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The Sweeper consortium was invited to hold the first live demonstration of its new sweet pepper harvesting robot at the De Tuindershoek greenhouse horticulture firm in IJsselmuiden. The so-called ‘Sweeper robot’ is the world’s first harvesting robot for sweet peppers to be demonstrated in a commercial greenhouse. An audience of over 40 interested parties watched the harvesting robot pick its first commercially-grown sweet peppers.

The Sweeper robot was designed to harvest sweet peppers in a cultivation system based on single plant stalks in a row, a crop without clusters and in little foliage near the fruits.

Commercial use

In earlier test set-ups in a commercial greenhouse with a V-type double-row cultivation system the harvesting robot achieved a harvesting percentage of 62%. Based on these test results, the Sweeper consortium expects to be able to bring the commercial sweet pepper harvesting robot to the market in about four or five years.

Further research required

Until then, further research will be needed to enable the robots to work faster and achieve a higher success percentage. Additionally, commercially viable cultivation systems must be developed that are more suitable to the robotic harvesting of crops. The test and research results are not only suitable for the automatic harvesting of sweet peppers; the data can also be used to robotise the harvesting of other crops.

International research partnership

Sweeper is a partnership between Wageningen University & Research (WUR), sweet pepper farm De Tuindershoek BV, the Umea University in Sweden, the Ben-Gurion University in Israel, the Research Station for Vegetable Cultivation and Bogaerts Greenhouse Logistics in Belgium. The study receives financial support from the EU’s Horizon 2020 programme and is also funded by the Dutch Horticulture and Propagation Materials Top Sector.

Successor of CROPS

The Sweeper robot is the successor of CROPS (Clever Robots for Crops), an EU project launched by WUR, in which WUR and the other participants developed a robot that can make a distinction between a sweet pepper plant’s fruit, leaves, stalks and main stems. As a result, the robot can harvest sweet peppers without damaging the fruit, leaves, stalks or stems.

Source and photo: www.sweeper-robot.eu. Video: Wageningen UR greenhouse horticulture.

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A household waste disposal company in Alkmaar (HVC Alkmaar) will be supplying the greenhouses of the NH Paprika sweet pepper farm in Heerhugowaard with residual heat. This sweet pepper farm is the very first business in the Alton greenhouse horticulture area to start using this sustainable energy at the end of this year. Both parties recently signed an agreement that will connect the farm to the heat network of the household waste disposal company.

With 11 hectares of sweet peppers and a consumption of 5 million m3 per year, NH Paprika is the largest consumer of natural gas in the Alton area, which is located in the Dutch province of Noord-Holland. As set out in the agreement, HVC Alkmaar will be investing in connecting its existing heat network with the Alton greenhouse horticulture area. By the end of this year, an 8.5 km connection will be made from the HVC Alkmaar heat network to the greenhouse site. This will enable the growers in the area around Heerhugowaard to use sustainable energy to heat their greenhouses in the years to follow: an important step towards realising their ambition to stop using fossil fuels for cultivation by 2030.

Green heat

According to Alderman Monique Stam the green heat supplied through the heat network will enhance sustainability throughout the entire municipality of Heerhugowaard. In addition to the greenhouses, other buildings such as private residences, offices and commercial properties will also be able to tap into the heat network in the following years. Last year, the Municipal Council expressed its commitment to connecting at least 2500 private residences to the heat network in the next decade to come. The heat network is part of a collaborative venture initiated by government bodies and the corporate community, who have joined forces to enhance the sustainability of the greenhouse horticulture industry in the Alton area, explains area coordinator Dave Vlaming of the Ontwikkelingsbedrijf NHN development corporation.

The future of the Alton greenhouse horticulture site

As an independent area coordinator, Ontwikkelingsbedrijf NHN has been engaged in making the Alton area future-proof for the past five years. This project is being carried out in collaboration with various entrepreneurs, the Province of North Holland, the Municipality of Heerhugowaard and Koggenland, the Rabobank, the Stivas foundation for improving agricultural structure, the Netherlands Enterprise Agency, the Dutch platform for greenhouse horticulturists LTO Glaskracht and Alkmaar HVC. “During the 2008 economic crisis the future of this greenhouse area was looking everything but rosy, but Alton is now starting to take on an exemplary function with regard to enhancing sustainability and promoting closed loop horticulture. The construction of the heat network is a tremendous breakthrough in future-proofing the Alton area. Thanks to the heat network, more businesses are expressing an interest in making investments here, and the area is also attracting entrepreneurs from outside. Several years ago Gootjes Allplant established itself in Alton, followed by Verver Export last year. Another four companies from outside have expressed an interest in relocating to Alton. Amigo Plant purchased a 10-hectare site here last year,” confirms Vlaming.

