In hydroponics, one of the largest contributors to plant growth (other than the nutrient solution) is the lighting. Plants need light to conduct photosynthesis, and they need specific frequencies in the light spectrum for different stages of maturity (plant growth and leafing uses a different part of the spectrum than blooming and flowering). Hydroponic lights(commonly called “grow lights”) are used to evoke these behaviors in plants. Here are the most common types of hydroponic lights, and their usage.
Metal Halide (MH)
One of the most common types of grow lights, this is a High Intensity Discharge (HID) bulb that leans toward the blue part of the spectrum, which encourages plant vegetation and leafy growth. If your garden has no natural lighting at all, this is a good choice for your primary lighting source. Plants grown under MH lighting tend to be thick and compact.
High Pressure Sodium (HPS)
Also an HID, this bulb covers the orange and red spectrum bands, which is good for the flowering and budding stage. This bulb is best used as supplementary lighting, either with natural sunlight or with another type of bulb, like MH. Plants grown strictly under HPS lighting tend to shoot up quickly, producing abnormally thin and tall plants.
Used as grow lights, standard fluorescent bulbs are good for seedlings and sometimes lettuce and spinach plants, but in most cases they don’t have enough intensity to give other plants the lighting they need. However, the newer high-output fluorescents make great hydroponic lights, because they can be combined to cover the full light spectrum. Also, they emit much lower heat than their HID counterparts, so they can be placed much closer to the plants with better results.
One of the newer options available, LED grow lights are touted as a great choice in hydroponics because they can cover a broad spectrum and because they emit intense light with very little heat. The down side is that they can be quite expensive, and many growers have determined that they can get just as good of a color spread from fluorescents or other less-expensive combinations. So while LED lighting can do a good job and are relatively simple, the jury is still out as to their cost-effectiveness.
Incandescent grow lights
Incandescent bulbs, while commonly available, are considered the least productive choice in hydroponic lighting. They emit a limited color range and burn so hot that they cannot be placed near enough to the plants to be very effective. Color-corrected incandescents are sometimes used as accent lighting or to supplement other lighting for individual plants, but incandescents should never be used as a primary lighting source.
Because hydroponic lights cover various parts of the spectrum needed at different stages of growth, many gardens will use more than one type of bulb in their lighting array to provide full coverage. Because different plants have different needs, what you grow will determine how your lighting should be arranged.