By: Due Diligence Horticulture

January 7, 2025

Ultraviolet (UV) radiation is often touted as an effective tool to increase cannabinoid production in cannabis cultivation. The premise is that UV acts as an environmental stressor, triggering plants to produce secondary metabolites like cannabinoids and terpenes as a protective mechanism. However, while UV stress has been shown to influence the synthesis of compounds like flavonoids and phenolics in some plant species, and cannabinoids do absorb UV radiation, recent research examining the effect of increasing UV in cannabis  cultivation has consistently demonstrated that UV exposure has a negligible or negative impact on  cannabinoid concentrations. 

Understanding UV Radiation

UV radiation is the region of the electromagnetic spectrum with shorter wavelengths and higher energy than  photosynthetically active radiation (PAR) and visible light. It is divided into three categories: 

  • UV-A (315–400 nm): The least energetic, it penetrates deeper into tissues and can influence  photomorphogenic responses but causes minimal damage in most plants[1]. 
  • UV-B (280–315 nm): A mid-energy radiation that induces DNA damage and stress responses,  leading to the activation of repair mechanisms and secondary metabolite pathways[2].
  • UV-C (100–280 nm): The most energetic and harmful, UV-C is filtered by the Earth’s atmosphere  and is not encountered under natural sunlight. UV-C lamps are commonly used to sterilize surfaces and minimize powdery mildew[3]. 
graphic depicting the electromagnetic spectrum
Figure 1: The electromagnetic spectrum. Ultraviolet radiation includes wavelengths from 100 to 400 nm

Among these, UV-B is most often studied for its role in inducing stress responses in plants. It can stimulate  the production of protective compounds like flavonoids, but its effect on cannabis-specific metabolites such  as cannabinoids remains controversial.

Theoretical Basis for UV and Cannabinoid Production

Cannabis is a prolific producer of secondary metabolites, including cannabinoids, terpenes, and flavonoids[4].  Cannabinoids such as THC and CBD are hypothesized to function as UV shields, absorbing harmful radiation  to protect underlying tissues[5]. Cannabinoids absorb UV radiation, and this idea has led to the assumption  that increased UV exposure might lead to elevated cannabinoid production. UV-B is known to trigger  metabolic pathways related to the biosynthesis of secondary compounds in plants, such as the  phenylpropanoid pathway, which produces flavonoids[6]. However, the effect of UV on cannabis remained  largely unexplored until recently due to prohibition.

Experimental Results: A Mixed Picture

Despite the theoretical basis, experimental studies have largely failed to confirm a significant impact of UV  radiation on cannabinoid production: 

1. Minor Increase in THC: 

  • Lydon et al. (1987)[7] reported a small (less than 10%) increase in THC concentration with increasing UV-B in a drug-type variety (Figure 2). However, the total  THC concentration of flowers at that time was less than 3% (30 mg g-1). These findings provided early evidence that UV-B may influence THC biosynthesis in low-cannabinoid genotypes, but the authors concluded that: The contribution of cannabinoids as selective UV-B filters is equivocal. Despite this conclusion, this study fueled decades of belief that UV-B is the silver bullet for increasing cannabinoid concentration under electric lighting.
graph depicting THC in flowers of a ‘high THC’ variety
Figure 2: THC in flowers of a ‘high THC’ variety increased from 2.5% to 3% with increasing UV-B.

2. No Effect on CBD or THC:

  • Rodriguez-Morrison et al. (2021)[8]: This study investigated the effects of UV-B radiation (0.01–0.8  μmol m–² s–¹) on two type II (equal CBD:THC)  cannabis cultivars grown indoors. Increasing UV-B  exposure reduced photosynthesis, leaf size, and  caused stigma browning. However, cannabinoid concentrations (THC or CBD) did not increase and,  in one variety, decreased slightly. The authors  concluded that UV-B offers no commercially  relevant benefits for enhancing cannabinoids in  cannabis inflorescence.
  • Llewellyn et al. (2022)[9]: This research examined  both UV-A (50 μmol m–² s–¹) and combined UV-A +  UV-B (3 μmol m–² s–¹) treatments in type I (high-THC) cultivars. While UV exposure caused no significant  impact on total THC/CBD concentrations, a slight  increase in sugar leaf THC was observed under UV A + UV-B. The overall findings emphasized that UV light did not enhance commercially relevant  cannabinoid production. 
  • Westmoreland et al. (2023)[10]: This study explored high UV photon fluxes (including UV-B and UV-A) on a type III (high-CBD) cultivar. There was no effect of UV on cannabinoid concentration across a wide range of daily UV doses (Figure 3). The authors concluded that UV light is ineffective for increasing cannabinoids in high-cannabinoid cultivars.
graph depicting the No effect of UV on CBD or THC concentration in a high CBD variety
Figure 3: No effect of UV on CBD or THC concentration in a high CBD variety

