"Growing under LED lighting is completely different to growing under HPS lighting". This is a phrase often repeated by greenhouse growers having made the switch to LED lighting after years of growing under HPS or SON-T fixtures. A major point of adjustment is climate management, as LED fixtures produce significantly less radiant heat than their predecessors.
While engineers are quick to say that HPS lighting is an inefficient way of heating a greenhouse, there is something to be said about the benefits of HPS' radiant heat on tomato and cucumber crops, especially in cold and temperate climates like Norway, Sweden, Finland and Canada.
Kristian Solberg
Solbergs Gartneri AS is one of many greenhouses that had made the switch from HPS to fully LED in their greenhouse. Solbergs Gartneri AS is a multigenerational operation located in Vettre, Norway and dates back to 1936, when the farm produced a wide variety of vegetables and flowers. In 2000, a 6.000-square meter greenhouse was built followed by another 6.700 square meters four years later. Today, the greenhouse produces 7% of Norway's cucumber production, or roughly 4 million units. This third-generation greenhouse is currently run by Tor and Kristian Solberg.
Growing under LEDs - a completely different approach
Given the greenhouse's Nordic latitude, supplemental lighting is essential during the darker months. After years of growing under HPS lighting, Solbergs Gartneri invested in LED lighting four years ago and initially considered installing a hybrid system to avoid losing too much heat at the head of the crop and having too low a leaf temperature. However, the older greenhouse's height made this difficult and as they felt the future is 100% LED, the Solbergs decided to grab the bull by the horns and install a full LED system in half of the greenhouse. Their first trials were under a broad spectrum of roughly 68% red light, with a lot of green light and relatively little far red light.
"At the time, we were having major discussions about whether far red was necessary or not for cucumbers and we were told that because these fixtures had a lot of green light, we didn't really need more far red light," says Kristian Solberg, CEO of Solbergs Gartneri AS.
However, the growers quickly noticed that far red light is quite important in wintertime for cucumbers and that the plants were entirely different without it, with the crop having smaller leaves, shorter petioles, slight leaf curling and but in our case, the yield was actually the same. It became clear to the growers that higher red and far red levels are critical to crop productivity.
LED lighting and dehumidification - a winning combination
When transitioning from HPS to LED lighting, a major concern was the loss of radiant heat at the head of the crop and the potential drop in plant activity.
"When we first switched to LED lighting, we had bigger issues with leaf temperature than we expected. The head of the crop was too cold and the humidity was high as we also kept the screens closed to prevent further heat loss," Kristian explains. "So our VPD dropped to zero and the plants weren't transpiring."
To address this, the Solbergs saw two options: find a way to increase the leaf temperature or to decrease the humidity in the greenhouse. While the rail pipe and roof pipe were primary and secondary heat sources under HPS, the growers had to switch this strategy to use the heating pipes near the roof as a primary heat source.
While gapping the energy screen is a good way to decrease the humidity, the drop of cold air onto the plants causes a whole range of issues. To reduce heat loss and cold air drop through gaping, the Solbergs pivoted to install dehumidification systems for humidity control after one season with LED lighting.
"Air movement is critical for cucumber crops and while I'd love to manage this with multiple screens some day, the low height of our greenhouse makes it really challenging as keeping screens closed quickly increases the humidity, while opening them chills the plant. With a dehumidification system, we can close the screens and insulate more without stressing the crop, " says Kristian.
Although Solbergs Gartneri initially collaborated with Fluence on the first installation, the company is currently working with Dutch Lighting Innovations (DLI) as a lighting provider for the rest of the area, with an increased level of red and far red light in the spectrum.
For more information:
Solbergs Gartneri AS
Kristian Solbergs, CEO
+47 6676 5150