Alton is the oldest of three areas in the northern part of the province of Noord-Holland to boast a dense concentration of greenhouse horticulture enterprises. Together with Het Grootslag near Andijk and Enkhuizen, and Agriport A7 in Middenmeer (where greenhouse horticulture enterprises cover 60 hectares of greenhouses on average), these three locations are responsible for the large-scale production of horticulture products in the Netherlands.

Source: HVC. Photo: HVC/Marc Dorleijn.

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When several of the sweet pepper plants at Zwingrow in Honselersdijk (Westland, the Netherlands) were found to be growing more slowly than the rest, the nursery decided to have an uptake analysis performed. An uptake analysis gives the grower a detailed picture of the actual uptake of nutrients by the plant. After all, as field researcher Ruud Kaarsemaker says, the drainage and drip values often don’t give a clear enough picture on their own.

It was in May that Zwingrow saw the first signs that the crop was ‘unhappy’. Cultivation manager Bart van der Valk noticed several plants across the crop that were not growing as vigorously as the others. They were wilting under high solar radiation and had brown, thickened roots. These plants were less dense, thinner and shorter than the others, prompting the growers to look into what could be going wrong. Were the systems still working properly or had something gone wrong with the nutrient tank schedules or the watering strategy? The grower couldn’t quite put his finger on it and called in Groen Agro Control of Delfgauw to help.
This research organisation was already sampling the nursery’s irrigation and drainage water on a weekly basis and, after visiting the site and talking to the growers, they recommended an uptake analysis. The analysis was carried out retrospectively to obtain a picture of the development of the nutrient uptake that led to the imbalance in the crop.

Nutrient status

A glance at the graphs illustrates what this analysis is about. The lines show the calculated uptake of all main and trace elements per week. The results are based on the concentrations in the irrigation and drainage water, the drainage percentage, watering volumes and a calculated estimate of the dry matter produced based on the amount of light and the CO2 concentration achieved. Another line in the graphs indicates the concentrations in the slab. Comparing the crop uptake with the slab concentrations and the ratios of the various nutrients provides additional information on the crop’s nutrient status.
The Zwingrow values showed that overall nutritional uptake was already very low in early April. Kaarsemaker: “When uptake is low, it is especially important to ensure the nutrients are properly balanced. But these sweet pepper plants were taking up much less potassium than other cations at the time. Potassium is important for opening and closing the stomata: if the stomata don’t close properly, the plants can wilt. Potassium is also needed for transporting sugars. A potassium deficiency can therefore result in insufficient transport of sugars to the root system.”

Iron, boron and zinc deficiency

The analysis also revealed that the values of at least three nutrients were well outside the desirable bandwidth. “The graphs show a distinct boron, iron and zinc deficiency. Low uptake of these nutrients fits in well with the picture of a weak plant. An iron deficiency will cause the young leaves at the top of the plant to turn yellow, a zinc deficiency causes yellow discoloration between the veins, and low boron uptake results in distorted, brittle leaves, weak plants and a brittle crop.”
The sweet pepper growers were surprised. These trace elements were present in the drainage water in sufficient quantities and in the right proportions during the period analysed. And you wouldn’t expect there to be an uptake deficiency if the parameters are correct. “Clearly the plants were not being stimulated enough to take up the nutrients,” cultivation manager Bart van der Valk says. So he significantly increased the irrigation volume – by 50% – but to his surprise, he found that the drainage values remained the same. That made it clear that something else was going on. Van de Valk also realised that from then on, he would need to start looking at the drainage and drip values from a different perspective. Uptake analysis has been a permanent fixture at this nursery ever since.

