Using a High Pressure Sodium Grow Light for Orchids
A high pressure sodium grow light (or HPS) is an effective choice for growing orchids under lights. These bulbs commonly come in wattages of 250W, 400W, and 1000W, among others; they can be very bright. There is plenty of intensity to grow things like Cattleya orchids. Obviously, types of orchids that need higher light intensity need to be closer to the bulbs, while low-light plants like Paphiopedilums can be further away so that the light can be elevated to cover more area. Plants directly beneath the light will get more light than plants off to the side, so with a bit of care you should be able to match each orchid's lighting needs.
These bulbs have low color temperatures compared to sunlight, so they'll be more yellowish or reddish than natural sunshine. (If you look at streetlights, the yellowish ones are HPS bulbs and the bluish ones are metal halide bulbs; both can be used for growing orchids.) The higher amount of red tones in the spectrum of these bulbs encourages stem growth in plants; this can help to encourage blooming, but it also tends to produce "leggy" plants that sprawl all over. HPS bulbs are often combined with metal halide grow lights to reduce this effect, as they tend to balance each other out. Bulbs specifically sold as grow lights will also tend to have color temperatures closer to natural sunlight.
These bulbs are have less energy efficiency than LED grow lights. One consequence of this is that the bulbs get very hot, potentially raising the temperature of your orchid growing area during the day. (It also means they will shatter if they get wet, so avoid that!) Some of the heat will also be produced from the bulbs' ballasts, which are sometimes part of the lighting fixture and sometimes in a separate unit. The most efficient ballasts are digital ballasts; the old magnetic ballast designs are inferior.
Efficiency is comparable to fluorescent grow lights, but the light comes from a more concentrated source so you can provide higher intensity to your plants.
As the spectrum of the bulbs gradually shifts and they slowly dim, replace the bulbs every 6 months to 1 year. The only grow lights where this isn't necessary are LEDs. The bulbs will be extra-bright for the first few days after replacing them, so be careful that your plants don't get too much light.
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