The cultivation of vegetables, herbs and flowers under artificial supplemental light offers tips and a point of reference for the cultivation of new plant species.
In Finland, the intensity of supplemental lighting over three-metre high cucumber and tomato crops is 300 to 430 micromoles per square metre and second, and 60 to 90 micromoles are added between the plant rows. This is equivalent to sunshine on cloudy summer afternoon. For lettuces and herbs 150 to 160 micromoles are used.
By providing these intensities for 18 to 22 hours a day, lettuces receive light 11 to 13 moles per square metre and the tall plants 21 to 31 moles. As sunlight increases in spring the lighting is reduced. On average day in March, the sun provides 17 moles per square metre, and in June 40 moles. From March to October, the sun and the artificial lighting provide together as much light as the sun on a summer day.
To produce 150 to 160 moles for the small plants, 90 to 100 watts per square metre of electrical power for high-pressure sodium (HPS) lamps are required. Lighting of the large plants requires 220 to 300 watts per square metre.
Replacing HPS lighting with LED lighting has enabled growers to reduce electricity consumption by 30 to 50 percent.
Replacing HPS lighting with LED lighting has enabled growers to reduce electricity consumption by 30 to 50 percent. Most of the reduction is directly due to higher luminous efficacy of the LED lamps, put better spectra helps as well.
Blue light from the LED lamps on top of red light boosts plants growth, produces more intense colour, and even improves taste. Green light penetrates deep into the crop and increases absorption of the spectrum in to the plants. Minor deficiencies in the spectrum of the artificial light are corrected by sunlight from March to October.
In Canada, at latitude 50°N, where climate is as cold as in Finland, much lower light intensities and lighting durations are used. In the Netherlands, a major greenhouse production hub in Northern Europe, supplemental light is usually used to the extent to help plants over cloudy periods during the spring and autumn.
Growing with intensive lighting is an entity that the greenhouse industry cluster has been developing over 30 years. In includes dynamic control of the growing systems that are adapted to crop cultivars. Without the refined system, far less would be obtained from the supplemental light.
Electricity saving over a few years and investment cost for the LED lamps are roughly matching. Therefore functioning LED lamps are not abandoned. They are used alongside LED lamps to create the right light intensity, spectrum and surface temperature for the plants.
The customised spectrum of LEDs and moderate heat production enable multi-layer growing of special lettuces and herbs without natural light. This is still scarce, though. Approximately 50 watts of energy, 95 percent of the electricity lead to the lamps, needs to be removed from the layers by cooling and ventilation. Utilisation of this energy as heat affects the total energy efficiency of this vertical growing system.
Published in Finnish in Maaseudun Tulevaisuus 3 of April 2017.