Combination of factors

Kaarsemaker: “As we know, the conditions for optimum growth are complex and a healthy plant is the sum of various factors. Nutrition is just one of those. We don’t yet fully understand what effect all the elements the sweet pepper needs have, but we can spot anomalies in the uptake pattern. What led to these phenomena at this nursery were most likely a combination of high plant load, too little watering and imbalanced nutrition. We could probably have avoided some of the problems by optimising our plant nutrition in good time. It’s so important to keep a constant eye on nutrient uptake.”
This realisation is gradually taking root among greenhouse growers as they discover that this analysis method allows them to steer their crops more precisely. The result: better quality, higher production, more vigorous plants and possibly even savings on fertilisers and water discharge. Kaarsemaker is seeing growing demand for the calculation tool, particularly among tomato and sweet pepper growers. They have a good understanding of their water flows and can therefore make excellent use of the analysis data.

Particular challenge

A lot is already known about tomatoes, and for the past year Groen Agro Control has been working more intensively with sweet peppers, a slower crop. The more data they can collect, including from new varieties, the more accurately the results can be interpreted. After all, all the information about the growth phases and plant stages of each crop type is used. That makes an uptake analysis in ornamentals a particular challenge, Kaarsemaker says. “We are doing them with plants such as rose and gerbera. Growers of these flowers often grow several different varieties and have plants of various ages in the same part of the greenhouse. That makes things complicated.”

Visibly improving

The adjusted nutrient strategy at Zwingrow is working. The cultivation manager was concerned that the weaker plants may have been permanently damaged, but even they have been visibly improving. More importantly, no new problems have arisen: the healthy plants are staying healthy. So the damage caused by the imbalance has remained limited. From now on, the grower will be allowing enough time to check the uptake graphs once a week and is hoping that this will avoid any problems in the future. However, he realises that not everyone is falling over themselves to get hold of a tool like this. It costs money and it’s also difficult to prove that it will help increase yields.
Kaarsemaker is also coming up against resistance. “Nurseries prefer to stick to the traditional method. They sample the drip and drainage water and decide how much to fertilise based on the target values in the slab. That’s fine as long as everything stays ‘average’, in other words if the plant and root system are healthy and the climate is right. But as soon as a plant starts taking up less for whatever reason, an imbalance can occur. An uptake analysis enables you to keep on top of things and intervene as soon as there is the slightest anomaly. That way you can prevent problems from escalating.”


An uptake analysis gives growers an accurate picture of which elements the plant is actually taking up, enabling them to fine tune the nutrient solution to the plant’s needs. The values from drip and drainage samples alone are sometimes not enough, as sweet pepper nursery Zwingrow discovered. The calculation tool highlights even the smallest anomaly and prompts the grower to take action in good time.

Text: Jojanneke Rodenburg. Images: Jos Bezemer.

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Sweet peppers can manage with 800 ppm CO2 in the winter months. Supplying more than that doesn’t boost photosynthesis: in fact, the sweet pepper plant simply gets used to a higher dose, resulting in ‘lazy’ leaves. Luckily, this is easy to reverse when spring arrives. The lazy leaves are back in action after six days.

These are the conclusions of Dutch research into sweet peppers carried out in 2014 and 2015 by Plant Lighting, Inno-Agro and Plant Dynamics. The research was conducted under the guidance of the Horticultural Technology Development growers association (TTO).

Greenhouse air quality

Many growers have a free supply of CO2 from their CHP units. But finding out what the ideal dose is doesn’t feature high on their list of priorities. “It may not help, but it can’t do any harm” is often the rule of thumb. Nevertheless, Stefan Persoon, innovation specialist at Inno-Agro, has noticed that this is changing.
“First of all, we are starting to use less fossil fuel as a sector. Of the 1,650 hectares of sweet peppers in the Netherlands, 300 hectares are grown using geothermal heat or another alternative energy source. Those growers pay for their CO2, for example via OCAP (CO2 from the port city of Rotterdam), and this encourages them to do more with less. A second reason for finding out the ideal dosage is the quality of the air in the greenhouse. On the face of it, this doesn’t seem to be a problem, but measurements taken inside greenhouses reveal that the air quality can easily come under pressure. Oddly enough, this mainly happens in spring when the vents are opened. Growers will then provide additional CO2 to compensate for the loss. This pollutes the air in the greenhouse, for example with NOx.”
Sander Hogewoning, a researcher at Plant Lighting, adds: “There is a third reason why growers should be taking a critical look at the dose. International research on arable crops indicates that prolonged exposure to higher CO2 levels can lead to ‘lazy’ leaves. When that happens, the key enzyme RuBisCO binds CO2 less effectively. Does this also happen in greenhouse horticulture? That’s precisely what this research was about.”