3. Reduced Plant Growth and Yield:

  • Rodriguez-Morrison et al. (2021)[8]:  The authors observed a decrease in  total dry inflorescence yield in one of  the two cultivars (“Low Tide”) as UV-B  exposure increased (Figure 4). The  reduction in yield was attributed to UV induced leaf curling that reduced  photon capture and reduced  photosynthesis.
cannabis plants being exposed to increased UV exposure
Figure 4: Increasing UV exposure reduced photosynthesis, growth, and plant size in two balanced CBD: cultivars
  • Westmoreland et al. (2023)[10]: At the  highest UV treatment levels, flower  yield was reduced by 12%. The  reduction was associated with  increased chlorosis (Figure 5) and  physiological stress caused by elevated  UV photon fluxes, including a reduction  in canopy photosynthesis. It is worth  noting that the reduction in yield was  small, and the daily UV dose was  exceptionally high, suggesting that  cannabinoid do act as UV filters, but additional UV exposure has not been shown to increase  cannabinoid concentration in high cannabinoid varieties.
increased uv exposure on cannabis plants
Figure 5: Increasing UV exposure reduced yield by 12% at highest dose compared to no UV exposure.

Practical Implications for Growers

Given the current body of evidence, growers should weigh the following considerations when deciding  whether to incorporate UV into their cultivation practices: 

1. Energy Costs and Efficiency: 

  • UV-B and UV-A LEDs have low efficacy, often below 1.0 µmol/J1, compared to 2.5–3.0  µmol/J for modern full-spectrum LEDs[11]. 
  • Fluorescent UV lights are even less efficient and require frequent replacements, further  driving up operational costs.
    • With no consistent cannabinoid gains, the added energy costs does not justify adding UV.

2. Potential for Plant Damage: 

  • Elevated UV can cause morphological changes that can reduce photon capture, photosynthesis, and yield. 

3. Health and Safety Risks:

  • UV radiation poses risks to human health, including skin damage and eye injuries. Proper  safety equipment, such as UV-blocking eyewear and shielding, adds complexity and cost.

4. Focus on Proven Strategies:

  • Optimizing photosynthetically active radiation (PAR), temperature, CO₂ enrichment, and  genetics remains the most reliable strategy for improving yield and cannabinoid production.

Conclusion

Despite the theoretical appeal of using UV radiation to increase cannabinoid production, current research  does not support its effectiveness as a reliable cultivation strategy. While UV-B can induce stress responses  in plants, its impact on cannabinoids is inconsistent and often offset by reductions in yield. Growers are better  served by focusing on proven techniques such as optimizing light intensity, photoperiod, and genetics to  maximize both yield and cannabinoid concentration.

References

  1. Zhen, S., Kusuma, P. & Bugbee, B. Photons at the ultraviolet-visible interface: Effects on leaf expansion and  photoinhibition. Scientia Horticulturae 326, 112785 (2024). 
  2. Jenkins, G. I., Christie, J. M., Fuglevand, G., Long, J. C. & Jackson, J. A. Plant responses to UV and blue light: biochemical and genetic approaches. Plant Science 112, 117–138 (1995). 
  3. Janisiewicz, W. J. et al. Use of low-dose UV-C irradiation to control powdery mildew caused by Podosphaera aphanis on strawberry plants. Canadian Journal of Plant Pathology 38, 430–439 (2016). 
  4. Andre, C. M., Hausman, J.-F. & Guerriero, G. Cannabis sativa: The Plant of the Thousand and One  Molecules. Front. Plant Sci. 7, (2016). 
  5. Pate, D. W. Possible role of ultraviolet radiation in evolution ofCannabis chemotypes. Economic Botany 37, 396 (1983). 
  6. Mackerness, S. A.-H.-. Plant responses to ultraviolet-B (UV-B: 280–320 nm) stress: What are the key  regulators? Plant Growth Regulation 32, 27–39 (2000). 
  7. Lydon, J., Teramura, A. H. & Coffman, C. B. UV-B radiation effects on photosynthesis, growth, and  cannabinoid production of two Cannabis saitva chemotypes. Photochemistry and Photobiology 46, 201–206  (1987). 
  8. Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. Cannabis Inflorescence Yield and Cannabinoid  Concentration Are Not Increased With Exposure to Short-Wavelength Ultraviolet-B Radiation. Frontiers in  Plant Science 12, (2021). 
  9. Llewellyn, D. et al. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet  radiation did not affect yield or cannabinoid content. Front Plant Sci 13, 974018 (2022). 
  10. Westmoreland, F. M., Kusuma, P. & Bugbee, B. Elevated UV photon fluxes minimally affected cannabinoid  concentration in a high-CBD cultivar. Frontiers in Plant Science 14, (2023).
  11. Kusuma, P., Pattison, P. M. & Bugbee, B. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7, 1–9 (2020).