Next Generation Growing
‘Good plant balance ensures lower plant burden and faster ripening’
Gardener’s Pride, the tomato nursery belonging to Cock and Marja van Overbeek, is starting to master the principles of Next Generation Growing. The new greenhouse has vertical fans and manager Tim Schinkel is completely in his element because he can manipulate the plant burden. The air movement ensures a steady climate and good moisture control.
At Gardener’s Pride, in Beetgum, the north of the Netherlands, the end of March signals the time for interplanting. The old crop, planted in July 2015, is topped and young grafted, topped plants are ready for the new season. The tomato nursery specialises in small tomatoes up to 20 grams, both loose and on the vine. It grows 14 different varieties of which three are produced in the new greenhouse. These tomatoes are mainly sold to regular retail customers.
This is the third crop since October 2014, when the 55,000 m2 greenhouse equipped for Next Generation Growing was completed. It has a SON-T lighting system of 200 µmol/m2/s that can be switched on over four stages. Cock van Overbeek, owner of the tomato company and manager Tim Schinkel explain the decisions they made.
Open system
In the quest to adopt Next Generation Growing in this new greenhouse, Van Overbeek considered several different systems. He visited colleagues who have air hoses under the gutter but decided he didn’t want so many hoses per bay. Bearing in mind the investment would be high he therefore considered other air circulation systems. In the end he decided on the Ventilation Jet, a vertical unit with two fans. The top fan sucks in cold dry air above the closed screen via an opening below. The lower fan sucks greenhouse air upwards from the crop below and mixes the cold dry air with the greenhouse air that has been warmed mostly due to the lamps.
The roof is of diffuse glass and there is a blackout screen just above which is an energy screen. “I had a good feeling about this open system,” says Van Overbeek, who runs horizontal fans in other sections of the greenhouse. He also wanted to change the way he managed the climate and limit the role of the minimum rail via the pipe rail net.
Teething troubles
Based on these principles Tim Schinkel set to work. Initially it wasn’t that easy and he had to deal with a number of teething troubles during the first winter. It was very difficult to drain off the moisture and the crop began very vegetatively. “It's hard to point to a cause afterwards,” he says. “We suspect that the ground under the new greenhouse was very cold and damp which meant we had to use the minimum tube more often than we wanted.”
More things emerged. The over pressure under the screen and the air movement caused the screen to shift. It had to be weighted down. In addition, the fans tended to swing due to the lightweight fixtures. Also the diameter of the plexiglass disc between the two fans had to be adjusted to enable better mixing of the air (within one meter from the fan) and its distribution through the crop.
Finally, the energy screen was modified to allow a gap to be made above the central path. Smoke tests proved that the air flowed towards the middle of the greenhouse where the greenhouse is the highest. This provided an extra opportunity to move the greenhouse air upwards.
Air movement
Meanwhile, the Ventilation Jets (one per 350 m2, 161 in total), which are interspersed with regular vertical fans, have been properly adjusted. Once again it was smoke tests that showed that the airflow through the crop was now good and was able to take air heated by the lights down through the crop. It worked so well that it was no longer necessary to use the minimum rail during the period of artificial lighting. Heating the greenhouse is now primarily with the growth tube.
“We’d seen previously in the other, conventional greenhouse that the fruits don’t turn colour so well when the fans are turned on. In that section we installed air hoses under the cultivation gutters for extra air movement underneath. In the new greenhouse they turn colour very easily,” explains Schinkel.
First ventilate above the screen
The next step in Next Generation Growing is to fine-tune the climate. The art is to achieve a good climate and save energy at the same time. Schinkel says that the climate in the greenhouse is now much drier than during the first cultivation year. “Due to the adjustments to the fans we can now run at full speed,” he notes. Last year he sometimes saw condensation on the energy cloth at the end of the afternoon. That happens much less now.
If the temperature in the greenhouse rises too much or the air contains too much moisture he first ventilates above the screens, preferably at a low stand on the sheltered side and more on the wind side. This enables the moving airflow to provide the best exchange of greenhouse air and outside air.
The next step is to make a small gap of maximum ten per cent in the energy screen. The blackout screen remains completely closed during the blackout period. It seems that more air exchange takes place through the black out screen than through the energy screen.
Grip on plant balance
The diffuse greenhouse roof results in a more meagre crop, notices the manager. This is probably due to the transmission of a broader light spectrum, including ultraviolet. On warm, sunny days, he tries to keep moisture inside. In addition, he allows the radiation level and radiation sum to determine the 24-hour temperature. This can vary between 17.5 and 19.5ºC in the early spring in the unlit part of the greenhouse. Schinkel: “In this way I can keep a grip on the plant balance. I want the sugars that have built up in the plant to reach all plant parts within 24 hours.”
