In hydroponics, natural sunlight is replaced by specialized "grow lights" which are designed to emit the spectrum that the plant needs most. What type of grow light (or combination of grow lights) you need will depend primarily on what plants you're growing, and what plant behavior you're trying to elicit.
First, it is essential to have a background about color temperature so as to fully comprehend the concept of grow lights. In the subject of plant lighting, "temperature" means color, and not heat. Just as in other applications of heat and cold, the color spectrum is measured in degrees Kelvin - but the difference is, higher temperatures are considered "cooler," and lower ones, "warmer." The temperatures that highly resemble that of natural sunlight are those ranging over 5000K, and is seen as blue on the color spectrum. Interestingly, the reds, oranges and yellows are lower temperature, ranging from 2700K and below. The importance of this spectrum is that flowering and fruiting plants tend to do better with slightly cooler color temperatures usually around 4800K, which is still considered in the "blue" zone but leafy vegetation does better in the 6000K range (in essence, full sunlight). Plants are also drawing more from the orange-red color temperatures, which are around 2700K, when they are flowering and reproducing. So you can understand why the appropriate grow light setting can be so important.
In hydroponics, the three most common types of grow light are veered towards the cooler temperatures - as this is most needed by the plants. The Metal Halide bulb is one of the most popular. If you like a grow light that resembles true sunlight the closest, then you must go for High-Intensity Discharge (HID) bulb, Metal Halide bulbs. As previously mentioned, this type of grow light is great for leafy plants. A second alternative (also a HID bulb) is the High Pressure Sodium bulb. These bulbs emit the orange-red part of the spectrum (around 2200K) and are most suited for flowering, but don't encourage full foliage. Normally, these bulbs are used in combination of other bulbs or natural light - and aren't utilized as a standalone. Of course, do not take your focus away from the 3rd most common bulb - the fluorescent, both high- and low-output. Not as intense as the HID bulbs, they can still cover the spectrum well, with the added advantage that they can be placed close to the plants because they emit so little heat.
Today, there are already available LED bulbs that claim to cover the full range o the color spectrum, without the issue on heat. However, they are a bit costly, and in some circles, it's believed that you can achieve the same results with the cheaper fluorescent bulbs.
For most hydroponic gardens, different bulbs are used to cover the full spectrum of blues and orange-reds that plants require for various stages of growth although leafy plants will lean more heavily toward the blues. Ask your retailer for the best grow light combination that can address your needs.
This article was added on Wednesday 23 December, 2009.
