High-pressure sodium and metal-halide lights designed for indoor gardening differ in the spectrum of light generated, energy efficiency and bulb life. They are considered the light sources of choice by indoor gardeners, but they excel in different phases of the plants' life cycles while being deficient in others.
Metal-halide bulbs emit light in the blue-white spectrum and at a much higher temperature than that of sodium lights. This spectrum of light more closely mimics the color of the light generated by the sun during the summer growing season. The result is a superior light for the vegetative plant stage, which produces shorter plants with thicker stalks and branches and leafier vegetation. That stage extends from germination until just before the flowering or fruit producing stage. During the flowering stage, metal halide lighting is inferior.
High-pressure sodium lights operate at much cooler temperatures and generate light in an orange-red spectrum. This spectrum is much closer to the sun's natural light in the fall, when many plants flower and produce fruit. Subsequently, plants under sodium lighting produce more and larger flowers and fruit. However, during the vegetative stage, sodium lights are inferior to halide lighting, producing taller, thinner plants with more space between branches.
Garden supply companies have recently developed bulbs and ballasts that work with both types of lighting so gardeners can use the best one for each cycle. But gardeners need to stock more of the metal-halide bulbs because they only last about half as long and use more energy than the sodium bulbs do.