Lower-energy lighting

A recent experiment out of Wageningen University & Research in the Netherlands has shown that there may be some more efficient options in supplemental lighting for growers to consider.

Comparing the traditional high-pressure sodium lights often used as supplemental lighting in greenhouses with a broad-spectrum white LED light, the university found that tomato crops have the same, or higher, yields under LED lights.

The experiment goals

Dr. Ep Heuvelink, associate professor at Wageningen, says the university was interested in comparing HPS lighting with white LEDs as more and more growers are using supplementary lighting in their operations. So they set out to see the role of the different wavelengths in the spectrum on crop development and crop physiology.

“For tomatoes, we’re talking about hundreds of hectares with supplementary light, and the norm is the high-pressure sodium lamp,” he says. “But everybody knows that LED is more electrically efficient than the high-pressure sodium lamps.”

Wageningen was particularly interested in the effects of broad-spectrum white LED lights because most growers using LEDs opt for 95% red lighting with a small amount of blue. “And that is not because that’s the best for the plant, but it’s because of economic reasons — because they’re LEDs, they’re most efficient in turning electricity into light,” he says.

“For tomatoes, we’re talking about hundreds of hectares with supplementary light, and the norm is the high-pressure sodium lamp. But everybody knows that LED is more electrically efficient than the high-pressure sodium lamps.” —Dr. Ep Heuvelink

The parameters

But there may be other advantages to the broad-spectrum white lights. “So we wanted to know, how does the crop grow and develop in the broadband white spectrum?” Heuvelink says. “Because surprisingly, there is very little research in the literature on this broadband white. And when there is research, it’s usually red with a little bit of blue.”

Heuvelink says he was also interested in the differences since growers sometimes complain about visibility issues with red-blue LED options. “It’s really difficult to see your crop and the color of your crop and even to judge whether the tomatoes are ripe or green,” he says, noting that white lighting is more attractive for workers in the greenhouse.

Looking at two cultivars, Merlice, a large size truss tomato with a high yield, and Tomagino, a cherry vine tomato that’s both sweet and crunchy, researchers wanted to find if the yields would be comparable, especially since more and more growers are using supplementary lighting,

From December to April, researchers studied the difference in yield for both cultivars using broad-spectrum white LEDs and high-pressure sodium lighting.

Researchers used two greenhouse compartments, each split into four units by white plastic foil to keep each treatment at the optimal temperature. “If you do it in one compartment you always have the problem with the fact that HPS will heat up your crop because of the radiative heat and that’s not a fair comparison because you’ll probably see a less developed LED-light crop, but it has nothing to do with the light — only the temperature,” Heuvelink says.

Shedding some light

At the end of the experiment, Wageningen researchers found an 11% higher yield in the Tomagino cultivar and a somewhat higher yield in the Malice cultivar, although Heuvelink says it was not significant enough to prove.

The study included a look at brix levels and found no differences between the two types of lighting. And while the experiment did not particularly look into quality issues like acidity or shelf life, Heuvelink says researchers didn’t observe any external quality differences.

If you’re thinking of making the move into LED lighting, there are definite cost benefits, Heuvelink says. “The numbers are always a little bit different, but I think it’s safe to say that it’s at least $40 more efficient with LED compared to HPS light,” he says.

But be aware of the investment costs as they can vary greatly, he notes.