Hydroponic gardens typically supplement or replace natural sunlight with specialized “grow lights” designed according to the parts of the light spectrum plants require the most. What kind of grow light (or combination of grow lights) you need shall depend largely on what plants you’re growing, and what plant behavior you are trying to encourage.
First, it is important to have a background about color temperature in order to fully comprehend the concept of grow lights. When speaking of “temperature” with plant lighting, we aren’t talking about heat, but about color. Just as in other applications of heat and cold, the color spectrum is measured in degrees Kelvin – but the distinction is, higher temperatures are considered “cooler,” and lower ones, “warmer.” The temperatures ranging above 5000K bear the highest resemblance to the intensity and color of sunlight, and are 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 under slightly cooler color temperatures typically around 4800K, which is still considered in the “blue” zone but leafy vegetation does better in the 6000K range (essentially full sunlight). Plants are also drawing more from the orange-red color temperatures, which are approximately 2700K, when they’re flowering and reproducing. With this, it’s clear that the appropriate grow light is very important to a plant’s health.
The three most common types of grow light used in hydroponic growing rooms are designed toward the color temperatures most needed by plant life. The Metal Halide bulb is one of the most popular. A High-Intensity Discharge (HID) bulb, Metal Halide bulbs range from 2700K to 5500K and are the closest to natural sunlight that money can buy. As mentioned earlier, this type of grow light is great for leafy plants. A second alternative (also a HID bulb) is the High Pressure Sodium bulb. Producing the orange-red part of the light spectrum (approximately 2200K), these bulbs are good for flowering plants but don’t encourage full foliage. They are not generally used by themselves, but in combination with other bulbs and/or natural light. Of course, do not take your focus away from the 3rd most common bulb – the fluorescent, both high- and low-output. Not as “powerful” as the High-Intensity Discharge bulbs, they can still cover the spectrum perfectly, with the added advantage that they can be placed close to the plants since 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’re quite expensive, and in some circles, it’s believed that you can achieve the same results with the cheaper fluorescent bulbs.
For many hydroponic gardens, different bulbs are used to cover the full spectrum of blues and orange-reds that plants need for various stages of growth although leafy plants will lean more heavily toward the blues. Your retailer shall be able to advise you on what grow light combination will be best for you.