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In Next Generation Growing, everything revolves around the needs of the plant. In the past, much of the way growers controlled the climate was based on green fingers and experience. But with the latest developments in greenhouse technology, the indoor climate has become a more complex factor. This also makes the relationship between conditions outside the greenhouse, the growing climate on the inside and the impact on plant growth less straightforward. The combination of plant monitoring and an advanced control system helps the Dutch company Van de Berg Roses to better match their irrigation to the needs of the plant.

Hoogendoorn Growth Management researcher Jan Voogt differentiates between three plant balances: the assimilate balance, the energy balance and the water balance. All three need to be in balance to achieve optimum growth. If you only factor in the energy from the sun and not the rest, then you’re not doing your job properly, he believes. He recognised that growers who work with lighting didn’t like the fact that they couldn’t input the energy they added from the lighting into the climate computer. And the same was true of the other factors that play a role in the energy balance.

In response to this, Hoogendoorn developed a new monitoring and control system: PlantVoice. This software module focuses on all three balances and takes the plant’s activity into account. Other factors apart from sunlight can be entered in this system, and irrigation is primarily determined on the basis of the energy flows. “The more factors you include, the more accurately you can fine-tune the amount of water to match the energy supply to the plant. That also gives you more control over the root environment and less unnecessary return water,” Voogt says.

First users

Van de Berg Roses has a 12 hectare rose nursery in Delfgauw, as well as sites in Naivasha in Kenya and Kunming in China. Maurice de Ruijt, cultivation manager at the company’s Dutch site, has been using PlantVoice for the past eighteen months. “We were pretty much the first users,” he says. “We used to irrigate based on our outdoor sensors which measured the amount of sunlight. We use a lot of artificial light here, so that wasn’t particularly helpful. We told them that we wanted to irrigate based on the PAR sum. The new module is a much more reliable measure of conditions in the greenhouse.”

PAR sum

De Ruijt had already been indirectly watering based on the PAR sum via the LetsGrow system, which had given him quite a bit of experience in this method of irrigation. He uses the measured PAR sum to determine when to start irrigating. Since he started using the new software module, he has started irrigating at the point when he measures a radiation sum of 10 mol/m2 after the last cycle. He knows from experience that the slab will have dried out by around ten percent by then. This point is reached between 2 and 6 am. He doesn’t want to start any earlier or later.

De Ruijt: “The start time can vary by a couple of hours. There could be various reasons for these variations. We can switch our lighting system on in three stages: 33, 66 and 100 percent. Sometimes we use 66 percent for a while.” What’s more, the outdoor conditions can vary after they have stopped irrigating. “Like if it’s a cloudy day but the cloud cover breaks up after you finish watering, and then you have an hour or two of sunshine.”

He waters around eight to ten times a day, stopping at 3 pm. De Ruijt still decides how often to water based on the PAR sum. For the time being he isn’t using any other factors such as the energy given off by the heating pipes to decide when to start watering, as that is more complicated and the amount of energy is negligible.


Also important, of course, is what this method of climate control can deliver. “We can see that the plants have nice white roots. The crop is healthy and we’re getting better yields.” De Ruijt won’t reveal any more than that. Using the module to control irrigation is a big step in itself, but the company has also made other changes to its irrigation regime. And this, too, is part of a greater whole. “It’s a great tool to work with. It was tricky to find the right settings to begin with, but now that we’re used to it, it works really well.”

More reliable trials

Bram van Haaster, trial manager at Wageningen University & Research in Bleiswijk, the Netherlands, has been using the new module since the summer. He looks after the vegetable, flower and pot plant crops in the trial greenhouses. For the research it’s important to keep the climatic conditions around the plant as stable as possible and to only allow the factors being studied to vary.

He used to control irrigation based on outdoor radiation, the amount of drain water and instinct. That was tricky because the percentage of drain water fluctuated. Now he keeps an eye on the energy balance using sensors and data from the climate computer. He measures the amount of radiation in the greenhouse with a PAR sensor above the crop. He can input into the computer how much energy he is adding via the pipes and whether or not he is screening. In this case, energy input from the heating pipes is left out of the equation as it is a constant, low factor. “If the heating were to fluctuate, we would need to take that into account,” Voogt adds.

