High pressure sodium or HPS lights are one type of lighting used to grow plants indoors. Plants exposed to natural lighting receive the full spectrum of colored light, and plants in particular need red and blue wavelengths of light to encourage flowering and vegetative growth, respectively. HPS lights encourage flowering, according to the University of Florida IFAS Extension, and can form part of an effective indoor plant lighting system. In addition, HPS lights are six times more efficient than incandescent lights, which provide a similar type of light supply to growing plants.
Learn the light needs of the plants you want to grow. Some plants prefer full sun--six or more hours in direct sunlight each day--while others grow in the shade and receive little light. Also consider the type of plant. HPS lights work best to encourage the production of flowers and fruit. If the plant you're growing isn't a flowering plant, another choice of lighting may be warranted.
Provide a source of blue light. HPS provides red light, and red light alone causes plants to become spindly. Blue light encourages fuller growth of leaves and stems. The University of Missouri Extension recommends providing 30 watts of red light--provided by HPS--for every 100 watts of blue light. Fluorescent and metal halide (MH) lights provide blue light. MH lights in particular work well in combination with HPS lights to form a complete grow light package, according to the University of Florida IFAS Extension.
Set up the lights about 6 to 12 inches from the tips of the leaves. Hanging the lights from chains makes it easy to adjust their height as the plants grow.
Provide 12 to 14 hours of light per day, if plants are not receiving additional light from a bright window. Be sure to provide darkness for your plants as well. Photoperiod, which controls flowering, depends on darkness, not light, to induce flowering.