Double screen plays key role in quest for uniform climate

Double screen plays key role in quest for uniform climate

Don’t throw away your old shoes before you get new ones – that’s an adage the Dutch sweet pepper grower Stephan Persoon firmly believes in. Now he and his cultivation manager Roel Klapwijk are putting out feelers in the direction of Next Generation Growing, and instead of replacing the old screen fabric they decided to install a second climate screen under it. It took a while to work out how best to install it.

The first green peppers have been harvested at Personal Vision in Bleiswijk. The new season is just around the corner and changes are in the pipeline in Stephan and Thea Persoon’s 7 hectare nursery with their son Roy having recently joined the team. They grow three main varieties of red peppers and they are also trialling several new varieties from various breeders. This broad palette is a good reflection of how the company works. At Persoon they manage their own sales without the intervention of a growers’ association. Every customer has their own preferences and with several different varieties in house, Persoon can meet all their requirements.

Crop manager Roel Klapwijk has been part of the team for ten years now and specialises in climate control. He attended one of the Next Generation Growing (NGG) courses and came away with several new ideas. Like many sweet pepper growers, Persoon listened to what was being said and let it all sink in for a while. “I built up my whole cultivation strategy around my own experience and intuition. It’s hard to just let that go. I can see that the younger generation doesn’t carry this ‘baggage’ around with them!” he says.

Second screen

Nonetheless, he is open to change and therefore to NGG as well. So last year Klapwijk started to experiment a little with their existing energy screen, above a variety that happily stays generative. He allowed the temperature to rise slightly higher there, except on the hottest nights.

Last winter the time had come to make some changes to the screen system, which plays such an important role in the new cultivation strategy. “We were planning to replace the old fabric”, Persoon explains. “But then we began to question whether that was such a good idea.” In their ten-year-old greenhouse with a six metre post height they have a Luxous 1243 D screen, says Ton Habraken from Svensson. It was actually working well in combination with a fixed AC foil at the start of production. Although it had seen a lot of use, it was not particularly worn and was technically still in a reasonable condition.

After the NGG course, it began to dawn on Klapwijk: why not add a second screen? It didn’t take them long to decide. Persoon: “We took the plunge and decided to leave the old diffuse screen in place and install a second screen, a clear energy Luxous 1347 H2no FR, underneath.”

Halfway up the trellis

That decision was not the end of it, however. The big question was where the second screen would go. The original wire bed is on top of the 12.8 m wide, 58 cm high trellis girder. Placing the second screen at the bottom would leave a gap of exactly the same width as the one above the first screen, allowing plenty of room for air movement.

“With the single screen, AC foil and horizontal fans we already get too many temperature differences. That’s what we want to avoid,” Persoon explains. He was already trialling moving fans around to better control air distribution in the greenhouse. He also fitted 150 sensors to measure the temperature distribution in the greenhouse.

Working with his regular fitters Steetec, he came up with a system in which the second screen would be installed halfway up the trellis, reducing the distance between the two screens to just 25 cm. The screens now close towards each other. Some modifications were needed to the drive shaft, the reverse wheels on the pull wires and the cross braces in the greenhouse. To reduce draughts above the screen, four vertical partitions were fitted along each bay in the 211 metre wide greenhouse. The growers hope that these measures will at the very least make things “calmer”.

Radiation meter

They also installed additional measuring equipment. There are now two sensor units measuring the temperature and RH above the screens. This enables Klapwijk to keep track of exactly what happens when the screens are closed. An RH meter and a radiation meter that records outgoing radiation into the sky were fitted on the weather station.

The screen can be closed on clear nights with high outgoing radiation, although that isn’t always necessary when there is cloud cover. The time of year is irrelevant: “Clear nights in summer sometimes produce more outgoing radiation than in winter,” Habraken explains. “On the other hand, in cloudy conditions there is sometimes very little difference between incoming and outgoing radiation. In that case, it’s often better to keep the screen open.”

Thermal imaging camera

The nursery has been using an infrared plant temperature sensor ever since it started out in 2006. A thermal imaging camera has now been added which measures the vertical temperature distribution in the crop. Klapwijk: “We know there are differences, but now I can really see what’s going on.” He doesn’t yet use the information from the camera to steer the crop because he wants to wait a while to see what happens in the crop first. “I expect to start using this information as a steering tool later in the season.”

