‘Interlighting boosts truss development and flowering speed’

Hybrid lighting in low’ Dutch greenhouse
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‘Interlighting boosts truss development and flowering speed’

Interest in artificial lighting with LEDs is on the rise. Recently tomato growers Verkade in ‘s-Gravenzande (Westland, the Netherlands) gained their first experience with a hybrid system consisting of SON-T HPS lights and LEDs in a greenhouse that is too low for intensive lighting using only SON-T lights. Growing manager Fred Schäpe is happy with the winter yields. Based on his first impressions, he believes this system could deliver even more. He is pleased with how things are going.

It’s the middle of summer. At Verkade the LED interlighting modules are often still on at 11 am, depending on the climatic conditions. They can be switched off after midday. In his first year with this lighting system the growing manager is already exploring how far he can take things. By taking a good look at how other growers are using artificial lighting, he is learning fast and making his own plan.

Low greenhouse

In the autumn of 2015, an artificial lighting system was installed at this ‘s-Gravenzande-based tomato grower’s 4.4 ha site with the support of the growers’ cooperative Van Nature, in order to meet rising customer demand for Dutch products of consistently high quality all year round.
The Venlo greenhouse has a bay width of 8 m and a post height of 4.85 m, which is too low for intensive lighting with SON-T lights because of the amount of heat they emit. “We could also have raised the height of the greenhouse, but a combination of SON-T lights and the cooler LEDs proved a better option in the end,” Fred Schäpe explains.
After much discussion, the two organisations decided to install a hybrid system. At the top they installed Hortilux 1,000 watt SON-T downlights delivering 135 µmol/m2/s. These lights are distributed in threes along the width of the trellis girder. The distance between the light and the tops of the plants is 1.2 m, reducing to around 1 m as spring arrives.
One strip of 55 µmol/m2/s LEDs was suspended between the plants in each row and can be moved as the plant grows taller. A total of 6,610 Philips GreenPower LED interlighting modules have been installed, bringing the system’s total output to 190 µmol/m2/s.

Start of growing

Verkade specialises in large cluster tomatoes. Fred Schäpe has always grown Merlice. In midsummer the plant load is spectacular – an image that is only enhanced by intensive leaf pruning which makes the trusses stand out even more.
The 64-day-old grafted and topped plants were planted at the end of October. At first the plant density was 2.3 per m2. A lateral stem was allowed to develop in week 46, increasing the density to 3.05 per m2. The last lateral stem was added in week 48, bringing the final density to 3.8 stems per m2. Production started in week 52.
Immediately after planting, the plants remained unlit for ten days, after which the light level was gradually increased. In the lighting season, which lasted until around 1 April, the SON-T lights were left on between midnight and 6 pm. They were switched off when the radiation reached 2,000 J/m2, so the number of lighting hours was lower in April.
Unlike the SON-T system, the LEDs are left on constantly from 4 am to 5 pm from 1 April onwards, except on very hot days when the system is switched off earlier.

Effect on truss development

At the start of cultivation the interlighting strips were hung as low as possible in the crop. Later on they were repositioned once, close to the developing trusses. The growing manager is already coming to the conclusion that he will have to move the modules several times over the course of the next crop to keep up with the trusses and to enable every set truss to get maximum benefit from the light. Schäpe: “We do this manually. We want the trusses to develop as well as possible with maximum plant load.”
By about week 50 the modules were in their highest position and were left there for the rest of the growing period. He soon noticed the positive impact of the LEDs on truss development and flowering speed. “Trusses seem to be developing a little better with LEDs than with only SON-T.”
In January last year, Schäpe was happy with the crop status. “It’s all going well,” he said at the time. “The crop has been steered generatively right from the start, and I can see that the plants are having no trouble putting their energy into the fruits.” He is happy with the winter production, and although it is only their first growing year, he looks ahead with optimism: “With the knowledge we gained last winter, I’m sure we will be able to do even better in the future.”
The crop manager has his own strategy for truss pruning and fruit load. Sometimes he prunes the Merlice to as many as six fruits if the truss is strong enough. By doing so he is looking for the ideal fruit load.

Heavy investment

Looking back at his first season, Schäpe is positive about the impact of the LEDs on production and quality. Although it’s a heavy investment for large cluster tomatoes, Schäpe sees even more potential in optimising the crop even further. Only time will tell. But for the time being he can look back over his first season of artificial lighting with a smile.

Summary

With support from the growers’ cooperative Van Nature, a hybrid lighting system was installed at one of tomato grower Verkade’s sites in Westland, the Netherlands, because the greenhouse is too low for intensive lighting using only SON-T lights. Crop manager Fred Schäpe is happy with truss development and flowering speed. Although this is only their first year of artificial lighting, the company is pushing the boundaries in order to optimise growing.

Text and image: Pieternel van Velden.

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