As a control, he uses an IR sensor that measures the plant temperature. Any rise in the plant temperature is a sign that the plant is not transpiring enough. In that case, he needs to adjust the irrigation or reduce solar radiation levels.

First experience

Van Haaster gained his first experience with this system in the summer, in a pot plant trial in which various substrates were tested alongside each other. Now a tomato trial under LEDs is underway, comparing various varieties and studying the effect of adding, or not adding, extra steering light in the form of long-wave red radiation. The light is on between midnight and 6 pm. The trial manager has been aiming for a stable drain percentage in both trials.

At this time of year, van Haaster mainly uses the PAR sensor to measure the light from the LEDs. “The outdoor light is around ten percent at best at this time of year. The increase in joules is easy to track on the computer,” he says.

On the screen he points out a neat, constant line representing the amount of drain water. Since he started using the module, the drain percentage has been stable. “If you don’t have to do much adjusting afterwards, that means you have got the settings right,” he says. “The root zone is nice and stable, both in terms of water content and EC. That’s good for the research, but it also benefits growers. You become more aware of your plants’ needs so you can get more out of them.”


With recent developments in greenhouse technology, the indoor climate has become more complex and more factors are influencing the plant’s energy balance. And that in turn impacts on the water balance. A software module is the answer. PAR sensors inside the greenhouse measure the amount of radiation in the greenhouse, and other energy factors such as the heat from the heating pipes, the screening factors of screens or coatings, diffusion and ventilation factors can be added in. Altogether they produce a better picture of the plant’s energy balance and therefore its water requirement.

Text and images: Marleen Arkesteijn.

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Fertiliser precipitate and biofilm in the water system can create a breeding ground for diseases and clog the system. Tomato nursery Varom is tackling these problems with a combination of a polyphosphate and biocide, focusing primarily on prevention.

The water system at brothers Guy and Dennis Van Rompaey’s nursery in Rijkevorsel in the Belgian horticultural area of Hoogstraten has grown as the business has expanded. “Originally we had a unit for 4 ha. When we added another 2.7 ha in 2008 we decided to use the same unit to service the whole area. For that we need a day’s supply, which we make up at night. It’s all a bit tight but we just about manage,” Guy Van Rompaey explains.
But this day’s supply has turned out to be a weak spot in the system. Bacteria can grow in the still water, giving rise to localised problems such as excessive root growth caused by Agrobacterium rhizogenes. “Two years ago we had such a severe problem that we suffered noticeable harvest losses,” he says. The brothers grow truss tomatoes (Merlice) and Coeur de Boeuf.

Two-pronged approach

Bacteria formation in the water system is a complicated matter. Fertiliser precipitate is a source of nutrients for biofilm, a slimy layer of microorganisms that are not hazardous in themselves. But the slimy layer forms a breeding ground and refuge for diseases. The problem can be tackled with a two-pronged approach: by preventing precipitation and by killing the biofilm-forming organisms and pathogens.
“At the end of the season we always used to clean the whole drip irrigation system through with nitric acid and bleach. We left each of them in turn in the system for 24 hours and then flushed it through. But nitric acid contains nitrogen which we aren’t allowed to discharge any more, so it’s better to avoid bleach if you can,” the tomato grower explains.
So he sat down with Guy Pluym, an advisor from suppliers Sanac, to take a closer look at the options. Besides the situation on site, there were also statutory measures and market developments to be taken into account.

Stricter laws

The EU Nitrates and Water Framework Directives are implemented in national law differently in the Netherlands and in Belgium. In Flanders, the Fertiliser Action Plan (Mest Actieplan, MAP) prohibits discharges of nutrient-rich water from greenhouses. There is a phasing-out plan in place for discharges in the Netherlands. So the law is currently stricter in Flanders than it is in the Netherlands, requiring Belgium growers to take action to be able to recirculate water problem-free.
“There is also pressure on the use of chlorine. The Knowledge Centre for Sustainable Horticulture in Belgium advises against the use of disinfectants containing chlorine unless the grower is certain that they won’t exceed residue limits if they do so. This advice was issued after the trade found chlorate in vegetables and fruit,” Pluym says.
Chlorate is a point for concern in Germany as well, where there has already been an outcry over this issue.


