- 1). Find the square footage of your growing area to determine the wattage of metal halide bulb to purchase. Use a measuring tape to determine the width and depth of the growing area. Multiply the width by the depth to determine square feet. Multiply the desired watts per square foot--use 40 for full sun plants and 25 for partial shade plants--by the square footage of the growing area to determine the appropriate wattage. Round up, if necessary, to the next wattage increment.
- 2). Purchase a lighting fixture that's compatible with the metal halide wattage you need to adequately light your growing area. Choose a reflector that's compatible with your grow light. Consider purchasing a grow light fixture with a switchable ballast that's compatible with both metal halide and high pressure sodium grow light bulbs.
- 3). Set up your grow light fixture so it can be raised and lowered as your plants grow. Position 250-watt metal halide lights 10 inches above plants, 400-watt metal halide lights 12 to 14 inches above plants and 600-watt metal halide lights 16 to 18 inches above plants. Keep 1,000-watt metal halide lights at least 24 inches from the tops of your plants to prevent the tips from burning.
- 4). Turn on your metal halide light and wait for it to warm up; this can take between 10 seconds and five minutes. Give your plants 16 to 18 hours of light per day. Consult the specific requirements for the type of plant you're growing to determine exactly how many hours of light to provide.
- 5). Run a fan in your growing area to keep the air cool and circulating. This is essential to keep the area at a growing temperature that's comfortable for your plants.
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