Cannabis growth rate depends on light. While outdoor growers utilize natural sunlight,indoor growers must optimize the harvest with artificial lighting.Let’s take a look at two pillars of cannabis lighting: quantity and quality.
Quantity of light is synonymous with light intensity.The measure of light intensity is photosynthetic photon flux density (PPFD), or the number of photons at wavelengths of 400-700 nanometers (nm) striking a square meter per second. This wavelength range is known as photosynthetically active radiation (PAR) because it can be used to drive photosynthesis. A PPFD of 2,000 is comparable to the summer sun’s noonday light.
Cannabis can handle intense lighting. Eaves et al. (2019) suggest that cannabis yields increase in a linear fashion up to 1,500PPFD, or about double the intensity more commonly used. Chandra et al. (2015) estimate an even higher beneficial PPFD of 2,000. Since more light requires more electricity, yield efficiency (grams per watt) may impose a barrier that limits the quantity of light.
Light quality refers to different wavelengths of light:
UV Radiation (100-400 nm)
Ultraviolent light (or radiation) can be harmful to humans and plants (e.g., sunburns).UV-B (280-315 nm) light has the potential to damage plant tissue, but in limited supplemental doses, may lead to increased tetrahydrocannabinol (THC) content in cannabis.
PAR (400-700 nm)
PAR drives plant growth and development. It consists of blue light (400-500 nm), green light (500-600 nm), and red light (600-700 nm). Sunlight emits roughly equal amounts of each one. Artificial lights, however, differ in the amounts emitted.
High-pressure sodium lamps emit mostly red and green frequencies. Metal halides and fluorescents favor green light with equal amounts of blue and red. LED lighting can be keyed for different frequencies but often emphasizes red.
Red light is generally most energy-efficient in terms of eliciting photosynthesis. However, blue and green lights have critical effects on plant metabolism and growth. Too much or too little of one type can be detrimental; excessive blue light may stunt plant elongation.Light quality is especially important in situations of low light intensity (quantity).Hawley et al. (2018) found that supplemental red-blue and red-green-blue subcanopy lights enhanced both yield and terpene profiles of indoor cannabis. 
Far Red Radiation
Far red radiation affects extension growth by enlarging/elongating leaves to capture more usable light. It mimics shade and may induce flowering in photoperiodic plants (like cannabis)that bloom in response to light variation.Incandescent lamps have high far-red frequency and can therefore induce premature flowering.
Quantity and quality are key factors when designing a cannabis lighting scheme. But the grower must also consider variables such as chemotype, space limitations, and light schedule. Sustainability dictates the threshold of energy/resources and yield. In short, the grower must analyze their circumstances and experiment with quantity and quality of light for optimum results.
- Eaves, J.et al. “The Impact of Light Intensity and Spectrum-Tuning on Cannabis Yields.” SSRN Electronic Journal.2019, doi:10.2139/ssrn.3310456. [Timed Cited = N/A; Journal Impact Factor = 0.31]
- Chandra, S. et al. “Light Dependence of Photosynthesis and Water Vapor Exchange Characteristics in Different High Δ9-THC Yielding Varieties of Cannabis Sativa” Journal of Applied Research on Medicinal and Aromatic Plants. 2015, 2(2): 39-47.[Times Cited = 1;Journal Rank = 0.355]
- Potter, D. J., & Duncombe, P.“The Effect of Electrical Lighting Power and Irradiance on Indoor-Grown Cannabis Potency and Yield.” Journal of Forensic Sciences. 2011,57(3): 618-622.[Times Cited = 7; Journal Impact Factor = 1.160]
- Lydon J, et al. “UV-B Radiation Effects on Photoynthesis, Growth and Cannabinoid Production of Two Cannabis Sativa Chemotypes.”Photochemistry and Photobiology. 1987, Volume 46(2): 201-206. [Times Cited = 27; Journal Impact Factor = 2.413]
- Bugbee, B.“Toward an Optimal Spectral Quality for Plant Growth and Development: The Importance of Radiation Capture.” Acta Horticulturae. 2016, Volume 1134: 1-12. [Times Cited = 20; Journal Impact Factor = 0.17]
- Hawley, D.et al.“Improving Cannabis Bud Quality and Yield with Subcanopy Lighting.” 2018, 53(11): 1593-1599. [Timed Cited = N/A; Journal Impact Factor = 0.83]