Always a breeze in greenhouse Van der Avoird, even when there’s no wind

Very distinct greenhouse sizes needed for hardening off
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Always a breeze in greenhouse Van der Avoird, even when there’s no wind

Van der Avoird Trayplant, in the south of the Netherlands, made such specific demands on the greenhouse ventilation that the builder’s computer program was not able to figure out the optimal design. Therefore, based on estimates, a unique greenhouse complex was built that appears to work well. The design of the new site was carried out together with the local residents.

The 1 ha greenhouse looks very normal but looking at its crossways you see two greenhouses each 48 meters wide and 135 meters long. In between these long narrow greenhouses is a four metre wide strip of grass. Next year another such greenhouse will be built along side, again separated by a strip of grass. As well as the vents in the roof, the sidewalls can also open for maximum ventilation.

Risk assessment

In 2006 the nursery that specialises in tray plants for soft fruits built a greenhouse for raspberry mother plants and cuttings. “We had to invent everything ourselves and couldn’t copy any other young plant propagator. It was the same when we built this greenhouse last year. That makes it difficult for the bank because they can’t refer to risk assessment data from anywhere else. That makes them cautious and it’s the reason why we have to build in phases," says owner, Peter van der Avoird.
His company comprises five locations in the province of North Brabant, which together total 31 ha of tray plants and 5 ha of glass for the propagation of raspberries and strawberries. The company is the largest producer of raspberry cuttings in Europe. The greenhouse serves to bring forward the season and to better manage the production process.

Discuss with the neighbours

Sales of soft fruits are growing in Europe. The demand for blueberries, raspberries and blackberries is significantly greater than the supply. The production areas are growing strongly and the propagator is growing steadily too, about 15% annually. However, on the older locations Van der Avoird has reached its limit for expansion. “Legally we can expand in the Molenschot area but the neighbours and council wouldn’t be happy about that. Running a business in the Netherlands increasingly requires you to look around the local area to discover new possibilities,” he says.
This was how he came to find a plot alongside the motorway in the neighbouring village of Bavel; grass and corn country but with a building permit. “We could have just gone ahead and built but we felt it was important to first communicate this with the surrounding residents. I personally visited all the neighbours to explain our plans and asked them to respond to our rough sketches. This did have consequences on the location of the office, other buildings and the green area,” he says. One neighbour wanted to have the office opposite his house; another neighbour preferred to have a fence instead of a green strip and a third wanted a piece of land to be left to nature. “This all worked out well. I wasn’t bothered about the exact location of everything. We could easily move things around on the plans.” The result was that none of the local residents objected to the plan, a unique situation.

Maximum ventilation

The building in Bavel started two years ago and will take four years to complete; within two years the site will have been expanded to 20 ha, and will comprise 13 ha of field trays for raspberries, 3 ha of field trays for strawberries, 3 ha greenhouse and 1 ha of stores and office.
The greenhouses required the most creativity because there was no previous practical experience on which to fall back on. The raspberry cuttings are rooted in the warm greenhouse (of 1 ha) and after two weeks are moved to two cold greenhouses which if necessary can be heated. Here they are hardened off. This means they have to get as much air as possible. “Our most important period for delivery is April/May. You can harden the plants off to meet this deadline but because of our volumes it is too big a risk: Just one bit of frost is fatal. Therefore we need to have a greenhouse that we can heat now and then as well as have maximum ventilation,” explains Van der Avoird. This last requirement caused many headaches and brain storming sessions with the builder. One possible option was a cabrio greenhouses; a greenhouse whose entire roof can open: This was not suitable concluded the grower after making lots of calculations. “Even if the roof is completely open you still have a 7 metre high glass wall. When there is not much wind there is far too little air movement.”

Guesswork

Discussions with the greenhouse builder Van Amelsvoort and advisor Looije Agro Technics led to a unique solution: Both the roof vents as well as the side walls of the long narrow greenhouses could open. This combination, with the open space between the two units, created a chimney effect. As a result there is always a breeze in the greenhouse even when there is no wind.
The problem was that such an unusual solution didn’t fit into the builder’s calculation program. What should be the exact size of the greenhouse? How much energy is lost through the large number of seals in the sidewalls? What width should you make the corridor between the greenhouses? “You can calculate everything for a square greenhouse of 5 ha. Here we had to make assumption after assumption after assumption. In the end the dimensions were mostly guesswork. It did lead to some surprises: Initially we hadn’t planned to have an energy screen in the cold sections but the heat loss was much larger than expected. Therefore we had to install one later.”

Optimal utilisation

The conclusion after one season is that in practise it works perfectly. Therefore they will go ahead with the planned third cold section next year in exactly the same way. “It is a very expensive solution,” he says. “It means we have to optimally utilise the greenhouse space by having at least three crops per year.”
Also the greenhouse equipment is quite expensive. The heating, as well as a ground net with ethylene hoses every 25 cm for above ground heating is hardly used. For 3 ha of glass he has a connection for 160 m3 and that is already an over capacity. The energy screen is hardly used. “For the average temperature in April we don’t need any heating. Actually the energy screen and the heating are a kind of insurance. But we do need them, otherwise we could run into big problems. It occasionally happens that we have to use heat on one day and the next day open everything up to maximise the ventilation.”

Summary

The new greenhouse built at Van der Avoird Trayplant is very special: long, narrow and all the walls can open up to achieve maximum ventilation. It is an expensive but effective solution. The company currently comprises 31 ha of field trays and 5 ha of glass for the propagation of raspberries and strawberries. The business continues to expand. Before building on a new location the owner contacted all the surrounding residents.

Text: Tijs Kierkels. Images: Wilma Slegers

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