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A number of different options for hydroponic lighting are available depending on the plant type and developmental stage. Plants typically require light from various parts of the spectrum, and many gardeners utilize several types of lighting for complete coverage. High intensity discharge bulbs such as metal halide and high pressure sodium cover the blue portions of the spectrum and are ideal for gardens where no natural light is present. Fluorescent bulbs are typically used for plants in the seedling stage of growth. Light-emitting diodes (LEDs) produce minimal heat and cover a broad range of the spectrum while conventional incandescent bulbs generally have little value as a hydroponic lighting source.
Metal halide bulbs produce blue light and are the most common type of hydroponic lighting. Plants that receive no sunlight typically benefit the most from metal halide bulbs. In most cases, metal halide bulbs are the primary source of lighting along with other bulb types employed as a secondary source. Plants tend to grow tight and thick when cultivated solely under this form of lighting. This type of high intensity discharge bulb is a relatively inexpensive lighting option.
Another type of high intensity discharge lighting is a high pressure sodium bulb. This bulb produces light in the red and orange spectrum and is often used in combination with the metal halide variant to produce light across a broader range of the spectrum. High pressure sodium bulbs are frequently used as supplemental lighting for gardens which receive only indirect sunlight. When employed alone, this type of hydroponic lighting usually causes plants to grow abnormally tall and thin.
Fluorescent bulbs are a good choice for plants during the seedling stage. Some gardeners use fluorescent lighting to grow plants such as spinach and lettuce. These bulbs give off very little heat and can be placed near the plant for maximum light exposure. Their low intensity light usually makes them a poor choice during advanced stages of plant growth. High output versions of this bulb are sometimes utilized with other forms of hydroponic lighting to broaden the spectrum range.
LEDs are a relatively new form of hydroponic lighting. They are very intense but produce only a small amount of heat. This form of lighting is usually the most expensive indoor gardening option. Standard incandescent bulbs typically produce a great deal of heat while covering only a small amount of the light spectrum. Incandescent bulbs are generally unsuitable as a lighting source for indoor plants.