Growing plants in a greenhouse environment can be tricky. Greenhouses need to have ideal growing conditions that mimic those the plants would otherwise experience outdoors. To this end, grow light manufacturers offer a variety of light bulbs with varying levels of luminosity. Generally speaking, the higher the wattage of bulb, the larger the area it will cover in the greenhouse. Therefore, the size of your greenhouse should ultimately determine what wattage you will use. A 1000-watt light bulb will cover a greater area than common 400- and 600-watt bulbs.
The coverage area for a 1000-watt light bulb will ultimately depend upon how high you mount the bulb above the plants you're growing. A light that is hung close to the plants will be limited in the amount of coverage it can provide, whereas one that is placed higher will be able to cover a larger area. However, the intensity of the light also diminishes with greater height as well.
The typical coverage area of a 1000-watt light bulb is roughly 8-by-8 feet, according to Plant Lighting Hydroponics. This computes to 64 square feet of coverage for a typical greenhouse. To estimate the number of lights and ballasts you will need to cover an entire greenhouse, divide the size of the greenhouse by this number. For instance, if you have a 640-square-foot greenhouse, you will need about 10 1000-watt bulbs to cover your plants.
The coverage area for a 1000-watt grow light is generally the largest of the available lights. By comparison, a 600-watt bulb will cover a 6-by-6 foot area or about 36 square feet. A 400-watt bulb will cover a 5-by-5 square foot area or 25 square feet. The coverage area drops significantly for a 100-watt bulb, which only covers 1.5-by-1.5 feet or 2.25 square feet altogether.
When choosing the lighting system for your greenhouse, consider the costs in addition to the amount of coverage area. A ballast for a 1000-watt bulb will typically cost $25 or more while the bulb itself will typically run more than $75 each, depending on the brand. The type of light fixture you use will also have an impact on how much coverage you will get. Some light fixtures help reflect and diffuse light better than others.
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