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Choosing the best plant grow lights depends on the types of plants being grown and the purpose for which they are being grown. Plants vary on the colors of light they prefer, but it usually is in the 5,600 Kelvin (K) range of natural daylight, and Kelvin ratings are sometimes imprinted on light packaging. The intensity level of the light and the ideal amount of light vs. dark hours in a day also vary per plant species. Ornamental plants require less intense light in the blue range of the spectrum, versus fruiting and flowering plants that mature best under red- and yellow-tinted light that simulates the end of the growing season.
To choose the proper plant grow lights, look first at the environment to which the plants are native. Desert plants require the most intense, white-blue light for up to 14 hours per day, whereas tropical and flowering plants that are often in shade can thrive on medium levels of intermittent light. Vegetables and fruit-bearing plants need a bright mixture of blue and yellow light for healthy growth of vegetation and to encourage reproduction. The age of the plant also makes a difference, with seedlings requiring mostly blue-white light that stimulates the broad growth of vegetation.
Home greenhouses use fluorescent tube lights that come in a standard cool-white, office-style color; but, a better choice are warm white-colored bulbs, as the light spectrum they emit more closely simulates nature. Plants requiring sunny or semi-sunny environments require between 500-1,000 foot-candles (ft-c) of light for at least 2-5 hours per day minimum to thrive. A 40-watt tube kept within eight inches (20.32 cm) of the soil surface will provide 700-1000 ft-c of light to the plant. Fluorescents last up to 20,000 hours, and are inexpensive. They also emit little heat, unlike incandescent bulbs, so they can be kept close to plant surfaces without burning the foliage.
Full-spectrum and high-output fluorescent bulbs and tubes are also a good option for plant grow lights. These lights replicate 98% of natural sunlight and emit twice as much light as standard fluorescents, while their average lifespan is halved to 10,000 hours. Compact fluorescents can also be used effectively for smaller growing areas or individually-lighted plants.
Metal halide or High Intensity Discharge (HID) bulbs are also good plant grow lights. Because of their intensity, ranging up to 400 watts per bulb, they most closely resemble natural light for plants, and plants grow to look identical to their outdoor counterparts under HID lamps. The HID design emits twice as much light as a fluorescent bulb for the same amount of energy used, but they are much more expensive, require special lighting fixtures, and can be a significant drain on power systems. Hydroponic plant growth and commercial nurseries are the primary market for HID lamps.
Another commonly-used form of plant grow lights are high pressure sodium (HPS) or sodium vapor lights. These lamps emit red spectrum light that is needed most by fruiting and flowering plants. To encourage even plant growth, HPS light systems are often mixed with HID or fluorescent lighting, as HPS alone will cause plant growth that appears tall, spindly, and unhealthy. One of the biggest advantages of HPS, however, is that they can last for up to two years, and have the highest light output for the energy they consume of any lighting system. Only LED lights exceed the brightness and energy efficiency of HPS, but trials with LEDs for plant growth have proven to be disappointing.
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