Choosing LED changes more than just the fixture. The cultivation also changes. Tomato growers realise this all too well. Moisture is a big challenge, as is keeping the crop properly generative, they revealed Wednesday afternoon at a meeting on LED in tomato. At the invitation of Glastuinbouw Nederland, about 50 growers and suppliers came to Bleiswijk, Netherlands, to gain new knowledge at Delphy, but above all to share knowledge.
For the knowledge sharing, the attendees were presented with a series of questions. Who already has LED? And what does the installation look like? And what are the challenges? In answer to the latter question, besides moisture and generative cultivation, achieving good setting came up a lot. As well as numerous other challenges, such as dealing with dimming of LED lights, but also watering, feeding and evaporation.
Making the most of technical possibilities
Many of the growers present already work with LED, in half the cases in combination with HPS. In other words, hybrid lighting. The number of micromoles hanging above the crop, or sometimes in between with intermediate lighting, varies. 300, 280 and even 180 micromol are the most common, according to a small sample of the room. Exposure hours vary even more. Not surprising, considering that last winter the energy issue demanded each grower create their own strategy.
What was striking was that many growers also already have access to dimmable LED lights. Even at Delphy itself, in trials with fossil-free tomatoes within the Kas als Energiebron programme, they are working with them these days, Stijn Jochems told us. He briefly discussed the results from five years of doing trials.
In doing so, he talked about technical changes in the trials, such as last season when air hoses hung at the top of the crop for the first time. The goal? Getting the relative humidity down. Even with this adjustment in the trial still a challenge he showed.
Dimming the LED lights is also relatively new for the researchers. What was noticed was that when the lamps are dimmed, the temperature in the head drops sharply, but deeper in the crop the drop in temperature is more limited.
The researchers learn a lot in the trials, which also include a guidance committee with growers. In their search for solutions to common problems, they emphatically also look at the state of the crop. According to Stijn, the key to optimal LED cultivation now lies mainly in applying the available techniques in a new strategy. He has no doubts about the spectrum with 90% red, 5% green and 5% blue. That is 'good enough'.
Do crops miss radiant heat?
In another presentation, Arie de Gelder of Wageningen University & Research discussed evaporation. Does the crop miss the radiant heat of HPS lamps when switching to LED, was the tantalising question he asked. Some specialists say it does. Arie showed why he thinks that is not the case.
Using data and (old) studies on evaporation, the researcher especially wanted to get the audience thinking. He cited studies from 1968 (!) and 2003, along with graphs showing what a tomato crop does under exposure. One of the points he raised was that between roughly 700-1400 nanometres, a tomato crop hardly absorbs the Near Infra Red radiation of LED lighting. Very different from HPS, where there are two uptake peaks in that zone.
Another point Arie pointed out was that with LED lamps becoming more efficient in recent years, less energy input is needed. That also affects evaporation. But how do you measure it? Arie knows plenty of ways, but really the ultimate way, especially for practical growers, he does not yet know.
The question is whether, before Arie retires in September, that way will exist. It would be very welcome for growers going into the new winter with LED then. With making calculations, it is possible to get a long way, though. 15 grams per square metre per hour is the minimum, he says, derived from an unlit tomato crop in August.
Master plan evaporation
After the presentations, the group dispersed for brainstorming sessions on four topics: setting (flower quality and inflorescence), fruit colouration (outgrowth duration and deviations), evaporation (and nutrient uptake) and plant temperature versus air temperature (generative versus vegetative).
In it, some of the topics already discussed in plenary reappeared, such as the 'cake-coloured tomatoes' caused by wan colouring. Growers recognise this from the time they were introduced to HPS in winter cultivation. They suspect a relationship with evaporation. The challenge now is to master it under LED as well. Robert Solleveld of Glastuinbouw Nederland informed that, crop-wise, several parties are working on an evaporation master plan. In that context, someone also suggested bringing the knowledge and experiences from the time of the switch to HPS to now, in order to make the translation to the LED situation.
During the sessions, it became clear that there are still plenty of questions, including about what is already being measured and what is not yet being measured (properly). What about air movement, for instance? Growers acknowledge that they don't really measure it, while it does matter whether there is still air or moving air in relation to the moisture deficit. And what about pollen release? Why does that sometimes not want to work properly? Again, moisture seems to be a cause, especially if it turns out that under the chosen spectrum, pollinator flights are fine.
Experiments and practice
The meeting gave the researchers additional insight into what growers are up against when growing LED in tomato. It is up to the researchers to find answers to the questions with trials. They often do this in small greenhouse compartments. Sufficient for spectrum or lighting selection, but the question for some growers is still whether you can replicate the situation from, say, 1000 square metres to 5 hectares.
With that increase in scale, the light from the lamps is still the same, but is air movement still comparable, for example? "Here I am still missing some answers," acknowledged one of the growers who is not yet going for full LED next winter. Other growers already did, or will next winter. So the specialists with large-scale LED tomato cultivation were definitely ín the room as well.