Having considered all the options, the grower and the adviser decided on a combination of the polyphosphate VitaPhos K and the biocide Huwa-San. The polyphosphate goes a long way towards preventing fertiliser precipitation and can carry any existing precipitate with it in solution down to the plant roots. The biocide kills the microorganisms.
Phosphate fertilisation can be done using two forms: orthophosphates and polyphosphates, Marco Molenaar of VitaPhos suppliers Haifa explains. The first form can precipitate with higher pH values and a high EC together with other elements such as magnesium, calcium and trace elements, making them less available to the plant. “You can counteract this by replacing some of these with polyphosphates. Not only do the elements remain in solution better, they are also available for uptake by the plant,” he says.
The ratio between ammonia and nitrate nitrogen also plays a role in the uptake of the various elements. The first has an acidifying effect while the second increases the pH around the root medium. Using more ammonia can encourage uptake of various elements. “We do that as well, but only to a limited extent,” the grower says. “We are very wary of blossom-end rot, you see. Coeur de Boeuf is extremely prone to that, and we are worried that a higher proportion of ammonia will cause problems.”

Blossom-end rot

This is a common concern among growers, Molenaar finds. “And yet we still advise people to use more ammonia, although you can of course stay on the safe side. But to avoid blossom-end rot it’s just as important to keep an eye on the potassium to calcium ratio. We recommend 4:7 or 5:8, in other words less potassium than calcium. Preventing precipitation while encouraging uptake at the same time therefore calls for an approach that not only involves phosphate but also nitrogen, potassium and calcium,” he says.

No chain reactions

The biocide they opted for is Huwa-San, a stabilised hydrogen peroxide. “Plain hydrogen peroxide also works as a disinfectant but it reacts away too quickly in the vicinity of organic contamination. So you will never get enough in the system to disinfect it right to the end,” Severina Windmolders of the product suppliers Roam Technology explains. “Our product is stabilised with a positively charged silver ion. The bacteria in a biofilm are negatively charged so they attract the positive charge. A reaction therefore only happens at that spot. You don’t end up with chain reactions as you would with ordinary hydrogen peroxide, and proportionally more active hydrogen peroxide passes through the system right to the end.”
Adviser Pluym confirms that there will still be active biocide at the end of the process. “If there is a big difference in levels between the end and the beginning of the system, that means your system is highly contaminated,” he says.

Starting early

The grower applies the mixture in three places: in the drain water, the mixing tank and the watering pipe. He does this to be on the safe side, as he is working with a day’s supply. He now has one-and-a-half seasons’ worth of experience to look back on. “We haven’t counted how many mats are affected by it, but I do think I had less damage from excessive root growth last season than two years ago,” he says. “I would have preferred to have been completely problem-free by now, but we are going in the right direction.”
Measurements show that Agrobacterium infections are significantly reduced, however. It may be some time before the old contamination has disappeared completely from the system. Windmolders emphasises the need to start early. “A few years ago, growers tended not to start disinfecting until around March or April,” she says. “We have taken measurements at nurseries and have seen quite a high degree of bacteria formation in still water as early as in January and February.” Pluym adds: “So it makes sense to start disinfecting when you fill the mats.”
The introduction of the system at the nursery was accompanied by regular measurements and advice. “This support is extremely useful,” Van Rompaey says. “The problem won’t be solved straight away, but we can see that it’s going in the right direction. I hope we’ve got a grip on it now.”


The Belgian tomato nursery Varom uses a combination of a polyphosphate and a biocide to control precipitation and biofilm formation in its water system. This method was chosen because of the risk of disease, the legal requirements and market demand. Disease problems have eased since they started using the new approach.

Text: Tijs Kierkels. Image: Wilma Slegers