J.J. Casal, Photoreceptor signaling networks in plant responses to shade, Annu. Rev. Plant Biol. 64 (2013) 403–427.
M. Götz, et al., PAR modulation of the UV-dependent levels of flavonoid metabolites in Arabidopsis thaliana (L.) Heynh. leaf rosettes: cumulative effects after a whole vegetative growth period, Protoplasma 243 (2010) 95–103
As cannabis growers, many of us find this response alluring. On the surface, the concept of more purples and secondary compounds such as anythrocynine seem likely to lead to a better terpene and cannabinoid suite in our final product. The science, however, indicates that UV and Cannabis relation effects are minimal at best. Yes, we get more vibrant colors and possibly better trichome head production as well. Ironically though, it seems this can also be achieved with any full spectrum LED on the market without the extra stressor of UV light. As the scientific studies are lacking here, anecdotal stories and results are all that we can rely on today.
An additional question we must ask is why are we not seeing more LED light manufacturers adding UV to the fixtures? With UV-A being 50-100x the cost of a full spectrum white or even a deep red LED, the cost for UV is definitely prohibitive. For a manufacturer to include this in every light they sell, it gives pause and begs the question of its validity and value. Especially as we look at commercial facilities who are purchasing hundreds of lights, is there enough evidence (as of the publishing of this article there is not) to support the increased cost?
The Dangerous Side of UV
The first question is, “Why is there such an industry wide belief that UV light is needed in the first place?” When plants of any type encounter UV, they produce stress responses that trigger protection mechanisms. Just as humans use sunscreen to protect themselves from the harmful rays of the sun (UV-A and UV-B), plants produce secondary compounds like anythrocynine to protect themselves from these same damaging UV wavelengths.
Soil and organic coco coir growers have some secondary things to consider. First and foremost, multiple studies have shown UV-B can degrade organic additions. Bacteria and fungal communities can be impacted and have been shown to actively avoid high UV-B areas. Research also shows that arthropods will be pushed further into the canopy or media to avoid the high UV-B light (more on this later). Secondarily, ask yourself if you’re getting 100% out of your plants already. Is that 3-5% bump from minor stressors going to be worth compromising your additions?
D.T. Krizek, Influence of PAR and UV-A in determining plant sensitivity and photomorphogenic responses to UV-B radiation, Photochem. Photobiol. 79 (2004) 307–315.
Is UV Worthy of All The Hype?
As our research into cannabis continues, we hope to see more data come out of future experiments. Bruce Bugbee has a project underway looking very closely at cannabis’ response to UV-A and UV-B and the UV and Cannabis relation. In a recent interview, shedding some light on his current work, he suggested some of the early research from heavy blues (400-450nm), just short of the UV-A spectrum, were showing similarly beneficial effects.
What is the true UV and Cannabis relation? There are few more heated topics in the cannabis industry than the potential of UV light to improve the quality of cannabis plants. Some growers insist that it is an undeniable fact that UV is necessary to grow high quality cannabis, period. The demonstrable data to date, however, is minimal at best. While there have been a handful of good studies conducted on UV light and cannabis, on the whole, their results are inconclusive and unremarkable.
Many of the spectrum wavelength ranges I mention above are visible to the human eye, but plants require wavelengths above and below the visual spectrum of light that includes ultraviolet (<400nm) and far-red/infrared (>700nm).
UVB (280-315nm) has a short wavelength, high energy and also causes sunburn in humans and plants. UVB is known to damage protein and nucleic acids in plant cells, causing decreased metabolism and decreased number of flowers. UVB can have positive effects for plants as well. Cannabis responds to the stress and sunburn from UVB wavelengths, by creating it’s own sunscreen in the form of trichomes. The more trichome production the higher the THC, CBN and CBD levels.