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Peppers can be grown without minimum pipe
The trial was funded by the Dutch program Kas als Energiebron, with contributions from Svensson and Cultilene. Kas als Energiebron is the innovation and action program of LTO Glaskracht Nederland and the Dutch ministry of Economic Affairs.
From mid-June 2016, Stefan Hendriks of Delphy followed the Next Generation Growing (NGG) pepper test as a consultant. The cultivation has now been completed. Time for a retrospect. "In the years I grew peppers myself, I was always at the upper end of energy use compared to my colleagues. When energy use is compared, you really need to know whether extra gas needs to be used for CO2. When comparing the screen hours, I was always at the lower end," he says. "So when I got the opportunity to join in the NGG pepper trial, I was very skeptical about the goals set in advance: Growing peppers with an energy use below 20 m3, with a practical production and quality." The cultivation has now been completed and stock has been taken: with 16.8 m3 gas, a 34 kilo/m2 (gross) production has been achieved. So the previously set goals have been more than cleared.
NGG and screens
"NGG focuses on optimizing the cultivation climate, resulting in significantly more screen hours and a higher daily temperature compared to practice," Stefan says. Generally, the upper screen could be opened already at 100 Watt irradiation per m2, the lower screen could only be opened when the temperature above the screen differed 4 degrees with the desired heating temperature. In the morning, the screens remained closed longer as well. "In order to prevent problems with humidity, we did ventilate above the screen, on two sides. Ventilating above the screen was preferred over pulling cracks in the screen. Cracks in the screen create cold drafts and temperature differences in the greenhouse."
The lower screen was closed for 5700 hours in total, the upper screen was closed for 6012 hours. Compared to normal practice, with 2,700 hours on average (single screen with fixed foil), that's significantly more.
NGG and the minimum pipe
A major step in the trial is leaving out the minimum pipe. "As a grower, I set the minimum pipe as a rule during dark days, in order to create enough activity," Stefan says. "With NGG, the idea is to only introduce pipe heat to achieve the set heating temperature. Vertical ventilators were placed to create sufficient crop activity this way." The cultivation was also completed without minimum pipe. "As a side note, the trial was done on 1,000 m2 with a relatively high number of gables, which may have caused increased drainage."
NGG and daily temperature
For former grower Stefan, the daily temperature was notable. "With NGG, this ends up significantly higher because of the warmer nights (around 20 degrees). The screen was always closed, to prevent emanation. The higher temperatures during the day were also noticeable. As a grower, I would avoid afternoon temperatures of more than 25 degrees. With NGG, it often rose to almost 30 degrees during the afternoon." He hasn't seen any crop damage because of this, however.
"The plant load was higher, however. While in everyday practice the plants had 40 fruits per m2, with NGG we often had 50 fruits per m2. The higher plant load is a cause of the lower average fruit weight, which was about 165 grams."
The top, young fruits and flower are also warmer due to the heavy screen use in NGG. "That means the plant takes less time to grow, which in turn means the plant roots faster, and you end up with a higher total production."
Vapor pressure deficit
Because of the high temperatures (particularly in the afternoon), the "vapor pressure deficit" (VPD) was lower. "Between 1 and 2, while in practice it's often over 3. This positively influences the stomata, which stay open more and longer this way," Stefan observes.
Inner rot only occurs sporadically, starting from October. "Because of the low gas use, I had expected inner rot earlier, and more of it. Because the screen opens later in the morning, the top of the plant is at room temperature, and I think this has a positive influence on preventing inner rot," Stefan ventures. "The high screen use also caused less irradiance, which also positively influenced the top temperature."
And what about the practice?
The trial has shown it's possible to grow without minimum pipe, but Stefan doesn't want growers to have it turned off just like that. "That step is too big for practice. A responsible way to get here is first of all by turning down the pipe temperature, and carefully looking at what moments it can be left out completely."
"As a grower, you're used to always letting in the maximum amount of sunlight, but with NGG we've seen that more screen hours is possible. You do need to ensure, as a grower, that there's still enough crop activity, for instance with ventilators, or by ventilating above the screens."
The trial showed that the upper screen sometimes only opened at 400 Watt of irradiance. "The focus was more on the temperature above the screen: if it was within 4 degrees of the heating temperature, the screen remained closed. In this case, the power of the sun caused enough crop activity. In the late afternoon, the screen was also closed quite quickly: as soon as the irradiance was less than 100 Watt, the screen closed. This meant pipe heat was used a lot later during the evening/night to achieve the desired heating temperature."
Follow-up trial with three screens
A follow-up trial has started now. The Allrounder variety, sown on October 16 and planted December 1. "The good thing is that now, with Maikel van den Berg's company, we can make an even better comparison with normal practice," Stefan continues. "In addition to gaining new insights in NGG, we also want to level off the energy spikes in the beginning of cultivation. We want to realize this by using a third screen." At the moment there's one single screen, and one double screen with a cavity - still plenty of challenges regarding NGG. "One thing's for sure: we will gain more insight and learn how a plant reacts in an optimum growth climate with NGG. And I'm looking forward to it!" Stefan concludes.
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