Polytunnels & Sundries - Stronger structures

Polytunnels and tabletops are being improved by manufacturers to withstand severe weather, Brian Lovelidge reports.

Pro Tech Marketing produces tunnels with extra long legs to enable them to accomodate cherry trees - image: Pro Tech Marketing
Pro Tech Marketing produces tunnels with extra long legs to enable them to accomodate cherry trees - image: Pro Tech Marketing

The area of polytunnels and tabletops being used for strawberry production is still on the increase. But there are pressures on this investment. Not least, the loss of EU grant aid where producer organisations have been delisted by the Rural Payments Agency, and the rising cost of steel - eight-to-ten per cent over the past 12 months - of which they are entirely constructed. Manufacturers are looking at various ways of reducing the amount of steel in their products, but they have a difficult task on their hands.

Among the main current developments in polytunnel construction is the strengthening of their supporting structure to make them more resistant to collapse due to the sort of high winds and heavy snowfall that occurred in parts of Scotland early this year. Another is making them more suitable for cherry production, which an increasing number of growers are investing in thanks to its potentially very high return per hectare.

Extra high specification

Arguably the first company to produce polytunnels with a frame with an extra high specification, reaching BS standards, is NP Structures. They are being marketed under the Robus name with two widths, 6.5m and 8m, and a gutter height of 2.7m. On particularly windy sites the frame can be further strengthened by reducing the hoop spacing.

NP Structures marketing manager Gemma Honeyman says the company believes that growers need a product that withstands winds and snow loads as directed by the British Standards Institute and European Committee for Standardisation.

National Polytunnels has also recently introduced a version better able to withstand severe weather conditions. Called the Superbay, it has box section steel legs that the company's Ken Bambury claims are significantly stronger than the conventional type. The structure has an eaves or gutter height of 4m with span widths of up to 10m and a post spacing of 4.8m.

"With this tunnel it's a lot easier to put in automated roof ventilation and with the legs about 16ft apart (much wider than normal) you get a lot more open space," Bambury reckons.

"Growers are looking for larger covered areas but you are limited for size because the bigger the structure the more difficult it is to control the temperature, although there are all sorts of add-ons to help such as automated roof ventilation and automated roll-up blinds around the sides and ends."

Polybuilds technical sales manager Jonty Swales affirms that they have experienced increased demand for new structures across the board. These have undergone considerable improvements in design and the same applies to their equipment.

For example, its automated ventilation systems now have completely waterproof integrated electric motors that slide into the roll tubes used for adjusting side and end ventilation. These motors are resistant to the water that tends to collect in the roll tubes and cause their malfunction.

"With our motorised vent system we tend to split the side curtains into smaller runs to give more control over temperature," he explains. "We believe that roof vents are really a false economy, though. If you have high enough eaves in a multispan, for example, it's best to put in louvred vents in the half-moon sections at the ends of the house."

He continues: "These are very efficient at achieving a good air flow through it. But for more positive ventilation, fan ventilation is the best way to go."

Increased automation

Haygrove managing director John Berry foresees that with the advent of planning constraints a lot of tunnels will be permanent so there will be a big need to increase yield/unit area with the help of a lot more automation. This includes automated control systems for managing, door, side and roof vents based on sensors inside and outside the tunnels for measuring temperature, relative humidity and rainfall.

"With the basic Haygrove structures that growers invested in 10 years ago, we can retrofit electrically operated doors, roller ends, sides and roof vents," he points out. "We have a gutter forming machine that we can take to farms. The gutters are used for sealing the leg rows and recycling rainwater.

"We have just launched an integrated control system that can be linked to a weather station to open and close vents according to the temperature and humidity in the tunnels," he adds. "This means that tunnels can in effect be converted into field-scale glasshouses."

With the expansion of cherry production on the back of very high fruit prices, the demand for Haygrove's specialist cherry house has significantly increased. To accommodate two or three rows of trees, the house usually has an 8m span and is 2.5m to the shoulder and 4.5-5m to the apex.

In conjunction with Andy Hunt, managing director of Clive and Sylvia Richards' Lower Hope Farms, Haygrove has developed a special cover for its cherry tunnel. The cover prevents the build up of excessive heat and humidity, which cherries do not like.

The farms grow 28ha of cherries, mostly on Gisela 5 dwarfing rootstock, in Ullingswick, Hereford. About 24ha of the crop are covered - 18ha with Spanish tunnels and 6ha with Voen covers. "The new cover is a new woven plastic material that's much stronger than pure polythene and has bird netting sewn into the top to allow hot air to escape, reducing peak temperatures," claims Hunt. "We've got 10 acres (4ha) of Spanish tunnels with this cover and we'll be increasing this area."

Ideal netting width

He has tried three widths of netting - 0.5m, 0.7m and 1m - and has settled for 0.7m. The 1m width allowed too much rain and drizzle to get into the tunnel and the 0.5m width was not effective enough, he explains.

Apart from improving the tunnel's ventilation, the new cover has delayed fruit ripening by at least a week - to give a price advantage for the fruit - compared with the standard polytunnel thanks partly to spatial filters in the cover. Its ventilating ability means that venting of the house is rarely necessary.

Pro Tech Marketing sales manager Rob Pasker predicts that the growth of both polytunnels and tabletops will slow down significantly due to the loss of EU grant aid and the rising cost of steel. The latter is being exacerbated by the loss of grant aid in Europe for rebuilding glasshouses, notably in Holland. As a result, the supply of second-hand 50mm glasshouse heating pipe, used for tabletop construction in the UK, is drying up.

"This pipe is used for making 50-70 per cent of tabletops here and it's getting harder and harder to source," says Pasker. "Five years ago we could buy it for 33 euro cents per metre, but now it's EUR1.5/m."

To reduce the amount of steel in its tabletops, Pro Tech Marketing is developing a version using much less steel that is not reliant on Dutch pipe. The company is also trialling a "V" support system comprising a steel trough on legs that can carry growbags or be loaded with substrate carrying a trickle tape. However, the latter has the disadvantage of allowing water-borne diseases to spread along the trough's entire length.

Pasker estimates that 15-20 per cent of strawberries are now grown in tabletops and, despite the constraints, the area is still increasing, mainly because growers are running out of clean land and the high cost of labour for picking soil-grown crops.

Faster picking rate

The picking rate for tabletops is much faster and the industry is continuing to lose soil fumigants to control soil-borne diseases. It is possible that the widely used fumigant chloropicrin might be the next to be withdrawn.

Pro Tech Marketing also produces a polytunnel for cherries. Depending on the spacing of the tree rows, the tunnel is 6.5m or 8.5m wide and 4m tall. The 10.5m hoops are fitted into 2.5m legs that are driven 0.7m into the ground to leave 1.8m exposed.

The firm is working on a new cover similar to Haygrove's to improve ventilation. The polythene will cover the trees and there will be a width of netting over the roadways that will also keep out birds.

Hortech Solutions managing director Richard Brown agrees that the loss of producer organisation grant aid and the rising cost of steel "are certainly not helping things". But he says while there is demand for strawberries there will be a need for tabletops due largely to the declining availability of soil fumigants. He finds that larger growers in particular are investing in tabletops, one of his customers alone having more than 40ha.

His company is also looking at ways of reducing the amount of steel used in its tabletops such as putting the growbags in a trough supported by legs. This arrangement has the added advantage that the nutrient solution run-off is easily collected for disposal.

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