Longer, nicely stretched branches with some blue and green

24 March 2022

Multiple years of research at Proefcentrum Hoogstraten have provided insight into the optimal color composition of LED lighting in winter cultivation. A spectrum with 5% far-red, 5% blue, 5-10% green and the rest red seems to be a safe approach, with possible variations.

Horti LED grow light

Strawberry researcher Peter Melis of Proefcentrum Hoogstraten (PCH) presented the results last week at the light symposium ‘LED there be light’ in Leuven (Belgium). The spectrum trials have been running for three years now.

“Winter strawberries only grow during the shorter, darker days. So it’s important to choose the spectrum carefully,” he noted. At the request of manufacturer MechaTronix, PCH tested the effect of eight different spectrums in the 2020-21 season. The variety chosen was Sonsation, because this variety has relatively few root problems. Lighting was switched on from November 20 tot March 30 (over 1,700 hours) with 140 µmol/m².s.

The eight types of fixtures differ in percentages of green, blue and far-red (in addition to the base color, red). With blue and green, at times they looked at extremes – for instance 28% green – to get a clear image.

Far-red necessary

“With 5-6 percent far-red, the extensive growth was noticeable. Branches were longer. The first flowering branch in particular grew very extensively. That leads to earlier fruiting, and is favorable because of the high market price,” he explained. The positioning of the leaf also benefited from far-red. The leaves were better able to position themselves next to each other, catching the light better. The mid-harvest date was a week earlier than with a more traditional LED spectrum (92% red, 3% blue, 5% green). “Blue ensures a compact crop with crispy leaves. The color is good for opening the stomata, but too much blue means the branches won’t stretch well”, he noted. So an important question is how much green there should be. A high amount of green ensures a tender, large plant with long branches. Fruits also tended to be bigger. But green is an expensive color, so the researchers are still careful in their conclusion about the optimal amount.

Special winter strawberry lights

“With the results from our earlier spectrum trials, we went to several light suppliers and asked them if they wanted to have research done into lamps adjusted for winter cultivation,” he said. Interest in that was high. Due to limited space, three trials were run last winter. Hybrid lighting of 130 µmol/m².s SON-T and 100 µmol/m².s LED (red/blue) was compared to fixtures from MechaTronix (5% blue, 6% green, 83% red, 6% far-red) and Colasse (11% blue, 22% green, 62% red, 5% far-red), both 240 µmol/m².s. The varieties Sonsation and Sonata were planted October 20; the harvest period ran from January 6 to March 5. “Across the entire cultivation, we aimed for a total light sum of 2,000 mol, including sunlight,” Melis said.

Not too much blue

“The visual results were as expected, based on our earlier trials. The plants under LED lighting with a low amount of blue and green had longer branches, which extended nicely. The LEDs with the higher amount of green and blue gave a more compact crop with shorter branches. The latter looked a great deal like those under hybrid lighting.”

The fruit quality didn’t show any differences – production did, however. That was better under the MechaTronix spectrum, particularly thanks to the faster growth. “The high amount of blue in the other LED lights delayed the harvest,” he noticed.

Susceptibility to root diseases

One thing to note is that Sonsation is more of a spring strawberry than a winter variety. But its limited susceptibility to root diseases makes it a good candidate for research, which Sonata, although a typical winter variety, can have a lot more issues with the roots.

The various spectrums did not influence the effect of predatory mites.

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