Horticulture Techniques

All the Colors of the Rainbow: Cannabis Photobiology, Chapter Two

Last week, we introduced the concept of photobiology, which regards how plants convert sunlight into chemical energy (photosynthesis) and how light interacts with plants, leading to growth, structure, flowers, and overall yield (photomorphogenesis). Now, we’ll get to the meat of it, namely, various colors of the rainbow, and how they impact plants.

Red: Red light impacts photomorphogenesis through phytochromes Pr (red) and Pfr (far red), leaf nutrient content, and stem growth. [1] Researchers have shown that a high Pfr/Pr ratio is optimal for flower stimulation in plants that require longer “days” via longer exposure to light. This occurs when red light (approx. 650–670 nm) is employed early in the photoperiod, giving way to far-red light (approx. 705-740 nm) at the end.

Interestingly, some cannabis cultivars have shown indifference to Pfr/Pr dependence for flower generation. Researchers reported that some cannabis plants thrive with 18 hours of light and 6 hours of darkness, or a low Pfr/Pr ratio. [2] So, it’s clear that red light affects the flowering quality, quantity, and duration. Terpene biosynthesis is also regulated by red-light phytochrome Pr. [3]

Blue: Blue light interacts with cryptochrome and phototropin receptors which oversee physiology and development, including germination, elongation, water movement throughout the plant, and exchange of carbon dioxide. [1] Blue light governs how plants respond to environmental stressors like bacteria and fungi. [4] One study found that wavelengths below 445 nm enhanced stem growth, plant height, and anthocyanin formation which causes those wonderful purple hues in cannabis. [5]

Cannabis plants grown under blue light with a 12-hour on, 12-hour off photoperiod showed enhanced cannabinoid concentrations. [2] These researchers also found a correlation between ultraviolet (UV)-A and blue wavelengths, that, when used together, augmented cannabigerol concentrations in flowers.

Ultraviolet: Speaking of UV radiation, while wavelengths below 400 nm are not heavily involved in photosynthetic processes, they are involved in photomorphogenesis. UV-B, while detrimental to plants in larger doses, can fortify a plant’s resistance to pests, increase flavonoid concentrations, and improve photosynthetic efficiency. [6] UV-B has also been shown to increase delta-9-tetrahydrocannabinol (THC) levels in leaves and flowers [7], which makes sense since cannabinoids have been called a natural sunscreen. [2]

Green: While plants reflect a lot of green light, finding little utility for these wavelengths, plant morphology can be affected. Like UV-B wavelengths, lower levels of green light can help plant growth while heavier doses can inhibit plants from developing. [8] And while green light can penetrate deeper into leaves compared to red or blue light [9], it can cause conflicting responses to important processes induced by blue light [10]. Additionally, green light can reduce THC levels. [2]

So, the composition of your light source has everything to do with the vitality, yield, and phytochemistry of your crops. Identifying which wavelengths are or aren’t needed can ensure you’re not illuminating your plants with unwarranted radiation.


  1. Bilodeau, S. et al. “An Update on Plant Photobiology and Implications for Cannabis Production.” Frontiers in Plant Science, vol. 10, 2019, pp. 296. [journal impact factor = 4.298; cited by 2 (ResearchGate)]
  2. Magagnini, G., Grassi, G., and Kotiranta, S. “The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L.” Med. Cannabis Cannabinoids, vol. 1, 2018, pp. 19–27. [journal impact factor = N/A; cited by 5 (ResearchGate)]
  3. Tanaka, S., et al. “Phytochrome-Mediated Production of Monoterpenes in Thyme Seedlings.” Phytochemistry, vol. 28, 1989, pp. 2955–2957. [journal impact factor = 2.547; cited by 22 (ResearchGate)]
  4. Schuerger, A. et al. “Anatomical Features of Pepper Plants (Capsicum annuum L.) Grown Under Red Light-Emitting Diodes Supplemented with Blue or Far-Red Light.” Ann. Bot., vol. 79, 1997, pp. 273–282. [journal impact factor = 3.454; cited by 199 (ResearchGate)]
  5. Lee, J. et al. “Shorter Wavelength Blue Light Promotes Growth of Green Perilla (Perilla frutescens).” Int. J. Agric. Biol., vol. 16, 2014, pp. 1177–1182. [journal impact factor = 0.802; cited by 9 (ResearchGate)]
  6. Wargent, J., and Jordan, B. “From Ozone Depletion to Agriculture: Understanding the Role of UV Radiation in Sustainable Crop Production.” New Phytol., vol. 197, 2013, pp. 1058–1076. [journal impact factor = 7.43; cited by 94 (ResearchGate)]
  7. Lydon, J., et al. “UV-B Radiation Effects on Photosynthesis, Growth and Cannabinoid Production of Two Cannabis sativa Chemotypes.” Photochem. Photobiol., vol. 46, 1987, pp. 201–206. [journal impact factor = 2.214; cited by 49 (ResearchGate)]
  8. Kim, H.-H. et al. “Green-Light Supplementation for Enhanced Lettuce Growth Under Red-and Blue-Light-Emitting Diodes.” HortSci., vol. 39, 2004, pp. 1617–1622. [journal impact factor = 0.623; cited by 24 (ResearchGate)]
  9. Brodersen, C., and Vogelmann, T. “Do Changes in Light Direction Affect Absorption Profiles in Leaves?” Funct. Plant Biol., vol. 37, 2010, pp. 403–412. [journal impact factor = 2.491; cited by 72 (ResearchGate)]
  10. Frechilla, S. et al. “Reversal of Blue Light-Stimulated Stomatal Opening by Green Light.” Plant Cell Physiol., vol. 41, 2000, pp. 171–176. [journal impact factor = 3.929; cited by 94 (ResearchGate)]

Image Credit: PL Light Systems

About the author

Jason S. Lupoi, Ph.D.

Jason S. Lupoi, Ph.D.

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