In this way he is steadily achieving a good plant balance, a slightly lower plant burden and as a result faster ripening of the fruits. He deliberately harvests larger fruits. “We prefer to pick fruits of 16 grams than 13 grams. We have of course a very hardworking crop that requires a lot of labour. We try to keep it as manageable as possible.”
Step towards sustainability
Everything requires effort, says Schinkel. A recent, short but intense, fire in the water house caused panic. Fortunately the damage was limited and the water supply was quickly restored. There are also some new challenges. In the new greenhouse they are currently running a trial with different substrate mixtures in containers, another step towards sustainable production.
Summary
In 2014, Dutch nursery, Gardener’s Pride, built a new greenhouse equipped with vertical fans. These fans and a double screen enable the nursery to apply Next Generation Growing techniques. The first year was not completely flawless due to teething problems. By adjusting the system the crop is now growing as cultivation manager Tim Schinkel intended.
Text/photos: Pieternel van Velden
‘In the greenhouse temperature differences not easy to solve’
In a large greenhouse with supplementary lighting the temperature differences in winter can rise so high it’s at the expense of quality and energy consumption. Berg Roses, of Delfgauw, the Netherlands, has already broken new ground with the Next Generation Growing and is fully committed to improving the climate. After a trial with vertical fans it is now running a trial with horizontal fans.
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NGG profits from new insight into air movement and evaporation
Over the last ten years the research and experience gained from Next Generation Growing has revealed many new insights about the complex relationship between air circulation, crop transpiration and plant growth. By applying this knowledge well, the crop can perform better even without the need for expensive techniques. “When the crop is at the centre of attention it automatically leads to energy savings,” says NGG pioneer Jan Voogt.
Copies of the Dutch book, ‘De basisprincipes van Het Nieuwe Telen’ (in other words The Basic Principles of Next Generation Growing which contains some 150 pages) are selling like hot cakes. Quite rightly it sets a new standard. Hundreds of copies are now circulating among growers and advisors in digital or print format. The creators are also considering publishing an English version. Co-author Jan Voogt (of Hoogendoorn Growth Management), who wrote the book together with Peter Geelen (of Plantmonitoring NL) and Peter van Weel (of Wageningen UR Greenhouse Horticulture), have a lot of respect for his work.
“That gives me a good feeling,” he says, sitting in the employer’s office. “And even more so if growers don’t have to invest in new greenhouses or other expensive hardware to enjoy the benefit. Any nursery can apply the principles of NGG but it does take a different way of thinking than we are traditionally accustomed to. A growing number of growers have started to recognise that. Some of them first installed new technology on a small scale to gain some experience and discovered that you can apply the principles just as well in a standard greenhouse."
Better understanding of transpiration
Why was it not done in the past? The answer is obvious: Because we didn’t know then what we know now. The gap in our knowledge was among other things related to the transpiration mechanism of plants and the role that air circulation plays in that. We only started to focus on that when things in the Closed Greenhouse occurred which couldn’t be explained by the then existing models.
Voogt: “It was strange that crops in the Closed Greenhouse transpired less on sunny days than those in a normal greenhouse and on overcast days even more. Now we can explain that because we know much better how transpiration and stomata work.”
In earlier models it was assumed that the stomata were either open or closed. Now we know that the opening varies as a result of the water and energy balance in the plant. The launch of the Hoogendoorn Stomata Sensor in 2007 made it possible to accurately observe that. “From that moment air circulation was also integrated into the models,” adds the climate expert. “That was a breakthrough and the most important initiative for the further development of Next Generation Growing.”
Balance of energy-, water- and assimilates
Stomata are the central control organs through which the plant can influence three balances: the energy balance, the water balance and that of the assimilates. The energy balance relates to the supply and removal of heat by means of radiation, convection and – after some give and take of these two – transpiration via radiation evaporation (referred to in the book as whistling kettle evaporation) and convection evaporation (similar to wet bulb evaporation).
The water balance is determined by the relationship between water uptake, transpiration and the amount of water stored in the crop. The correct ratio is important for good uptake and transport of water and nutrients and of course how much the plant is allowed to increase its fresh weight. If the balance is tipped, for example, due to high radiation and low RH, then the stomata close and transpiration and CO2 uptake are slowed down.