Ultimately the cultivation manager will be able to decide what action to take based on measurements taken in the canopy and at the head of the crop. He will also be able to ascertain whether the temperature of the flowers or fruits is dropping below the dew point. The LetsGrow climate monitor tells him what is going on in real-time.

New climate regime

Last year the growers decided to apply a diffuse coating to some of the glass. Klapwijk: “The crop has become slightly more vegetative under the coating.” With this experience in the back of their minds, they will soon be applying a diffuse coating to the whole greenhouse to allow the light to penetrate further into the crop. The next step is to gradually implement a new climate regime. “We are already closing the screen at the end of the day more often, and we are making sure there isn’t too much vegetative growth.”

The question is whether the sweet pepper growers are ready to try growing at a higher temperature, as advocated in NGG. According to Persoon, that decision depends on the plant balance. If the plant load is too high, he doesn’t think they will. Or perhaps the higher temperature might actually help the fruits ripen more quickly. All in all, it looks set to be an interesting pepper season.


Personal Vision in Bleiswijk in the south of the Netherlands installed a second screen with clear energy fabric in March. The old diffuse fabric in the existing system has not yet been replaced. With this combination, the sweet pepper growers want to gradually apply the principles of Next Generation Growing. By installing the second screen halfway up the trellis girder, they aim to reduce air movement between the screens and ultimately improve the temperature distribution in the greenhouse.

Text and images: Pieternel van Velden.


Greenhouse growers across the world must focus on water

Greenhouse growers across the world must focus on water

Water. It’s essential for everything that grows and blooms anywhere in the world. And of course greenhouse horticulture is no exception. But with the advent of new, improved cultivation techniques and innovations, our sector is perfectly placed to use the good quality water we have at our disposal sparingly and carefully.

Columnist Herbert Stoker in South Africa has already written about the water problems in the Cape Town area in the south-west of the country, which are forcing growers to revisit the way they use water. As a result, drip irrigation is very much on the up. And even though there is plenty of rain in other parts of southern Africa, filling farmers’ reservoirs and gardeners’ water butts, attention needs to be focused on water there too. The burning issues are: How do you handle water flows to which nutrients have been added and which contain crop protection residues? And how will this impact on the soil, flora and fauna, and human health?

Here in the Netherlands, water treatment is one of the key issues facing greenhouse growers. As of 1 January this year, all Dutch growers are required to treat their water before discharging it into surface water. They can do so using their own fixed water treatment unit or a mobile unit, or they can use shared facilities. The third option is a closed system in which the water is reused and not discharged at all. This government measure currently only relates to crop protection products, but fertilisers are due to be added over the next few years.

In the European Union, environmental commitments such as these are still not harmonised but every member state will have its own targets and regulations in this area. Some countries have gone quite a bit further than the Netherlands and others are set to follow soon. Although the 1 January 2018 deadline was known about for some years, many growers only took action at the last minute. Researchers, advisers and suppliers delivered a masterpiece which growers on various continents can benefit from. After all, as Bob Dylan once said, “The Times They Are a-Changing”.

text: Roger Abbenhuis.


Good filter technologies greatly reduce volume of residual drain

Good filter technologies greatly reduce volume of residual drain

Wasting water is so yesterday! The water treatment requirement that entered into force in the Netherlands at the beginning of this year is forcing Dutch growers to reduce the volume of drain water they discharge. But there is scope to reduce it even further by optimising filter output. Making minor adjustments at the front end of the system can have a significant impact on the volume of residual drain water. What’s more, keeping this volume as low as possible can ultimately save you a lot of money on the cost of water treatment.
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The number of fixed and mobile water treatment plants approved for greenhouse horticulture use in the Netherlands is constantly on the rise. This equipment has to meet strict requirements. The treatment efficiency of these systems is at least 95%, with as much as 99.5% for imidacloprid, allowing growers to comply with the water treatment requirement that came into force on 1 January 2018. Problem solved, as far as they are concerned.
Complying with the water treatment requirement has been the focus of every Dutch grower’s attention in recent years – so much so that some have relatively little interest in improving their existing systems. And that’s a pity, because there is so much more they could do to reduce the volume of residual drain water.

