Sunlight is undoubtedly the cheapest source of light for growing plants. However, there are times when there is insufficient or no natural light available, for instance, when growing indoors, in winter or at night.
The High Intensity Discharge (HID) lamps that we are most concerned with are High Pressure Sodium (HPS) lamps and Metal Halide (MH) lamps. HID lamps are very bright, require external ballasts and can be used as a substitute for natural sunlight, as they have a similar colour spectrum.
HPS lamps are used by commercial growers, to ‘supplement’ the natural light and to extend the ‘day length’ that a plant is subjected to. This type of lamp puts out a ‘redder’ light and can be compared to the light of an autumn sunset. The addition of HPS lamps enhances flowering in many crops and are the preferred option, if natural daylight is also available.
Metal Halide lamps give off a ‘blue’ light which is more suitable for young plants and vegetative growth.