Artificial lights cannot accurately reproduce natural sunlight, but they can produce a spectrum that provides for the needs of plants. Plants need light in the right spectrum and intensity for a long enough period each day. The exposure period is easily controlled by turning lights on and off daily. Control intensity by using more lights and placing them closer to the plants. Reflectors also increase light intensity by focusing light back onto the plant. The light spectrum varies with the light source, and is the main difference in artificial lighting sources.
Fluorescent lights come in different spectrums. While white light or daylight tubes seem light the best choice for plants, they are less effective than warm-white or cool-white fluorescent tubes. Cool spectrum tubes produce light mainly in the blue range and are good for the vegetative growth phases of plants. Warm-white fluorescent tubes produce more of the red rays needed during the flowering and fruiting stages. Mixing warm-white tubes with cool spectrum tubes provides a good balance of light for low light plants and seedlings, notes the University of Missouri Extension. Fluorescent lights are not as intense as the sun and may not produce sufficient light for plants needing high light levels.
Grow lights are specially developed fluorescent lights that produce a mixture of red and blue spectrum lights. These lights can be used alone to produce all the light for the plants, or they can be mixed with cool-white tubes for a less expensive but adequate light source. Use one grow tube with two cool-white fluorescent tubes, advises the University of Missouri Extension.
High Intensity Metal Halide Lamps
Metal halide lamps are high-intensity lights, emitting plenty of light in the blue spectrum at a higher intensity than fluorescent tubes. While metal halide lamps have a more balanced spectrum than sodium lamps, they are not as effective in the red spectrum. High intensity metal halide lights are best for plants that need high light conditions during the leafy vegetative growth stages. High intensity lights are more expensive than fluorescent lights and not cost effective for most small home gardens. They require a special ballast and lamp fixture that adds to the cost.
High Pressure Sodium Lamps
High pressure sodium lights are high in red and yellow light and low in the blue spectrum. Plants grown exclusively under HPS lights tend to be tall and spindly, searching for more light. These lights are a good choice in situations where some daylight is available. The red and yellow spectrums of HPS lights mimic the light spectrums available in the fall and are good for plants that usually flower in the fall. HPS lights also require a special fixture and ballast and are a more expensive option.
Combination Metal Halide and HPS
Commercial growers often use a combination of metal halide and HPS lights to give the full light spectrum needed by most plants. Blended lights are available, using both lamps eliminating the need for separate ballasts and fixtures. While less expensive than purchasing both HPS and metal halide lights separately, this is still an expensive light source for the home gardener.
Light Emitting Diodes
Light emitting diodes, or LED lights are becoming popular for plant growing. LED lights can be built to produce the exact spectrum of light needed and produce the perfect light for plants. Good results has been reported with these lights. They are expensive, but unlike the other options, the bulbs last for many years, reducing the overall cost of the system. LED lights are also more energy efficient than the other options, further reducing costs.