Next Generation Growing has played a major role in the choices Dutch orchid grower Rob Olsthoorn of OK Plant is making for the new greenhouse he is building in Naaldwijk. “We want to grow in a closed environment as far as possible. The three screens in the greenhouse are helping us achieve that.”
The bottom screen is a transparent, energy-saving fabric. The middle screen is a shading fabric with an open structure which shades 55% of the light. For the top screen, Olsthoorn opted for a light reduction fabric. This screen keeps 99% of the assimilation lighting inside the greenhouse and he can also use it as a blackout when necessary.
The glass is highly diffuse. “This way, I get good light distribution with my triple screen,” the Westland-based orchid grower says, walking through his new greenhouse. “The middle screen shades 50% of the sunlight. That’s plenty. I’m not a fan of screening too heavily. It’s better to have too much light and to have to add in the other two screens than to have too little and not be able to adjust it. It’s important for us to be in complete control so we can create the growing conditions that are best for us. Everything revolves around quality.”
As closed as possible
Olsthoorn grows Phalaenopsis in 9 cm pots on 6 hectares, the last 2.5 hectares of which are currently being built. He has another 2 hectares on which he grows seasonal products like cyclamen, campanula and Primula obconica. “We are targeting the higher segment, such as wholesalers and garden centres. We don’t supply the mass market but specialise in plants with solid added value. And that needs us to be completely focused on quality. Some customers are so strict that they raise the alarm as soon as they see a bad leaf or a mark.”
Next Generation Growing plays a major role in improving quality in Olsthoorn’s greenhouse. “The climate, RH and energy consumption must be as stable as possible so that we can grow in as closed an environment as possible.”
So the nursery opted for a triple screen system which was built into the structure of the greenhouse. You won’t find any end strips made of fabric there. In this part of the nursery, the greenhouse builders Technokas built white steel plates into the greenhouse structure at each end of the screen installations. As a result, the greenhouse is completely energy-efficient and light-proof for its entire service life.
“Sitting down with the right partners in the preliminary phase makes a big difference. This system is actually a screen system 2.0,” says Jeroen de Jonge of Peter Dekker Installaties (PDI). “It saves us a lot of work. Our installers used to only be able to start making the fixed strips once the construction stage was finished. They had to squeeze in around heaters, water pipes and lamps to secure the strips for the three screens to the greenhouse separately. With this new system, the greenhouse builder hangs up the metalwork as the building work gets under way. Then all we need to do is pull up the screen, secure it – and that’s it. This system is much less prone to breakdowns and it’s maintenance-free.”
The grower is also hugely impressed with the system. “I’m ready for the next 20 years now. It’s an investment but it will easily pay for itself in the long term. The closed system keeps the greenhouse climate much more stable and it’s quite a lot easier to maintain. I no longer have any flaps that keep coming loose when I spray down the greenhouse.”
The triple screen they went for in combination with highly diffuse glass also serves a purpose. “It means we can use a transparent energy screen that lets in maximum light. It allows the sunlight in in a much more evenly distributed way.”
More resilient crop
The plants under the screens warm up more slowly, enabling them to absorb more sunlight and therefore more UV radiation, the orchid grower believes. “That produces a stronger plant and therefore a more resilient crop.” He has seen it in Asia with his own eyes. “They grow in plastic greenhouses there. I took my light meter with me and I discovered that they let in a lot more light than we usually do here. And yet the plants were still a rich green colour.”
To prevent the climate from becoming too humid, he is installing a dehumidifier unit in the new greenhouse which blows dry air in from outside. “This means I won’t have to adjust the screens so much and the temperature inside the greenhouse will stay more stable.”
Double top wires
The triple screen has one downside: the screens are very close together. On a 60 cm high truss there are three axles which can rotate independently. If the fabric blows up there is a risk that it can become trapped in the turning axles. “Of course, three screens are always riskier than a single system,” says De Jonge. “So we have installed systems that mitigate that risk. For example, we fitted an axle guard on the axles, like a kind of emergency brake. If there is a malfunction, the motors stop, preventing any consequential damage. Fitting double sets of top wires to prevent the fabric from being blown up is also a must with a triple screen.”
No matter how well the greenhouse is equipped with screens, diffuse glass and a dehumidification system, vents remain a problem. To solve this, Olsthoorn has opted for one-sided ridge ventilation. “Fortunately we were able to build the greenhouse in such a way that the vents could be fitted on the north-eastern side of the roof. If the sun shines at noon and the vents are open, no direct sunlight enters the greenhouse, so the plants don’t get burned.” He is not bothered about the wind. “Ninety percent of the time it comes from the west.”
Not only are they building a new greenhouse, they are also adapting the existing one to improve the quality of the crop. The grower also has a triple screen there: an 88% screen fabric, a 66% fabric and a diffuse, transparent screen. “This plus the combination of float glass and 50% chalk screening isn’t enough to further optimise crop quality,” Olsthoorn explains. So fitters are also installing an outdoor screen above the greenhouse roof. The space between the greenhouse roof and the screen can keep the temperature inside the greenhouse around 6-7ºC cooler than outside.
Olsthoorn opted for a 50% screen fabric with an open structure. “That means I don’t need to use forced cooling so much and I need to use less artificial light in dark weather in the summer. That saves a lot of power,” he says.
This may be difficult to justify financially compared with his chalk screening, he admits. “But it will pay off in the long term. The outdoor screen gives us complete control over the weather conditions. If it only helps us deliver fractionally better quality, we will have achieved what we set out to achieve with the outside screen.”
Grower Rob Olsthoorn of OK Plant in the Netherlands deliberately opted for a triple screen for his new greenhouse so as to make it as closed an environment as possible. To improve climate equality even further, he integrated the screen into the greenhouse structure, producing a greenhouse that will be completely energy-efficient and light-proof for its entire service life. The existing greenhouse is being fitted with an outdoor screen which will make the weather conditions completely controllable. The aim of all these measures is to improve quality.
Text and images: Marjolein van Woerkom.