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.

Text and images: Pieternel van Velden