"It’s not about how bright the sun shines, but how much light reaches your crop."
Eric van der Horst on light strategies in greenhouses around the world
You’ve worked in many different climates. How do local conditions, such as sunlight and day length, influence how growers handle light?
“The differences are significant. In Northern Europe and North America, the day length varies greatly between winter and summer. As a result, light levels are low in winter and increase toward the longest day. This has a big impact on stem density per square meter. In the spring and summer, when there’s more light, there can be more stems per square meter, while in autumn and winter fewer stems are needed to distribute the limited light more effectively.
In southern countries like Spain, Morocco, and Mexico, the number of sunlight hours is more consistent throughout the year. Growers there often work with a fixed plant density, assuming there’s always enough light. But that doesn’t automatically mean the light is used optimally.”
And how is light data used around the world?
“In countries with a lot of natural light, such as Spain, Morocco, and Mexico, light data isn’t always used efficiently. There, light is often seen more as a source of heat rather than a growth factor. Growers might apply coatings to block out heat without precisely measuring how much light remains afterward. As a result, too much or too little sunlight may be filtered out compared to what the plants actually need.
In cooler climates, you see the opposite. Growers try to retain every bit of light, especially during the darker months, for example by cleaning the greenhouse roof regularly. But even there, situations can arise where there’s too much light combined with high outdoor temperatures.”
What do you see as the biggest misconceptions about light?
“A common mistake is removing too much light to reduce temperature, without realizing what that means for the plant. In warmer regions, many growers produce through the winter, but without light measurements outside and inside the greenhouse, it’s hard to know whether the plant is getting enough energy.
Many growers in those regions continue winter production with too high a plant load: too many stems and/or too many fruits per truss. The plant simply can’t keep up with the available winter light. As a result, it enters spring weakened and can’t handle the rapidly increasing day length and amount of Joules. The plant becomes exhausted and produces fewer kilograms.
More plants or stems don’t automatically mean higher yield. Always adjust the number of stems and fruits to the amount of light that’s available in the greenhouse at that time.”
Finally, what would you like to see change in how growers view light worldwide?
“Measure, measure, measure. Whether you have a lot or little light, you need to know how much actually enters your greenhouse. Many growers make assumptions, and those are often wrong. Some think their greenhouse lets in a lot of light, or that a coating blocks too much or too little. You only know for sure when you measure both the outside and inside light amounts.
And think ahead. Many decisions are made based on what’s happening today, while the light in the coming weeks and months determine how well your crop develops. If you plan ahead based on light, you’ll have more control, a healthier plant, and ultimately a stronger crop.”
About Eric
Eric works in customer support at ProJoules, helping growers worldwide get the most out of their data. With his background in labor consultancy and efficiency, he understands how cultivation, light, and data can strengthen one another.
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