Airtight cabins

The trials took place at the Westland Demo Nursery (Demokwekerij Westland). They built six 1.4 m2 glass cabins in which CO2, RH and temperature can be precisely controlled. In the first phase, both sweet pepper and tomato plants were studied. The researchers put young plants from a breeder in cabins with 400, 700 and 1,000 ppm CO2. Using a photosynthesis meter, Sander Pot of Plant Dynamics recorded in detail how the leaves use the different concentrations for photosynthesis.
CO2 saturation in tomato plants was found to be 600 to 700 ppm, while in sweet peppers the figure was 700 to 800 ppm. “The step up from 400 to 600 ppm provides far more additional photosynthesis than the step from 600 to 800 ppm,” the researcher says. The amount a grower has to dose for that second step is also much more than for the first step, especially when the vents are open a crack. So the rule “the more, the better” doesn’t hold water. “And yet there can be downsides to high CO2 concentrations. Not all growers take that on board,” Hogewoning explains.

Lazy leaves in sweet pepper

The next important issue the study looked at was whether this high dose would yield “lazy” leaves. The answer? Not in the short term. Leaves that had formed in the breeder’s nursery did not turn lazy. But the same was not true of leaves that had developed entirely in the trial cabins. A CO2 dosage of 1,000 ppm did not produce lazy leaves in tomatoes, but the outcome was different in sweet peppers: photosynthesis was just as high in plants grown at 1,000 ppm and dosed with 900 ppm as it was in plants grown at 400 ppm and dosed at 600 ppm. This is clearly illustrated in Figure 1. In other words, plants that were “pampered” with 1,000 ppm needed a sustained high dose to keep their productivity up.
Hogewoning: “This is caused by the enzyme RuBisCO, which is the key to photosynthesis. The capacity of this enzyme drops. In this case the ‘laziness’ has nothing to do with the stomata, as some growers believe.”

Reactivating quickly

The follow-on research focused exclusively on sweet pepper plants and looked at whether the lazy leaves could be reactivated. “We call that ‘reversible’. We wanted to see whether and how quickly the leaves could get used to a lower CO2 dosage. That is actually what happens in practice. After a winter with a high dosage, the vents are opened a crack in spring. The question is how long the leaves stay less productive then,” Hogewoning explains.
To test that, in 2015 they carried out a trial with sweet pepper plants with doses of 400 and 1,000 ppm. Once the researchers had identified the lazy leaves, they switched two cabins of 1,000 ppm to a varying regime of between 500 and 1,000 ppm, similar to spring in the greenhouse. Using the photosynthesis meter, they determined CO2 uptake in the leaves after six and fourteen days.
Good news for sweet pepper growers: the laziness turned out to be reversible after just six days. Therefore, lower CO2 levels only cause the crop to be less productive for a very short time. “We were quite surprised by that. It means that it isn’t necessary to adjust the dosing strategy in winter. But what is important to remember is that growers who opt for a high concentration need to keep it up right through the winter. The plant gets used to it. And dosing above 800 ppm has very little added value,” Hogewoning concludes.

Eye opener

Whether or not sweet pepper growers can make use of the findings in practice depends on their situation. We ran this past Bart van der Valk of Zwingrow, who grows orange peppers on three sites in the Westland area of the Netherlands. He is positive about the outcome of the research. “We use geothermal heat and we pay quite a lot per square metre for the CO2 we source from OCAP. So we are keen to use it more efficiently.” For him it was an eye opener to discover that the level could be lower in winter.
“We are now dosing 600 to 700 ppm in winter. It’s just as effective as 1,000 ppm. I prefer to keep the CO2 for the spring. What the research also revealed is that lazy leaves can recover again quickly. That’s good to know. Of course, there are still some unanswered questions. For example, I would like to find out what time of day is best for dosing CO2. More research is needed in that area.” But a survey among growers using geothermal heat reveals that there isn’t enough money for practical research yet.


