The latest LED trials with tomatoes at the GreenQ Improvement Centre have given rise to a yield of no less than 100 kg per m2. This was achieved with the help of intensive light from Philips GreenPower toplighting and interlighting. The combination of a thick Rockwool growing slab and a sophisticated irrigation strategy had a favorable effect on the taste of the tomatoes during certain periods.

In this research with the large vine tomato variety Komeet, a combination of 104 µmol/m2.s of toplighting and two rows of interlighting − each delivering 53 mmol/m2.s − was installed, which together delivered a light intensity of 210 µmol/m2.s. Cultivation commenced in week 41. The grafted and topped plants showed good generative growth right from the start of cultivation. After setting out with 2.8 heads per m2, extra stems were kept in weeks 44, 49 and 1, taking the final spacing to 4.2 heads per m2. The trial was finally concluded in week 39, which meant the total cultivation period was 50 weeks.

Long lighting period

Both the toplighting and the interlighting were used right from the start of cultivation. Up until 1 May the toplighting was only not used after an expected total radiation of 1,750 joules, and after this date the toplighting ceased to be used at 1,250 Joules. The interlighting modules were used for a few hours every morning throughout the summer, except on very hot days. This is in contrast to companies using conventional lighting, which have to stop much sooner because they would otherwise be hampered by the heat of radiation from the HPS lamps.

Less watering

The LED trial at the GreenQ Improvement Centre was combined with various watering strategies in order to reduce the total amount of water given to the plants and, in so doing, to reduce plant transpiration. This is because less transpiration leads to substantial energy savings. It is for this reason that in the trial department 10-cm thick growing slabs were used in combination with 2- and 3-liter drippers. The trial area with 2-liter drippers received a higher EC than the trial area with 3-liter drippers. This also gave rise to differences in yield at the end of the cultivation period, i.e. 97.1 kg per m2 and 100.6 kg per m2 respectively. In the winter six months, the tomatoes grown with the highest EC were the sweetest. The quality of the fruits was very good.

In control

The good quality of the fruits was partly thanks to the use of toplighting and interlighting, which led to a better distribution of light over the entire crop, enabling every leaf on the plant to help produce the maximum yield. Piet Hein van Baar, a plant specialist in high-wire crops and former greenhouse manager at GreenQ Improvement Centre, explained it as follows: “Compared with conventional lighting based on HPS lamps, as a grower you are in control. HPS generates a lot of heat in the greenhouse, which you ultimately have to get rid of via the air vents. When you open the air vents the valuable CO2 escapes from the greenhouse. With LEDs the air vents stay closed more. What's more, the leaf remains in much better condition.”

Sharing knowledge

The trial was commenced at the initiative of Philips, together with GreenQ, Koppert Biological Systems, Monsanto, Grodan and Stargrow Consultancy. The objective was to achieve maximum yield and to gain a greater understanding of cultivation under LED lighting. The various participants wanted to strengthen their position by sharing knowledge with one another.

Piet Hein van Baar says the latest lighting strategy based on Philips GreenPower toplighting and interlighting looks extremely promising. Complicated trials like this one show that cultivation really is an advanced science. There is no doubt in his mind that LEDs play an important role in the cultivation of tomatoes under artificial lighting. Or, as he puts it: “To LED or not to LED, that’s no question.”

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