be hard-pressed to beat the yields produced by crops on raised beds on the best-quality soils.

The big bonus with tabletops is that picking is about 20 per cent cheaper because the fruit is far more accessible, he maintains. But it is not just their ease of picking, he asserts. They make picking a far more attractive job - an important factor because good-quality labour is getting more difficult to attract.

Some advisers are even more bullish about the picking advantage of tabletops, putting the savings as high as 60-70 per cent. This means that two people can do the work of five or six on raised beds and so recoup a substantial part of the capital investment in just one year by savings in picking alone.

Also much in their favour, tabletops obviate the need for soil sterilisation, which generally is essential unless growers have an unlimited supply of clean land of the right quality. Even with sterilisation, the threat of Verticillium wilt is not completely eliminated.

Furthermore, the other major problem for strawberries is more effectively controlled with nematodes in substrates, although they can work reasonably well in soil under favourable conditions.

Not surprisingly what tends to put growers off tabletops is their high set-up cost, although the pros almost certainly outweigh the cons when compared with raised beds. The cost of the supporting structure can vary depending on the deal growers can negotiate with their suppliers, says Creed. He finds that with a 6,200m row length per hectare, the average cost is £40,000 per hectare.

The cost of bags at £1.30 per metre or £8,000 per hectare must be added to that outlay. Unlike the tabletops' steelwork, they need regular replacement. Most growers get two-to-four crops from bags before they need replacing thus significantly reducing their annual cost. Multiple use is easier with coir bags because unlike peat it does not slump after several cropping seasons.

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