Plants need good quality light in a high-enough intensity to stimulate growth. Sunlight is high in intensity, provides a full spectrum of light and is the best light source. Artificial lights used for plants are designed to supplement or replace sunlight. Plants need both blue and red wavelengths for healthy growth, they reflect the green and yellow wavelengths rather than using them. The needs of the plant changes during its lifespan, requiring more blue wavelengths during heavy growth periods and more red light during flowering and fruiting.
High-Intensity Discharge Lamps
High-intensity discharge lamps are good at delivering the intensity of light that plants need, but are not full spectrum. Metal halide lamps produce light in the blue wavelengths needed by plants for strong vegetative growth. High-pressure sodium lamps produce more of the red light that stimulates flower and fruit production. Commercial growers often use these two lights in combination to provide a full spectrum light at high intensity. These lights are expensive for home applications unless you are planning a large indoor garden space.
Combination of Fluorescent Tubes
Cool-spectrum bulbs mostly give light in the blue light range. Warm-spectrum bulbs produce mostly red wavelengths. The combination of a warm-spectrum fluorescent tube and a cool-spectrum tube used together provides the light spectrum needed by plants. These bulbs are not high intensity and need to be close to the plants to deliver enough light. Since light intensity falls off as the bulb ages, fluorescent tubes used for growing plants need to be replaced every six to 12 months, even though they still give plenty of visible light.
Grow lights are a special type of fluorescent tube designed to deliver both blue and red wavelength. Newer fluorescent bulbs produce a full spectrum of light that is close to natural light in wavelength. The light intensity is not as good as the high intensity discharge lights. Like other fluorescent lights, the bulbs need to be located as close as possible to the plant. These grow lights are similar to using a combination of cool and warm-spectrum tubes.
Compact fluorescent grow lights give a more concentrated light than fluorescent tubes. Full-spectrum and warm-spectrum lights are available. These bulbs can be used in a regular incandescent fixture, such as a spotlight, for more flexible placement.