Keep windows closed in sun
Without describing all the background and scenarios in detail, these new insights into the dynamics of the plant balance have considerably changed the view on cultivation. “For the New Way of Growing a completely different way of thinking is required,” adds Voogt. “Is it a sunny day? Keep the windows closed as much as possible because then more moisture and CO2 remain in the greenhouse. At a high RH the plant can better maintain its water balance and the stomata in the leaves stay open wider so the crop takes up more CO2 and speed of growth increases.”
Previously, ventilation was based almost exclusively on the greenhouse temperature. When the radiation was high the windows were opened wide to remove the surplus energy, but the crop could only maintain the associated high level of transpiration for a short time. As a result the RH in the greenhouse fell, the stomata closed and the crop took up less CO2, while the greenhouse temperature still often remained too high.
“The New Way of Growing takes the RH in the greenhouse much more into account,” says Voogt. “Often this can be much higher, just like the greenhouse temperature. In this way the air vents and the screen can stay closed for longer. In the end you’ll have to drain off excess moisture or energy but the critical limit lies much further away then previously thought.”
Screen longer, ventilate less
Air movement plays an important role in the microclimate around the plant. It promotes the removal of moisture that arises from transpiration by the crop and supplies energy by convention so that, in the dark period, transpiration can continue which is necessary for the uptake of, for example, calcium. Keeping the plant active by means of the fans costs less energy than using the minimum pipe rail.
An active crop with a balanced and stable plant balance can endure a lot. By monitoring the crop with greenhouse sensors and plant sensors (which these days are fairly affordable) and steering it based on this data, by screening for longer and ventilating less, the energy bill decreases while the crop becomes more productive. An indispensable tool for steering the crop is the psychrodiagram, a practical version of the Mollierdiagram. By not only looking at the RH or moisture deficit but also at the Absolute Humidity (AH), dew point and energy content of the greenhouse air compared with the outside air, it is possible to grow more effectively and energy efficiently.
Radiation
Another point to consider, which previously hardly received any attention, is radiation. “In 2009 this was revealed as the cause of flower head rot in gerbera,” says the NGG pioneer. “Until then it was thought that a high RH was the main culprit. Due to radiation the plant temperature at the top of the crop can fall to several degrees below the greenhouse temperature, so that transpiration comes to standstill and moisture condenses more easily. The message from this is: close the energy screen in good time and keep it closed for longer. The top of the crop remains more easily at the right temperature, the plant grows better and receives more nutrients. This is true for most crops.”
Summary
Ongoing insight into the mode of action of transpiration and plant balances has led to a different view on cultivation and climatic control. As long as plants are in balance and the stomata are open, so that transpiration can continue, they remain more active even when the RH and greenhouse temperature are higher. Thus the air vents and screens can stay closed for longer, the crop takes up more CO2 and energy is saved. This is sustained by good air circulation in the crop.
Text and images: Jan van Staalduinen
‘The multitude of solutions is too complex and hinders exchange of knowledge’
Roughly 60 to 70 greenhouses in the Netherlands have invested in air handling systems and they are all different. Growers who are interested in the Next Generation Growing can no longer see the wood for the trees. Specialist advisers believe it can be much simpler and cheaper as well as more standardised.
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Less energy input yields as many or more gerberas
Since 2014 Dutch gerbera grower Reijm Nieuwerkerk has been using a system to actively ventilate the greenhouse air as part of its Next Generation Growing strategy. The continuous refreshment of the air provides an optimal microclimate at the bottom of the crop. The company is producing the same quality gerberas with less energy input.
Reijm Nieuwerkerk cultivates 5 ha of pot plants and 3.5 ha of gerberas at three locations in Nieuwerkerk aan den Ijssel, near Rotterdam, the Netherlands. Annually the company produces 4.5 million pot plants and 16 million gerberas in many colours, from large flowering to minis.
Cultivation is in stone wool substrate in a gutter system. A double screen, comprising a black out and energy cloth, has been installed at the top of the greenhouse. A CHP cogenerator with a flue gas cleaner for CO2 production also generates electricity for the lighting (95 µmol).
Active ventilation
Last year, for the Next Generation Growing, the Active Ventilation System (AVS) by Van Dijk Heating was installed in 7,500 m2 of gerberas. This year the area has been extended by 17,500 m2. The system comprises a wall unit made of plastic which contains a fan, mixing valve, heating element and a filter. From the unit and running along the greenhouse wall is a PVC pipeline which acts as the distribution system. Air hoses that contain small holes for air injection are connected to the pipeline. The air hoses hang under the rows of plants.
The greenhouse is dehumidified thanks to active ventilation of the greenhouse air that is mixed with relatively dry outdoor air. The heating element provides low value heat (40ºC water from CHP) for warming the external air to greenhouse temperature.
