Last winter, Prominent's greenhouse participated in a trial with extra far red light. The installation of 30 μmol/m2/s of far red light as inter-lighting resulted in 11% more tomato production.
In the lighting season 2015/2016, Wageningen University & Research Unit Greenhouse Horticulture has grown tomatoes under LED, with an additional of 30 and 55 μmol/m2/s of far red light. This resulted in a 6% increase in production at 30 μmol/m2/s and 15% in the treatment with 55 μmol/m2/s of far red light, mainly because of the higher average fruit weight. These results were, for the tomato growers who had visited the test a number of times, enough reason to also want to see a test in practice, whereby the far red light is used as additional lighting to SON-T lights.
Therefore, in the 2016/2017 season, a practical test was carried out at Prominent Groeneweg I, in which the Siranzo variety, in addition to the SON-T lighting, was lighted additionally with far red LED light. In the greenhouse, four test sections were lit with the following treatments:
- 10 μmol/m2/s far red LEDs above the crop (top lighting)
- 30 μmol/m2/s far red LEDs between the crop (inter-lighting)
- 10 μmol/m2/s far red LEDs between the crop (inter-lighting)
- Reference (no far red LEDs)
From the use of 30 μmol/m2/s of far red light, a higher production was expected based on the results of the previous season. Besides, growers liked to know whether the same effect would be achieved with less far red lights and thus less electricity, and whether the lights should be hung above or in between the crop. Therefore, the treatments with 10 μmol/m2/s of far red light above or between the crop were added.
At the beginning of the cultivation, the plants with extra far red light stretched more than the reference crop. However, that difference did not become more than 10 cm, and disappeared from the beginning of 2017. At the moment when the far red lighting was switched off (at the end of April) there was no measurable difference in plant length anymore between the plants of the four treatments.
Findings
It was found that far red light can have an effect on the stretching of the leaves: if the plant load becomes too tall, this can lead to smaller leaves, less light interception and eventually less production.
The leaf length was measured weekly by Prominent. From January there was a difference in leaf length, whereby in particular the treatment with 30 μmol/m2/s of far red inter-lighting resulted in slightly shorter leaves. At the end of the test, this difference in leaf length was about 3 cm. Because this difference can be solved with or without the removal of leaves from the top of the plant, the treatments have continued. The far red lights were switched on according to schedule until the end of the test, at the end of April.
An important question in this research was what the addition of far red light means to production. It was found that the treatment whereby 30 μmol/m2/s far red inter-lighting was used had 34.6 kg/m2 (week 17) or 11% more production than treatment without additional far red light. To obtain a measurable effect of far red light, 10 μmol/m2/s was found to be too low; this resulted in only a 1-2% increase in production.
During the test, the quality of the tomatoes were measured a number of times. In January, there were no measurable differences yet. In February and March, the taste of all the tomatoes treated with far red were found to be better than without far red lighting, and the tomatoes from the 30 μmol/m2/s far red lighting treatment were tested best. Towards the end of the lighting season, these effects disappeared again.