Definition - What does Lumens (lm) mean?
Lumens are the amount of light a bulb provides. Essentially, lumens mean brightness. The higher the amount of lumens, the brighter the light that will be achieved. Plants, especially cannabis plants, require adequate light to grow. Typically, one cannabis plant needs at least 2,500 to 3,000 lumens.
Ideally, a marijuana plant should have 4,500 lumens to ensure maximum growth. A 4,500-lumen bulb will cover a 3x3-foot space with adequate lighting.
Cannabis plants are dependent upon precise light/dark ratios in order to adequately flower and produce an ample harvest. Growers must ensure that the plants receive sufficient light so the plant can complete its life cycle.
Hydrolife explains Lumens (lm)
Most standard fluorescent tubes produce 3,000 to 3,300 lumens per tube. There are various tricks that a cannabis grower can do to increase the lumens of a grow light, such as placing a metal or foil reflector behind the bulb or tube to reflect even more light down onto the plant. This trick increases the amount of lumens generated by the light source that reach the plants.
Many growers first starting out tend to get confused between watts and lumens. Watts are the amount of energy a bulb consumes. Even if a bulb has a high wattage, it does not necessarily mean that it will produce high lumens. This is especially true with new high-energy LED lights because the amount of wattage does not guarantee a brighter light.
A common expression in the cannabis cultivation community is "lumens are for humans, PAR is for plants." This is a way of saying that for grow lights, growers should be most concerned with how much photosynthetically active radiation (PAR) a plant puts off, rather than it's level of lumens.
Plants need adequate light in the blue and red spectrums, which is considered the PAR range. Lumens is a good place to start, but PAR levels will take your consideration of grow lights to another level. Make sure your grow lights have the right level of lumens in the right PAR range.