If sweet pepper plants receive a high dose of CO2 over a long period in the winter, they get used to the high level. That produces “lazy” leaves which use the CO2 less efficiently. But this is reversible: when the dose is reduced in spring, the plants adapt within just six days.

Text: Karin van Hoogstraten. Images: Studio G.J. Vlekke.

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The adage that every downside has an upside even applies to the regulations on discharges, which are becoming ever stricter. Full recirculation in sweet pepper crops is now achievable in the lab, and apart from providing a much better understanding of the crop, it can also save money. Growers can already largely avoid emissions without investing in expensive cleaning equipment.

The trial department at the Water Innovation and Demonstration Centre (IDC) in Bleiswijk, the Netherlands, may be only 150 m2 in size but it produced a lot of new information in 2015. In the first trial year the researchers compared zero-emission recirculation in a sweet pepper crop grown on stone wool with conventional cultivation and discharging. A consortium of nine suppliers, the Dutch water boards, the province of South Holland and the Dutch government, are not only financing the research but are also supplying new knowledge and techniques. “Stronger together” is certainly true in this case.
After the successful results achieved in the first trial year, a second trial was run last year, this time with stone wool and coco and with zero emissions. Why coco? Eelke Hempenius of Grodan and Erik van Os of Wageningen University & Research explain: “Following on from the stone wool trial, we also wanted to test other, non-inert substrates. Basically, we need to get a complete picture of zero emission growing.”

Good irrigation water

Coco naturally contains high levels of sodium chloride. At the start of the new season, the drain water already contained 3 mmol/l sodium, compared to less than 1 mmol/l in stone wool. Therefore, the researchers filled the coco slabs with CaNO3 (EC 3.5) for the sodium and calcium exchange and the slabs were buffered.
By always using clean, low-sodium irrigation water throughout the crop cycle, they found that they could obtain almost the same yields with coco substrate as they had with stone wool. The difference was ultimately 6% in favour of stone wool. With the buffered coco slabs, a total of 8 kg nitrate per ha was discharged over the whole crop cycle, excluding the residual water and the slabs. This is a very good result, considering that the total for conventional cultivation in 2015 was 153 kg per hectare.
This positive result provides food for thought. Would it be possible, for example, to meet the discharge requirements without having to spend money on expensive systems? Yes it would, the researchers believe. Hempenius: “Good, low-sodium irrigation water is essential. If you have very good quality water, you don’t actually have to worry about what happens on the backside.” You will need plenty of rainwater storage for that clean water: 1,500 m3 per ha is the recommended amount. Reverse osmosis is also a useful addition.

No need to rinse

Traditionally, growers have drained off the first slab water before planting the crop, in the belief that it contains substances that are detrimental to the young plants’ roots. But according to Hempenius, that isn’t necessary. “Our emission tests have proved this. I have noticed that this idea is still very much ingrained in people’s minds.”
Ingrained – that’s an expression that crops up regularly. The researchers often hear growers saying they prefer not to recirculate because it means the plants are constantly getting the same nutrient solution. That’s an argument Van Os dismisses out of hand. With 30% drain, for example, 70% of the nutrient solution is refreshed, so the plants always get plenty of new nutrients.

Concerns about sodium

By far the biggest concern growers have is that sodium will accumulate in the process water. Sweet pepper is known to be able to tolerate up to around 6 mmol/l sodium, but the figure may be even higher, the researchers claim. Salt tolerance is an area that is not yet fully understood and new research is needed on it. “A lot of what we do is still based on instinct and we tend to err on the side of caution,” Hempenius believes.
This 6 mmol/l sodium level increases the EC by 0.6. With an EC of 2.5, that still allows enough leeway to create a well-balanced nutrient solution. Van Os: “So starting to drain at 4 or 5 mmol/l sodium is really not necessary.”
The nutrient solution must be matched to the plant’s needs, however. During the trial the process water was analysed once a week. In the laboratory of partner Groen Agro Control, uptake was also analysed weekly, based on irrigation, drain, temperature, light sum and CO2. This shed more light on the plant’s needs. This intensive method of monitoring resulted in a much better match between supply and uptake of nutrients.
Van Os: “By doing this, you also gain in terms of growth and possibly even yields, because you have a better picture of what the plant needs.” He also expects things to move in that direction in practice. Growers who currently only take samples every two or three weeks could substantially increase their sampling frequency. This would result in more growth, less discharge and ultimately a better understanding of the crop, eliminating imbalances in the nutrient solution.

