Hans Houben’s initial reason for trying out Next Generation Growing was to save energy. “But that shouldn’t really be your main objective: you need to focus on the plant,” the cucumber grower says. This has led to a higher 24-hour temperature, growing with the light, more screening and adjusting for outgoing radiation. Oh, and lower gas bills too.
Every year is different when you use the Next Generation Growing (NGG) method. You keep taking one step further and all of a sudden you’ve created a completely new way of growing. “I am always very keen to keep up with the latest developments, but to begin with I was quite sceptical. It all sounds very logical but it takes courage to do it. Right now I’m starting venting on the wind side, for example. It is working out well: you get a much more even climate and it’s easier to keep the humidity at the right level. But I really wouldn’t have done it this way two years ago,” Hans Houben says.
His screening hours have also increased: he is now screening twenty per cent more than in his second NGG year. “During the first couple of years you tend to be a bit wary of doing things this way. Before I started I used to mainly keep an eye on relative humidity, but now it’s all about absolute humidity, humidity deficit, vapour pressure and outgoing radiation.”
Back to the plant
Hans and Carla Houben’s cucumber business Mellantas in Sevenum (4.7 ha) in the south-east of the Netherlands is on its third season of high-wire cucumbers. At their old site they had two crops of cucumbers per year, followed by autumn tomatoes. After moving to their new location they introduced high-wire cultivation with two crops per year, first Topspin and then Kurios. The plants grow in rockwool that lies on the ground.
Gas consumption is currently at 28.5 m³/m² for production of 230 cucumbers per square metre. A traditional crop would use 34-35 m³/m² for 180-195 fruits. “The power of Next Generation Growing lies in the fact that you are going back to the plant,” Houben says. “We have started growing more quickly, with a higher 24-hour temperature, but we keep the plant load at no more than 6-7 cucumbers per plant. From 11 am onwards we allow the temperature to get higher than before, light permitting.”
For example, with 1,000 watts of incoming radiation the 24-hour temperature is 21.5ºC, and with 500 watts it is 19.3ºC. In his first two years of NGG, Houben allowed an extra 1.5ºC per 1000 joules of incoming radiation over and above the basic temperature of 18ºC. Now it is 2.5 to 3ºC extra – so quite an increase. He achieves this with a combination of heating, screening and ventilation.
“An extra 1.5ºC saves more energy, of course, but it makes the crop more sluggish. When it’s sunny we want a higher temperature, preferably 28ºC after 11 am rather than 25ºC, light permitting.” Before 11 am he aims to achieve a moisture deficit of 1.5-2 g/m³ to activate the crop; after that he works up to a higher temperature in a gradual line. “I used to turn the temperature down sometimes if there was a lot of light. But I don’t do that any more. You can tell by the top of the plant whether you are doing the right thing. If it is getting too thin, the 24-hour temperature needs to come down.”
If the temperature is higher during the day, the night temperature can be reduced slightly, although it is the overall 24-hour temperature that counts. Less use of minimum pipe prevents excessive evaporation and limits night-time energy consumption. Incidentally, the main source of heat is the grow pipe, which is always level with the fruits, and not the pipe rail.
More outgoing radiation
Over the past few years the grower has started screening twenty per cent more to limit outgoing radiation. He uses a very light Luxous energy screen from Svensson which only screens out twenty per cent of the light. A radiation meter (pyrgeometer) on the roof helps control the screens. There is also a thermal camera pointing at the crop. This isn’t connected to the climate computer but is used as an additional adjustment tool. Houben demonstrates how it works on the computer screen. “This morning there was a rain shower just after we opened the energy screen. You can see on the thermal image that the temperature at the top of the plants dropped to 15.5ºC at that point. You want activity but the tops of the plants are cold. So I closed the screen again and within ten minutes the temperature at the top of the plants had risen by 4-5ºC. That’s because you are eliminating outgoing radiation.”
The principle is simple. When outgoing radiation is higher than what is coming in, the screen is closed, even on a warm summer’s day. “In that case you close it ninety per cent. Then you can control the temperature easily and control outgoing radiation at the same time,” he explains.
The numbers always add up. For example, if there is 200 watts of radiation coming in, the screen blocks out 40 watts of that. But with a clear sky, outgoing radiation from the crop soon reaches 80 watts, and because that is more than 40, the screen has to be closed.
Houben has invested in a pyrgeometer, a thermal camera, a leaf temperature sensor and an extra sensor unit above the screen, but not in air handling units, extra fans or a second screen. “I could save an extra 2-3 m³ of gas with a second screen, but then I’d need a dehumidification system as well. The maths wouldn’t necessarily work then. So we decided not to do that just yet. We are waiting for dehumidification technology to move in a clearer direction,” he says.
With the experience he and other growers have gained, Houben sees potential to improve the system even further. “There is definitely scope to optimise the light/temperature ratio. You might be able to grow even faster with more light. If you can pluck up the courage, you could turn the temperature down more in the evening because you would still be achieving the 24-hour temperature, and that’s what counts. So an extra pipe during the day and not at night. But you could even raise the 24-hour temperature, which would enable you to maintain a higher temperature at night and make better use of the screen. And on sunny days you could also extend the day by switching to the night temperature later.”
Houben is also trying to gain a better understanding of the minimum level of evaporation needed at night. He is very happy with the knowledge shared on the LetsGrow platform. “I am learning a huge amount by looking over other growers’ shoulders. You don’t have to find it all out for yourself. You can see exactly what time other people open their screens and what that achieves. Next Generation Growing is really still in its infancy. You can get much more out of it if you focus primarily on the plant.”
Text: Tijs Kierkels, image: Wilma Slegers