Thrown away costs

Rob Zwaard, Senior Account Manager at Uvar Holland, sees it as his mission to encourage his customers to use water responsibly. “There is still so much to be gained at the front end of the plant, by making better and more efficient use of your existing filter system. Every cubic meter of water you throw away costs you €1. Some growers end up with too much residual drain water.”
Greenhouse growers tend to use three different types of filters to prepare irrigation water for use and to clean process water for recycling. Sand filters, in which the water sinks through a sand bed, remove suspended solids from the water. The amount of drain water left to be discharged is around 2%. Screen filters, which remove less coarse particles from the water, produce less than 1% drain water for disposal. The latest generation of ring filters, suitable for depth filtration, are cleaned with high-pressure air and leave less than 0.5% drain water to be discharged.

Cleaning by backflushing

The process of flushing sand filters and earlier generations of screen filters itself generates a lot of drain water for discharge. Yet not everyone is aware of this. “When I ask customers how long they flush their filters for, all too often they tell me they flush their sand filters for 15 minutes. That’s completely unnecessary. Five minutes is plenty,” says Zwaard. “Not a lot of people know that.”
Many growers flush their filters at set times. That’s also not ideal. How often they need flushing depends very much on how contaminated the water is. If it is heavily contaminated, the pressure soon builds up in the filter and can cause damage if the filter is not flushed in time. It’s better to clean them when a pressure difference of 0.5 bar is reached. “If you flush your filters to a fixed schedule, you’ll invariably be too early or too late,” Zwaard says. “And while we’re on the subject, also think about a maintenance programme for the filters. That way you can frequently prevent problems before they arise. We often find that taking out a maintenance contract is cheaper than having the filters repaired.”

Reducing drain water discharge

There are still things you can do to reduce the volume of drain water to be discharged once the contaminated water is in the wastewater buffer silo. You can run it through a small sand filter again, for example. Some of the filtered water can even be returned to the drain water silo and routed from there back into the system via the desinfector. This is particularly practical for nurseries that manage their nutrient solutions well and don’t get chemicals accumulating in the wastewater. This again reduces the volume of residual drain water in the wastewater silo.
Ultimately, once the options for “thickening” the residual drain water using filtration have been exhausted, then it is time to clean the water in an in-house or mobile purification plant. The small volume keeps costs down.

Set an example

“Water is a scarce commodity,” the account manager says. “Freshwater makes up just two percent of all the water on the planet. Almost 90% of the freshwater is not immediately available because it is trapped in ice caps. In the Netherlands, enough water falls in the form of rain, but often at times when you don’t need it. This requires us to bring an awareness to the way we deal with process water. We also have plenty of technologies at our disposal to enable us to set an example for the world. By approaching the subject in a positive way, we are making advances that also improve our image.”
Zwaard sees the Dutch water treatment requirement as an interim step towards full recirculation at horticultural businesses. “I expect that ten years from now, we will no longer be throwing out any water at all and we will be able to keep the cultivation process completely closed.”


In their efforts to comply with the Discharge Decree, Dutch growers are focusing their attention on purification systems. They sometimes forget that the volume of residual drain water can be reduced with intensive filtration. A lot of water is lost due to excessive backwashing when cleaning filters. Running residual water through a small sand filter a second time also reduces the volume to be discharged.

Text and images: Pieternel van Velden.



Book about grafting fruiting vegetables

Book about grafting fruiting vegetables

Levels of production of crops such as tomato and aubergine have increased in recent years with the use of rootstocks. In a number of other crops, such as pepper and cucumber, rootstocks are mainly used to prevent problems with soil diseases in organic production. The benefits of using grafted plants are already well known, but there is still a lack of knowledge of the interactions between scion and rootstock in different conditions and how to control them.

A group of experts have now compiled all the available knowledge in this area in a book called “Vegetable grafting: principles and recent practices”. It not only covers the latest scientific insights but also provides a lot of practical information on grafting methods, choice of rootstock and the use of grafted plants. It also features recommendations from researchers at the Greenhouse Horticulture business unit on how to best utilise the benefits of grafted plants during production.
This book can help to increase the production, quality and sustainability of grafted vegetable crops.