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Growers updated on screens, moisture and evaporation

NL: "Growing at 90% RH is a breeze"

After several winters of growing under LED, dealing with moisture and energy remains a challenge for growers. During the EnergiekEvent, various workshops discussed this topic once again. The growers and advisers present got (new) insights, but not always the answers they might have secretly hoped for.

Looking back at (old) research is nice, but we have to grow a good lighted LED crop again this winter. This was mentioned by a young tomato grower when a workshop was a bit too much about the past for his taste. He pointed out that LED wasn't available back then. Shouldn't we try to understand the big picture before we get lost in details? He posed this question during a workshop on dehumidification. This question reflects that growers want to quickly become better at exposed LED cultivation with the help of research results.

A lecture by Silke Hemming during the EnergiekEvent. Click here for a full photo impression (in Dutch).

Making good screen choices
In her workshop on screens in the greenhouse, Silke Hemming of the WUR focused on energy, and the role of (new) screen cloths, also when it comes to exposed crops. The WUR researcher did so using insights from an ongoing Energlik project.

Silke showed which screen properties are important, and she stressed the importance of the behavior of screen cloths under wet conditions. After all, screen cloths in greenhouses tend to be wet rather than dry, for example because of condensation. In their research, the researchers look at different screen properties, which they measure and quantify for light transmission, heat radiation properties, air permeability and moisture transport.

During the workshop, the WUR researcher insisted on the possibility of increasing the RH set point. By doing so, significant energy savings can be achieved. If you go from 80% RH to 90% (much higher is not necessarily advised because of growth inhibition for crops, and other factors), you can save 35-40% energy at night. With day screens that percentage is lower, but still significant at 15-30%.

Many growers have started to replace screen cloths during the energy crisis or have had an extra cloth pulled in. Researchers are happy to receive the old screens, Silke said. "But one meter is enough," she laughed when there was a bit too much enthusiasm from the audience.

In some cases, Silke showed, for night screens it's worth trying to pull in two regular cloths instead of one "very good" newer cloth. During the day, the differences are smaller, even between the various canvases studied.

Film rolls
Research has shown that growing at 90% RH is 'a breeze', though with active dehumidification, Silke shared about the first studies in the Energlik project that continues until early 2026. Then again, a 'disadvantage' of growing with higher RH is that the active dehumidification system does not have to run as often, Silke laughed. "That's good for operational costs, but perhaps less so from an investment point of view."

A new development in the screen field is a transparent low-E screen. Another point of attention is research into screen cloth that allows growers to avoid CO2 loss, while the screen cloth continues to play a role in dehumidification.

The low-E screen is a screen that's not yet commercial, and that the researchers are testing with. This year and next year, Silke doesn't expect the screen to be on the market. She herself refers to it as 'a transparent Gore-Tex fabric', without actually being Gore-Tex. The material used comes from outside the industry, from film rolls. Disadvantage for now: the mechanical properties of the material are still a challenge for screen installations.

Click here to watch Silke Hemming's entire presentation on screens (in Dutch).

Silvester de Nooijer (Delphy), Stefan van den Boogaart (Plant Lighting), and Jos Paul (WUR) during the workshop on evaporation. Click here for a full photo impression of the day.

How much evaporation is needed, and how can you control evaporation?
In the session on dehumidification, workshop participants could attend three presentations that discussed studies on evaporation. Stefan van den Boogaart of Plant Lighting asked two questions, to which both answers were 'no'.

The first question: Do we know how much a crop needs to evaporate to grow within safe limits? And the second question: Do we know at what times of the day a crop should evaporate? He had preliminary conclusions from a chrysanthemum study. Leaf quality in chrysanthemum seems to be more of a climate issue than an evaporation issue. Further research will look for the lower limit for evaporation while maintaining quality.

When it comes to tomatoes, the hypothesis in Delphy research is that too low evaporation in commercial crops caused problems in recent winters. The researchers are specifically looking at watering and nutrition as solutions. A follow-up trial in 2024-2025 will also focus on air movement.

After the third presentation in this workshop on cucumber research in an indoor growing cell there was a vivid discussion. One of the ideas: Let's stop setting watering purely based on joules to avoid too much focus on radiation evaporation.

Click here to see the entire presentation of the evaporation workshop (in Dutch).

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