Perhaps the most important step in setting up your indoor growing space is selecting the right lights, to ensure your plants will thrive. The absolute lowest output you’ll want to use is 2000 lumens per square foot. Any lower than this and you risk stunting plant growth, and increase the chances of growing plants that are all stem and little bud.
Optimal output for your lighting is 2500 lumens per square foot, 3000 if you are using soil enriched with CO2. The best way to achieve optimal lighting is using High Intensity Discharge lamps, of which there are three main varieties: High Pressure Sodium, Mercury Vapor and Metal Halide.
High Pressure Sodium (HPS) bulbs are by far the most efficient of the three varieties of HID lamp. A 400 watt HPS bulb has an output of roughly 45000 lumens, and can be used constantly throughout every stage of growing. As well, the spectrum produced by HPS lights is high in red light, so it produces above-average results during the flowering stages of growth.
Metal Halide (MH) bulbs have demonstrated faster crop maturity than HPS bulbs, but with smaller yields. A 400 watt MH bulb will output 36000 lumens, and again can be used throughout the growing process. The spectrum produced by a MH bulb is high in blue light, which accelerates growth during the plant’s vegetative growth stage.
Mercury Vapor (MV) bulbs are inferior to MH and HPS bulbs in both efficiency and light output. MV bulbs are similar in output to fluorescent bulbs, however you are not able to position them as close to plants as with fluorescent bulbs, so there is an increased risk for stem-y plants and below-average harvests. A 400 watt bulb will generate 20000 lumens (compared to 30000 by a fluorescent bulb of the same wattage), and produces a blue-toned light.
When setting up your lighting rig, aim to position the lights horizontally above the plants, as opposed to lighting them from the sides. This will increase the amount of light that reaches the plants and ensure that plants are lit evenly. Consider the size of the space you are working with, and make sure to choose an appropriately sized lamp for your growing area. HID lamps generate an excessive amount of heat and using a lamp that is ill-suited to your growing area will make constant ventilation a necessity.