Metal halide grow lights are a popular choice for growing orchids under lights. They provide good intensity, and the relatively high color temperatures enhance foliage growth.
Common wattages of these lights are 250W, 400W, and 1000W, though others are available. They put out quite a lot of light, and can effectively light several square feet or more depending on the lighting needs of the types of orchids you grow.
They are less energy-efficient than LED grow lights, and so will produce more heat. For warm-growing orchids, you might like this; for cool growers it can be a nuisance. Fortunately, as the lights are only on during the day (probably on a timer), this will help to create a temperature drop at night (you want it to cool off by 10-15°F, or 6-8C).
Metal halides typically have a warm color temperature, so they will be more bluish than natural sunlight. Plants only use certain frequencies of light, with an emphasis on blues and reds. Blue light encourages foliage, while red light encourages stems and flowers. If you want more of the reddish colors, you might pair a metal halide light with a high-pressure sodium grow light, which is typically skewed the other way. People that just use the sodium bulbs tend to get very leggy plants, though. Some brands of metal halides are closer to the color temperature of natural sunlight (6500K) than others, so you can also just look for a bulb that reasonably approximates the sun. There are also many fluorescent grow lights that work well, especially for plants with lower lighting needs.
These bulbs don't go into standard light bulb sockets; you'll need a special reflector, and a ballast to power them. Usually, these ballasts are only suited to a certain brand of bulb. Modern digital ballasts will outperform the older magnetic ballasts; in particular, they will make the bulbs last longer, and will increase energy efficiency.
For safety, make sure not to position electronic components anywhere they might get wet; for example, if you have an automatic misting system position it below the bulb, and keep the ballast out of the way as well.
Some metal halides produce significant ultraviolet radiation; it's actually possible to get a sunburn from some of them, so use sunscreen, wear a hat, and otherwise protect your skin when working in your under-lights orchid growing area. And avoid looking directly at the lit bulb to prevent cataracts.
The spectrum of these bulbs will shift over time and they will gradually become dimmer. It's best to replace the bulb every 6 months to 1 year, well before it burns out. Be aware that it will be significantly brighter than normal for the first few days after replacement, so you may have to watch out for sunburn.
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