Using up the last residues

In the research into crops grown on stone wool, a great deal of attention was paid to the end of the crop. The aim was to reduce residues from 50 to 20 m3 per ha, without discharging and with no nitrate or phosphate in the residual volume. For example, the total amount of irrigation water was gradually reduced based on the radiation sum.
The nutrient solution also had to be adjusted so that the plants would use up the last remaining nitrate and phosphate residues. This was done by replacing one-third of the nitrate with chloride. The pH was lowered to keep phosphate and trace elements available to the plant. Van Os: “This has to be done very precisely, because you can’t let the crop wilt. Tomato fruits stay firm for quite a while but with sweet pepper, the fruits tend to wilt more quickly than the crop.”
This reduction strategy doesn’t apply to coco. With this substrate, you have to drain to maintain a good nutrient balance right up until the end of the crop.

Affordable solution

Proof that recirculation with very low levels of discharge is possible is not the only insight this research has yielded. For example, the researchers have learned more about the composition of the nutrient solution and have made progress in identifying the plants’ actual needs for nutrients right through the growing season. They have also shown that sweet pepper growers can make substantial savings on water and fertilisers, ranging from five to ten percent of their total water consumption, equivalent to between 400 and 800 m3 per year. This can deliver savings of €2-€3 per m2.
By far the greatest gain can be achieved by reducing total emissions by reusing the first water released when the slabs are pierced and from the first drain. During cultivation, smart filter technologies, such as a flat-bed filter, and efficient watering, including the use of a fast ring main, help reduce the total amount of process water. Making these adjustments can save growers from having to purchase expensive cleaning equipment.
And if they do ever need to discharge drain water, they can collect it in a silo and perhaps bring in a contractor to purify it. So compliance with the new regulations doesn’t have to be a costly exercise.


Having grown sweet pepper on stone wool with zero emissions in 2015, researchers trialled coco and stone wool last year. Stone wool produced good results, with coco also heading in the right direction. What’s more, with a smart watering strategy it is possible to reduce emissions to such an extent that there is no need to invest in costly cleaning equipment. Growers still need to be willing to accept that this is a safe growing method.

Text and images: Pieternel van Velden.

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Dutch growers who are directly involved in research into Next Generation Growing soon discover that they can apply small parts of it to their own nurseries. At first sweet pepper grower Danny van der Spek was sceptical about the high energy target set in the research, but he is gradually adopting some aspects himself. However, it is difficult to translate the results from relatively small research sections into real life situations.

Every Monday, Danny van der Spek from Bergschenhoek visits the Next Generation Growing sweet pepper trials being run at the Delphy Improvement Centre (IC) and Wageningen University & Research in Bleiswijk. He discusses the condition of the crop and the climate settings with his colleagues Kees Vijverberg, Maikel van der Berg and Ard Ammerlaan and the other advisors on the research supervisory committee there. “It is extremely interesting and informative,” he explains. “It doesn’t take up much time but it does give you a good picture over an extended period of time. What’s more, you can adopt some of the positive results in your own climate control.”

Similar situation

Van der Spek has 9 ha of orange peppers split into two sections. His Venlo greenhouses are equipped with a double energy screen. His situation is quite similar to the one at the IC. There are fewer similarities with the parallel trial at Wageningen University & Research as the VenlowEnergy greenhouse has a double glazed roof. “That’s not a situation you will find very often in practice, simply because it’s too expensive,” he comments. But the comparison between double and single glazing is nonetheless interesting.
The pepper selected for both trials was the heat-loving red standard variety Maranello. The 1,000 m2 section at the IC has two transparent energy screens and is equipped with horizontal and vertical fans. Heating is kept to a minimum in this section.
Whereas energy consumption at commercial nurseries is usually about 30 m3/m2, the trial participants want to demonstrate that energy consumption of 20 m3/m2 is achievable. That is a saving of more than 30%. This target was achieved at the end of 2016, with energy consumption as much as 2 m3 less than projected. Maximum use of the energy screens and omission of the minimum pipe therefore had a massive impact.

