High Pressure Sodium bulbs create intense light that can be used for indoor growing. They produce red wave length light and are most effective during the vegetative growth stage.
High pressure sodium lamps are widely used by growers and produce an extremely intense light along the red/orange end of the color spectrum. They are considered High Intensity Discharge lights and can produce a great deal of heat when used. The shorter wavelength red/orange of light is most effectively used by plants when flowering or producing fruit. Growers often report significantly larger yields when they apply high pressure sodium lighting during later growth.
High pressure sodium bulbs only create red/orange light. If only high pressure sodium bulbs are used during a plants early life growth will be taller and faster. These plants will likely be unable to support the substantial flowering and fruit growers prize. By using other light sources, such as metal halide or fluorescent grow lights, during the early stages of growth plants develop sturdier structure. The ideal set up or many growers is long wavelengths of light during the early stages of growth and then high pressure sodium lighting as the vegetative stage begins.
This tends to grow sturdy plants that are then able to support the abundant fruit and flowers produced by high pressure sodium. There are combination light designs that include both metal halide and high pressure sodium bulbs in one one fixture, providing all the different wavelengths plants need throughout their growth.