I highly recommend watching these videos (https://www.youtube.com/watch?v=ID9rE5JewVg and https://www.youtube.com/watch?v=dXkYPN3HD6A ) by Dr. Bruce Bugbee, a professor of crop physiology at Utah State University, if you are interested in the effects of lighting (intensity and spectral quality) on cannabis growth. A summary of these videos describing effects of lighting on cannabis are listed below.
Cannabis requires a very high light intensity to grow to its full potential. On a sunny day at noon in the summer the light intensity outside is approximately 2000 µmol_photons/(m2*s). The light intensity of a well-lit office is around 7 µmol/(m2*s). This intensity is based on the distance from the light source. As the light source comes closer the total amount of photons are concentrated over a smaller area which leads to a higher intensity. Heat is often a consideration with placements of lights, however LEDs tend to mitigate this heat problem. Some crops such as lettuce have a maximum amount of light that they can take advantage of which is in the 500 µmol/(m2*s) range and its net photosynthesis will even decrease if the light intensity gets too high in the 2000 range. The “devil’s lettuce” on the other hand has a net photosynthesis that increases to 2000 µmol/(m2*s) and possibly more.
The amount of light per day also has an effect on the cannabis plant. For plant growing phase (also referred to as veg phase), it is best to have 16-18 hours of light. This duration of light should decrease during flowering phase to around 12 hours. Periods of complete darkness are also needed for cannabis. There are some literature research that say that cannabis is extremely sensitive to light pollution and can negatively affect the plant. Light pollution can cause your plant yield to decrease or the plant might not even flower.
Research says that light quality does not strongly effect the synthesis of cannabinoids. There might be some effect, but the effect is minimal. Photosynthetic radiation (the spectral range that photosynthetic organisms are able to use in the process of photosynthesis) exists predominately in the 400-700 nm range. However, the ranges of 350-400nm of ultraviolet light and 700-750nm also can effect plants. The optimum selection of wavelengths to grow plants depends on the stage of plant growth. Traditional wisdom has said that you want to grow with metal halide light in the veg phase and use high pressure sodium (hps) light during flowering phase. Metal halide lights have around 30% blue light which is good to limit cell expansion and keep the plant stems and leaves compact. High pressure sodium lamps during flowering phase are good to help with energy efficiency and aid in plant growth. However, the decreasing cost and energy efficiency of LEDs are becoming a more attractive alternative.
Spectral quality effect of plant growth
Blue photons inhibit cell expansion. The less blue light a plant receives the more compact the cannabis plant will be. Green photons facilitate human vision and allow for identification of issues wrong with plant such as bugs, disease, etc. Red photons are good for efficient photosynthesis. Far-red photos enhance cell expansion and aid in overall plant size. Unfortunately, far-red light typically just results in tall plants and without significant effect on leaves or flowers. In terms of cannabis flower output, experimental results have shown there is no statistically significant difference between plants grown at 4%, 10%, 16%, and 20% blue light. In terms of cannabinoid content, there is also no statistically significant effect on CBD output or THC output with medical hemp plants. This goes against the myth that color spectrum effects cannabis plant output. The primary choice in lamp selection should be based solely on cost and energy efficiency.