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.