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Hydroponic gardens normally supplement or replace natural sunlight with specialized "grow lights" designed according to the parts of the light spectrum plants require the most. The type of grow light you require, obviously depends on several factors, like the kind of plant and the plant behavior that you're trying to encourage.

First, it is essential to have a background about color temperature so as to fully understand the concept of grow lights. When referring to "temperature" with plant lighting, we aren't talking about heat, but about color. The color spectrum is measured in degrees Kelvin, in the same way as some applications of heat and cold are except higher temperatures are considered "cooler", and lower ones are "warmer." The temperatures that largely resemble that of natural sunlight are those ranging above 5000K, and is seen as blue on the color spectrum. Fascinatingly, the reds, oranges and yellows are lower temperature, within the range 2700K and below. The importance of this spectrum is that flowering and fruiting plants tend to do better with 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 are flowering and reproducing. With this, it is clear that the appropriate grow light is critical to a plant's well-being.

The three most common classifications of grow light utilized in hydroponic gardens 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. The High Pressure Sodium bulb (also another HID bulb) is your second option. Emitting the orange-red part of the spectrum (about 2200K), these bulbs are most appropriate for flowering plants but do not encourage full foliage. Normally, these bulbs are used in combination of other bulbs or natural light - and aren't used as a standalone. The third most common bulb is the fluorescent either high-output or low-output. Even if they're not as intense as the HID bulbs, they remain useful especially due to the fact that since they emit so little heat, they can be placed close to the plants.

Today, LED bulbs have been designed, some of which claim to cover the full range of color temperature for plant life with almost no heat. The only major fallback is, they're usually very expensive - and many cheaper alternatives are available that can do the job just fine.

For most hydroponic grow rooms, different bulbs are combined 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. Your retailer shall be able to advise you on what grow light combination will be best for you.