‘Improving our approach to lighting can keep the greenhouse climate more constant’
Van Marrewijk Tomaten
Importance of a lighting strategy
Artificial lighting is not usually the first aspect growers think of when applying the Plant Empowerment principles. But as this Dutch grower of tomatoes has discovered, the lighting strategy can have a valuable impact on the greenhouse climate and hence the plant balances.
Van Marrewijk Tomaten is a family company that has grown and expanded as it has been passed down through the generations. Today, owner Jan van Marrewijk has a 19.4 ha greenhouse facility for the year-round production of high-quality tomatoes in the Dutch town of Dinteloord. Looking ahead to the next generation, three of Jan’s sons are already involved in the business, including grower Lars van Marrewijk who has been there since 2015.
Lars went to agricultural college when he was younger and is now doing a part-time degree in greenhouse management at InHolland University of Applied Sciences. “I’ve already got lots of practical experience, so this is the ideal way to bring my theoretical knowledge to the same level,” says Lars. “The Plant Empowerment book is a big part of the literature we have to study.”
A long line of growers
Coming from such a long line of horticulturalists and having done so much on-the-job learning, Lars is a strong believer that a grower needs what he describes as ‘fingerspitzengefühl’ or gut feeling: “I think it’s really important to walk through the greenhouse to see and feel the crops for yourself. Data will never be able to completely replace that, but your instincts can sometimes be wrong so it can definitely add something,” he states.
In that context, the company’s relationship with Hortilux, one of the Plant Empowerment Implementation Partners, is a good example of how they are increasingly using data at Van Marrewijk. “The starting point came around three years ago, when Van Marrewijk purchased a high-pressure sodium (HPS) lighting system from us in 2018,” recalls Hans de Vries, consultant at Hortilux. “Since then, we’ve helped them to make optimal use of artificial light in several different ways, many of which are linked to Plant Empowerment.”
The importance of the right data
Data is at the heart of achieving sustainable horticulture through Plant Empowerment, according to Hans: “But accurate data can’t be taken for granted, so it’s important to ensure that you gather the right data in the right way. For example, at Van Marrewijk we tested that the Photosynthetic Active Radiation or ‘PAR’ sensor was installed in the right place in the greenhouse and that it was measuring the right things. It’s about getting a complete and reliable overview of every aspect of your climate and your crop, and light is an essential part of that.”
“When you’re trying to work out how much PAR a plant needs, for example, the data from HortiSense – Hortilux’s monitoring platform – gives you insight into the current situation as extra input for your decision-making,” comments Lars. “And what I like about the relationship with Hortilux is that we complement one another, because we’re tomato experts and they’re light experts. They are great sparring partners who help us to take a critical view of whether we’re making optimal use of the light. They also advise us on how improving our approach to artificial lighting can keep the greenhouse climate more constant to achieve a better yield.”
When it came to measuring the light generated by the HPS lamps at the top of the crop, the plants were too close to the fixtures to use a PAR sensor beneath the top-light lamps. “So we advised the grower on the best method for calculating the light level instead, based on a diagnostic field measurement. That way, the data is still reliable for use in the rest of the crop strategy,” adds Hans.
He continues: “The data revealed that the CO2 values in the Van Marrewijk greenhouse were sometimes dropping below a certain level which was limiting photosynthesis. This is linked to how the plant utilizes the light it receives, so we worked with the grower to analyse the lighting strategy in connection with the CO2 and made some adjustments. By maintaining a better balance between artificial light and CO2 during the night, we were able to improve the situation.”
Climate in a wider perspective
A Plant Empowerment-based approach revolves around taking a wider perspective to consider every aspect that can have an impact on the climate and therefore plant health – and, for Hortilux, that includes the power supply. “Before the lighting season started, we noticed that some lamps were failing sooner than normal,” says Lars. “We didn’t want to have to keep spending money on replacing them, of course, but above all we wanted the certainty that the lamps would keep working as expected, otherwise it could affect the quality of our tomatoes.” Hortilux took a closer look at the power supply and discovered that the voltage was too high during lighting, causing the lamps to fail. “We arranged for the transformers to be adjusted so that the voltage was in the right range. We also installed HortiSense Asset to continuously monitor the situation in real time and issue an automatic notification if the voltage or current deviates,” states Hans. “If an electrical problem causes some lamps to stop working in the middle of the night, the greenhouse climate will no longer be homogeneous – in terms of both light and heat – and that goes completely against the Plant Empowerment principles so we want to avoid that at all costs.”
In a future-looking lighting project, they are currently running a trial together in the Van Marrewijk greenhouse to evaluate the source sink ratio based on differences in the leaf area index (LAI): normal, medium and high. “The aim is to find the optimum LAI that achieves the best balance between the plant parts that absorb light and produce assimilates (i.e. the leaves, which are also called sources) and the plant parts that attract and store the assimilates (mainly fruits, which are also called sinks). The key in this is to maintain an assimilate balance in the plant which will result in a strong, productive and resilient plant. We should know more about the impact on overall production by the summer. This is an interesting example of how artificial lighting can be used to steer the assimilate balance, which is of course one of the key balances in Plant Empowerment,” concludes Hans.