The RWB spectrum - red with a little blue and green light - still seems to be the most efficient LED spectrum for tomato cultivation. In addition, with closed screens, bumblebees seem to fly better under LED than under HPS. These are the conclusions from an LED trial that is being run in the Delphy Improvement Center . Stéphane André from Hortilux and tomato grower Jasper Oussoren from Roots are convinced that growing tomatoes under LED offers opportunities, but at the same time indicate that there are still many research questions.
Dutch greenhouse horticulture has set itself the goal of fossil-free cultivation by 2040. In order to realize this, quite a few challenges still have to be tackled, also in tomato cultivation. “Tomatoes are hardly grown under full-LED in our country, for example. However, there are now several growers who work with a hybrid grow light system,” says Stéphane André, Consultant Grow Light Performance at Hortilux.
In order to take steps towards fossil-free tomato cultivation, the ‘Tomato fossil-free’ trial was set up four years ago. Delphy Improvement Center took the initiative for this project, which is financed by Kas als Energiebron (Greenhouse as a source of energy, the innovation and action program of the Ministry of Agriculture, Nature and Food Quality and Glastuinbouw Nederland) and various suppliers. Hortilux is one of these parties. “As grow light specialists, we think it is important to form the basis of innovation. In addition, it is crucial that we make tomato cultivation ‘future-proof’, also in terms of energy,” explains Stéphane.
Input from practice
The trial takes place in a 1000 m2 department at the Improvement Center in Bleiswijk. Various techniques are used here to minimize the energy requirement, including dehumidification, sensors and LED lighting. The lines with everyday practice are short: the trial is closely followed by several growers. Jasper Oussoren is one of them. “We monitor whether the trial is sufficiently in line with the practical conditions and provide input from practice,” Jasper explains. He is co-owner of tomato company Roots, with two locations in Zevenhuizen and the Westland area. Cherry truss tomatoes of the Piccolo variety are grown for Looye on the 26-hectare company.
Grow light is a hot item for the company, which has 12 hectares with HPS lighting and 8 hectares with hybrid lighting. “But because of the rising energy prices and the challenges that lie ahead for the future, growing under LED does have our attention. Also because there are still many questions in this area. For example, what impact does the loss of radiant heat have and how do you deal with this? For these reasons I think it is important to be involved in this project; I want to know what possibilities LED offers in the future.”
Light spectra comparison
The necessary knowledge and experience has now been gained. Initially, in the test greenhouse with the Merlice tomato variety, cultivation was done under a hybrid grow light system, but in the autumn of 2020 the switch was made to full-LED. “HORTILED Top Summit fixtures have been installed with a light level of 280 µmol. This is higher than usual in tomato; the standard light level is 180 µmol”, says Stéphane.
In the first year of cultivation, the most common light spectrum in tomato was chosen. This concerns the so-called RWB spectrum, consisting of 8% blue, 6% green and 86% red light. Since September last year, the spectrum in a third of the test greenhouse has been replaced by the so-called Daylight + FR spectrum, consisting of 12% blue, 31% green, 47% red and 10% far-red light. “We especially wanted to know whether this spectrum results in a better crop balance and higher production. This is also necessary, since this spectrum requires 20 percent more electricity than the RWB spectrum.”
The production differences between the two spectra have so far (March 2022) been minimal. However, under the Daylight + FR spectrum there is a higher water consumption and a better settlement. In addition, the brix content is higher. “This is probably due to the addition of the far-red light,” says Jasper. “On balance, however, it seems that growing tomatoes under this spectrum is not yet feasible, mainly because of the higher energy costs. For the time being, the RWB spectrum appears to be the most efficient in tomato. Personally, I would also invest in this spectrum, in case of a switch to LED. The 6% green light contained in this spectrum, when only the LED lighting is on, is sufficient to work well in the dark.”
Double screen and dehumidification
The trial has also provided growers with new insights in other areas, he says. “There is a lot that can be done with LED in tomatoes, that has become clear to me. You save fifty percent on electricity, but on the other hand we have to use 10 to 15 percent extra heat at our company to absorb the loss of radiant heat. On balance, however, you still have less costs.”
Jasper emphasizes that investing in a double screen, in order to better keep the heat in during the day, is a must with the switch to full-LED. “And when you invest in a dehumidification system, you can make extra profit in this area. In this way, you save maximum energy and use the LED investment as optimally as possible. It is important that we gather additional knowledge in the field of dehumidification, especially about the impact of such a system on the cost price.”
Whim of bumblebees
The trial yielded another interesting insight: bumblebees seem to fly significantly better under LED lighting than under HPS with closed screens. “This is a big gain. And this also makes the business case of LED lighting a lot more interesting,” says Jasper. “It is now important to further investigate the minimum amount of light a bumblebee needs to be able to fly. In short: this project has shown that LED offers opportunities, but that there is also a lot to investigate. Hortilux is an expert knowledge partner in this regard, who thinks along well and actively. I am extremely positive about the collaboration!”
From hybrid to LED
Despite this, Jasper does not expect tomato growers to switch en masse to full-LED in the short term. “They will probably first switch to hybrid and then make the step to full-LED. Because however you look at it, it remains a substantial investment. And to make it profitable, you have to be assured of optimal production. That is difficult in today’s tomato cultivation, because of the high disease and virus pressure. But there is no doubt that full-LED will be the only option in the long run.”