Minimum pipe

The sweet pepper grower describes himself as a “pretty heavy stoker”. He sees the minimum pipe as a way of keeping the crop active. That’s how he learnt it and that’s how he has always done it in practice. “I actually want to get away from the minimum pipe,” he explains. So following these trials and comparing them with a down-to-earth vision of professional practice appealed to him. The project, which is based on the principles of NGG, offers opportunities to approach climate control from a different angle.
When he heard about the high energy target, his ears pricked up. “I thought the target was extremely high and didn’t expect it to be met without loss of quality or production. The fact that it was, is very special,” he says.

Energy screens dominate

In the trials, the use of the energy screens was the dominating factor. The top screen opens when radiation reaches 100 W/m2. The second screen follows when the temperature above the screen differs from the desired heating temperature by four degrees. This small difference should prevent a cold dump.
There were times in the spring of 2016 when the bottom screen was still closed with radiation at 300-500 W/m2. The intensive use of the screens led to a 14% light loss between the start of cultivation and early April. That was difficult for the growers to get used to as they prefer to allow in as much light as possible.
Van der Spek: “The screens were closed a lot. They are even used at night in the summer to protect against outgoing radiation. The windows are left closed as long as possible so that the greenhouse is warmed up by the sun, not the pipe. With a net radiation meter at the top of the greenhouse, we can monitor that closely. You would expect the crop to suffer but it didn’t seem to. I have always been very impressed with the condition of the crop. Although you do have to take the small surface area of the trial into account. In a large greenhouse it could be harder to get rid of moisture because there is proportionately less wall surface area.”

Higher 24-hour temperature

It soon became apparent that production and fruit quality were keeping up well with commercial nursery standards. Losses were low and no internal rot was observed. The crop in the trial with double glazing was slightly stronger and showed more growth. In fact, blooms were slightly ahead of those in commercial nurseries. The 24-hour temperature was also a little higher.
That brings the grower back to the situation at his own nursery. He has gradually started applying some of the principles from the trials there. “For me personally, the greatest benefit comes from screening more and from the slightly higher 24-hour temperature. The more light you can let in, the more the plant can do with the sugars it creates,” he explains. “We still have to find out how high the 24-hour temperature needs to be for that.”

Translating into practice

The intensive use of screens in the trial resulted in significant energy savings, particularly in April. The fans – first the horizontal ones and later the vertical ones – ensured an even distribution of temperature and humidity. This has taught Van der Spek that January and February are not good months for saving energy, whereas March and April are. From then on and throughout the summer, he uses very little gas.
Nonetheless, it is difficult to translate the results of the trials straight into practice. “We growers have to leave the CHP running, you see, even when we need less heat,” he explains. “I myself have an OCAP connection which delivers us CO2 from industrial plants around Rotterdam, but not every nursery has that. In these situations the CO2 supply is the bottleneck.”
The research into NGG and the different approach to climate control whets the appetite for more. Van der Spek: “We growers are actually keen to continue with the research as there are still a lot of unanswered questions. I was very sceptical to begin with, given the results of previous studies. I will be looking at things differently after this season.”


Sweet pepper grower Danny van der Spek is a member of the supervisory committee overseeing the NGG sweet pepper research. He had expected the high energy target to impact negatively on crop, quality and production. Having discovered that it doesn’t, he is becoming more and more keen on NGG, although a lot of knowledge is still needed to translate the principles into practice.

Text and images: Pieternel van Velden.

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Hyperparasites in the crop can completely disrupt biological crop protection. Sweet pepper growers in the Netherlands know a thing or two about that. But where there’s hope, there’s life: in October 2015 a multi-year, fundamental research project was launched that aims to unravel the interactions between hyperparasites and their environment. Once these are sufficiently well-known, attractants may turn out to be an answer.

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Calcium is a delicate mineral in the plant. A deficiency can cause problems such as blossom end rot in sweet peppers and tomato, burnt edges in leafy crops and ‘tip burn’ in some ornamental crops. The other way round, excess leads to gold spot on tomatoes and stip (colour spotting) in peppers. Just paying attention to the nutrition is not enough. The distribution in the plant is crucial. This can be regulated but it requires a lot of diligence.

