Light has a great influence on plant growth. It supplies the energy for photosynthesis, the accumulation and growth of organic substance. Light triggers certain (photoperiodic) development processes in plants, e.g. flower formation, start of growth and growth dormancy. Light also influences the external appearance of plants.
Through photoperiodic plant lighting (photoperiodic stimulus lighting), plant growth can be controlled, with the botanical day varying according to plant type. A short-day plant, e.g. chrysanthemum or kalanchoe (flaming Katy) starts blossoming when its typical, critical day length is undercut, creating a correspondingly long dark period. In order to e.g. impede blossoming, an extension of daylight (shortening of the dark period) beyond the critical day length using artificial lighting at an illuminance of 20 lx to 200 lx (values depend on the species) is necessary.
The artificial day extension can be implemented using interval lighting instead of continuous lighting, which can save over 50% of lighting energy. For example, reflector luminaires with fluorescent lamps are turned on for 10 minutes and then off again for 20 minutes. For the photoperiodic processes, blue, red and IR spectral regions of light sources play the most important role. There are also day-neutral plants, whose flower formation occurs independently of artificially controlled day length.
The economic exploitation of ornamental plants makes photoperiodic lighting nearly obligatory. To promote growth (photosynthesis), illuminance values between 1.500 lx and 6.000 lx are required, with 1.000 lx being the minimum value. High-intensity discharge sodium vapour lamps or lamps with specialised spectrums promote the growth process in a particular way; detailed recommendations can be provided by the lamp manufacturers.
The (German) association Arbeitsgemeinschaft Elektrizitätsanwendung in der Landwirtschaft e.V. (AEL) also provides planning information for lighting in horticultural farms which are incorporated intable. The horticulture trade association has, in accordance with DIN VDE 0100, specified that greenhouses are “wet and saturated rooms” and luminaires must be at least drip-proof (IP x1); splash-proof around the impact area of irrigation systems (IP x4); and at least jet-proof where harsh washing down with hoses occurs (IP x5, see also chapter “Luminaire selection table”). High requirements apply to luminaires in greenhouses regarding resistance to changes in temperature, humidity and chemical exposure through pesticides.