Learn from practise
The reason for installing this ventilation system is to produce the same or more gerberas with less energy. “The art is to input the right amount of energy at the right moment. In the past we used to turn on the minimum pipe for this but actually we didn’t know exactly what we were doing,” says Jaré Reijm. He runs the family business together with his brother John. Over the years the grower has carried out several trials with gerberas and therefore has learned a lot about the greenhouse climate without having made too much investment in his nursery. “But then it’s not easy to further improve the greenhouse climate, although also not easy to make an investment and then to earn it back. However, installing this system has worked well. Last year we achieved a good cultivation climate. We were able to keep the climate under the plants somewhat drier and above the plants more uniform,” says the grower.
Better temperature distribution
Joek van der Zeeuw, of Van Dijk Heating, explains: “With traditional methods of growing you always need some gaps in the screen and you have a cold dump were you don’t want it. Now, by using this system to inject outside air into the greenhouse you obtain a small amount of over pressure. That is enough to ensure that no cold air enters through a hole or gap so the temperature distribution in the entire greenhouse is more uniform.”
The grower indicates that the temperature difference used to be 3ºC and now is just 1ºC. Further, he would like to better control the microclimate between the plants. The holes in the hoses where the air comes out are therefore set to point upwards. The air that is injected into the greenhouse has been mixed with air from outside and so contains less moisture. Due to a lower RH the crop is less susceptible to disease. “By continuously refreshing the air, we now have a good climate at the bottom of the crop. As a result we think that the flowers are less susceptible to fungal diseases such as Botrytis. And we can achieve this without using the minimum pipe,” says Reijm.
Drive controlled fans
The system has an air-displacement capacity of 6 m3 per m2. The fans in the wall units are equipped with energy-efficient EC motors that are controlled by a computer network. The fans can be reduced to 50% of the capacity, so they only consume 100 Watts of electrical energy for dehumidification. At full output 800 Watts is needed. Reijm tries to run it for as long as possible at low power, until the valve for the outside air is fully open. If then the humidity in the greenhouse rises, the speed of the fan is increased gradually. In this way the grower makes the most efficient use of the system.
Normally, the ventilation units are fitted in an outside wall to be able to draw in outside air. This was not possible in a partitioning wall with the pot plant greenhouse. Here separate units were made that draw in outside air from the top and have air passage through the roof. This suction hole in the roof is covered with a flat sheet so that the roof cleaner can drive over it.
Winter and summer
“Due to the active ventilation it is possible to use the energy screen more in the winter months because we can better control the climate, the humidity and the temperature distribution,” says the grower. Last summer he used the system for the first time for cooling. This worked well. Cooling was achieved by sucking in outside air and injecting it into the greenhouse.
Reijm: “Despite the tropical temperatures outside the greenhouse remained cool for longer. Even when it was 30ºC outside, it remained 25ºC under the crop. Because we measure everywhere under the crop, we see a difference with the traditionally grown crop. Because the temperature is lower for longer, we can open the vents later. This keeps the CO2 inside. Due to the cooling, the crop remains more active so that the quality of the flowers is better. The diameter and stem length remained the same during the summer. However, this was not the case with the traditional cultivation. The flower was smaller and the stem was shorter.”
Pure CO2
Reijm uses the system for dehumidifying and cooling, but not for heating. The grower doesn’t dare to use the latter for Next Generation Growing. “By heating from underneath we would push the crop too much. As a result the crop would transpire more, which would result in a higher humidify. Then we would have to get rid of the moisture in one way or another and that costs energy.”
A disadvantage of the system is that because less heat is required, the grower has a CO2 shortage. Because the CHP cogenerator produces heat, which the grower would have difficulty getting rid of, it only runs to produce electricity for the lights. However, in the summer months it doesn’t run for long enough to provide sufficient CO2. This summer the grower will have to buy and inject pure CO2.
Uniform growing conditions
The grower would like to have the same growing conditions for all the gerberas. Because they change the crop every three years, they will be able to install this system in the last hectare of gerberas next year. Installing it while a crop is present is not wise, according to Reijm. In addition, they will replace the entire cultivation system so they can do everything at the same time.
Summary
Since 2014 Dutch nursery Reijm Nieuwerkerk has been implementing the Next Generation Growing, a system that includes active ventilation. The grower can dehumidify the greenhouse air by mixing it with air from outside. He can produce the same number of gerberas with lower energy consumption. The continual refreshment of the air provides an optimal microclimate at the bottom of the crop. As a result flowers are less prone to fungal diseases such as Botrytis.
Text/photos: Harry Stijger