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Rhizopus stolonifer is a fungus that can cause stem and fruit rot in several crops. It is also a well-known bread mould. Rhizopus is usually visible as fluff on the surface of damaged plant parts. Often the black spore heads on the fungal hairs are visible with the naked eye. The fungus occurs naturally in the soil.

Contamination can occur through soil contact or contact with infested fruits. The fungus can grow rapidly under warm and humid conditions and the spores travel from plant to plant via the greenhouse air. The initial infection often occurs as a result of plant damage. In horticultural crops Rhizopus is most prevalent on fruits such as strawberry, tomato and eggplant but it can also cause rotten spots on the leaves and stems of pot plants.

Primary cause

Although Erwinia is often the primary cause of stem rot in sweet peppers, Rhizopus is also found in these rotten areas. Infected fruits can completely rot away within a few days, especially when products are packaged in foil. Infected fruits often leak sap, a typical characteristic of Rhizopus fruit rot.

Text and images: Groen Agro Control

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Schut Papier, based in Heelsum, the Netherlands, brought a new type of paper made from raw materials such as tomato and sweet pepper plant fibres to the market. The company has long-term plans to use other agricultural by-products in the production of its paper.

With production volumes of only 3,500 metric tons, Schut Papier is one of the Netherlands’ smallest paper factories. ‘This will make us the most flexible factory in the country,’ affirms General Director René Kort. ‘Our flexibility is one of the factors that got us involved in a research project initiated by the Dutch Bio Refinery Cluster. Smurfit Kappa wanted to develop packaging that uses the fibres of tomato plants as a basic raw material. Their own equipment was, however, too big to accommodate the necessary research. Our paper machine was perfect in terms of size; and so we were able to contribute. We spent two years developing the packaging. In the meantime we gained a great deal of knowledge about how fibres derived from agricultural waste flows can be processed for use as a raw material for the production of paper.’

Making use of by-products

Schut Papier subsequently deployed this knowledge to develop its ‘Valorise by Schut Papier’. The paper is made, in part, from the fibres of tomato and sweet pepper plants. Kort: ‘We want to contribute as much as we can to achieving a biobased economy. After all, by-products form one production process can easily become raw materials for another. By treating our raw materials sensibly we will be giving future generations a better world to live in. In concrete terms, it means that we aim to replace as much wood fibre as we can with fibres derived from waste products. After tomatoes are harvested the plants themselves are treated as a waste product. This is ridiculous, of course, because the stems contain interesting substances that can be used to make paint, glue, or crop protection agents, and are additionally chock-full of fibres that can be used to produce paper. This is of benefit to all parties involved. The growers appreciate use being made from their by-products - and this contributes to sustainable working practices at the same time.’

Off to a good start

That Valorise doesn’t reflect only Schut Papier’s corporate philosophy but also appeals to that of its customers is already apparent from the initial sales figures. ‘Valorise has already achieved a production volume of 6 to 7 per cent of all paper produced in part from agricultural by-products. This means we’re off to a good start. I’m also delighted to report that world-renowned companies have been showing interest in Valorise. We were recently selected as a supplier of paper by an advertising agency whose clients include Louis Vuitton and Moët Hennessy. We aim to achieve a production volume of 15 per cent with paper made in part from agricultural by-products within the next three years. Valorise is helping us to take a giant step in the right direction.’

Cost price

The cost price of Valorise is a little higher than that of wood-based paper. Kort: ‘The purification and grinding of the fibres is still a comparably expensive process. On the other hand, as Valorise is a decorative type of paper that is used primarily in luxury applications, people are willing to pay a higher price. Fortunately, an increasing number of customers understand that a big step towards increasing sustainability is inevitably linked to a higher cost price. We ultimately aim to arrive at a production process in which the cost price is at the same level as that of conventional paper.’

Schut Papier has long-term plans to use other agricultural by-products in the production of paper. Kort: ‘We have already used by-products from tulip growers, other crops and water plants left from cleaning waterways or lakes.’ The question remains whether it is possible to make a unique line of paper out of every by-product. ‘These could be used for special editions, such as environmental reports or books about circular economics.’

Source: www.agro-chemie.nl. Photo: Mario Bentvelsen.