Dr. David Katzin has been a renowned researcher at the WUR for many years. He specializes in the use of simulation models to estimate resource use and crop production under different conditions. In one of his recent studies, he examined the impact of movable screens on sweet pepper production, in comparison to the use of traditional whitewash. The research was based on data collected by Svensson and was conducted under Spanish greenhouse conditions. To ensure that these conditions accurately represented real‑world operational practice, Dr. Katzin carried out the study using KASPRO. This is a greenhouse simulation model that has been validated multiple times against data from real operational conditions.

Prior to this research, Dr. Katzin identified several key parameters to assess the research outcomes. “We focused specifically on temperature, humidity, and PAR levels within the greenhouse, as these factors are essential determinants of crop growth and development,” he notes. “Since actual production depends on the growth conditions, we used PAR as a direct indicator of potential yield. If the same light‑use efficiency (LUE) can be maintained, then 1% more PAR results in 1% more yield.”

Whitewash is a familiar method to reduce heat entering the poly greenhouse, and to keep light levels under control in the middle of the day. However, it also comes with clear limitations and lacks the flexibility growers need. According to Dr. Katzin, the main issues with whitewash primarily occur during the morning and afternoon, especially in the early season. “Our research examined a spring cultivation (May-July), a time of year when shading is generally unnecessary in the early morning and late afternoon,” he says. “Once a fixed layer of whitewash is applied on the greenhouse, growers lose valuable light during those hours, even though there is no risk of overheating. This unnecessary loss of light reduces the crop’s yield potential.”

This raises an important question: could movable screens be a more flexible and profitable alternative to whitewash? The research delivered a clear answer. According to Dr. Katzin, one of the key findings is that greenhouses equipped with movable screens can receive up to 30% more PAR light over the growing season, compared to using whitewash. “Although movable screens slightly reduce light intake when closed at noon, they allow growers to retain the crucial early‑morning and late‑afternoon light that would otherwise be blocked by whitewash. This leads to a total light increase of around 30%, which suggests a 25-30% rise in potential yield, using the rule of thumb of 1% more light is 0.8-1% more yield. To support this conclusion, earlier trials already revealed that substituting whitewash for movable screens resulted in a significantly higher crop yield.”

While closed screens can reduce ventilation and potentially challenge humidity management, the study revealed that movable screens often improve it. They can serve effectively as an energy screen in early spring. “When closed at night, the screens retained heat within the greenhouse, resulting in nighttime temperatures up to 4°C higher than in greenhouses where whitewash was applied,” says Dr. Katzin. “Their ability to maintain higher nighttime temperatures lowers relative humidity inside the greenhouse, reducing risks of fungal development. During these moments, the benefits of improved heat retention seem to outweigh the risks associated with reduced ventilation.”

“Another advantage of these higher nighttime temperatures is that growers may be able to start the growing season earlier, giving them the opportunity to further increase production,” Dr. Katzin adds.

By enabling 30% greater PAR light, movable screens provide a significant advantage that can translate into a 25–30% increase in potential crop yield. In addition, their ability to maintain higher nighttime temperatures contributes to lower overall relative humidity, supporting healthier crop development. Movable screens also offer greater flexibility in early spring by allowing growers to capture essential early‑morning and late‑afternoon light while simultaneously functioning as effective energy screens. This flexibility makes it possible to start the growing season earlier, further increasing total production.

Overall, the research demonstrates that movable screens deliver full climate control, improved light management, and substantially higher yield potential, making them a more effective and future‑proof solution than whitewash.