How Important Is UV Light for Cannabis?

By Stiina Kotiranta
Published: April 18, 2022 | Last updated: May 12, 2022 11:25:12
Presented by Valoya
Key Takeaways

Different spectrum colors serve different purposes in the growth and development of the cannabis plant. Light, which is one of the most crucial factors in cannabis cultivation, comes in different colors. How does one know what effects UV light will have on cannabis? Well – we know quite a bit, but a lot is still unknown. Here’s what we know so far.

UVA light has been shown to increase secondary metabolite activity in many plants, and this is also the case with cannabis. The most important secondary metabolites from a cannabis grower’s perspective are cannabinoids such as THC and CBD, as well as terpenes which give cannabis its distinctive aroma. Short wavelength irradiation, such as UVA and blue light, trigger the plant’s stress response system and the plant starts to protect itself from the abiotic stress i.e. short wavelength irradiation. Increased stress level results in increased metabolite activity and therefore higher THC accumulation in flowers, when compared to light sources lacking UVA or blue light (Magagnini et al. 2018).


As plants cannot move, they read signals about their surroundings from temperature, light spectrum, soil moisture content, etc. Short wavelength irradiation, such as blue and UVA light give the plant a signal that it is under a clear sky without competition from the neighboring plants. A no-competition environment indicates that the plant it is in no hurry to re-produce (make seeds) or stretch towards light. Plants which are grown under rich blue and UVA spectrum often have short internodes, small leaf area, and thick leaves. These responses can be reversed by green or far-red light, which induce shade-avoidance-syndrome symptoms such as stretching of the stem, increased leaf area, and enhanced flowering. Therefore, by adjusting the amount of blue and UVA in the light spectrum into a perfect balance in relation to other wavelengths, we can manipulate the size and biomass accumulation.

young cannabis plant under lightUVA light has been shown to increase secondary metabolite activity in many plants, and this is also the case with cannabis.


Perhaps it then comes as no surprise that in the nature, the most potent cannabis plants are typically found on high altitudes of mountain regions. In such areas plants have unobstructed access to an abundance of clear sunlight whose spectrum is higher in UV wavelengths than at lower altitudes.

As pointed out earlier, UVA increases the metabolite activity, such as elevated THC or terpene content in flowers, however the grower can benefit from the increased secondary metabolite activity in other ways as well. A plant which is mildly stressed by the short wavelength irradiation, constantly produces secondary metabolites, such as antioxidants and phenolic compounds, so as to protect itself down to cell level from the abiotic stress. These secondary metabolites protect the plant not only from light irradiation, but also from pathogens and pests. The result is a compact plant with increased THC concentration, and it is furthermore stronger against fungal pathogens, such as Botrytis (Kim et al. 2013) and pests. UV light has also been shown to directly decrease fungal pathogen growth by inhibiting sporulation. However, more studies are needed to better understand the exact effects of UVA and UVB on the most common pests and fungi.

Webinar: Latest Developments in Research on LED for Cannabis


Data from several plant trials will be condensed and shared to help growers better understand how light affects cannabis and drives its growth.

close up of cannabis trichomesUVA increases the metabolite activity, such as elevated THC or terpene content in flowers.


There are not so many lighting companies providing grow lights with UVA incorporated into the fixtures. In some horticultural light spectra, the type of UV used is UVA (radiation between 315-400 nm). Often so, the range doesn’t go lower than 400 nm. A little bit of UVA creates gentle stress that drives the plant to develop better defense mechanisms – cannabinoids. However, a little too much and we will stunt the plant’s growth and it will underperform. Valoya has tested UVA LED chips of different peak wavelengths to find the sweet spot where we get maximum cannabinoid output, without harming the plant’s natural processes.

According to Valoya’s studies, the floral and leaf cannabinoid concentrations in cannabis can be optimally increased by using the 385 UVA wavelength. Note that, even though this is actual UVA light, that it is very mild and would not have adverse effects on humans exposed to it. However, adding a separate UVA or UVB lamp to cultivation is potentially dangerous as UV light is strong and can damage the plants, workers exposed to it and even the plastic benches that hold the plants. A small excess in UV can inhibit plant’s development and eventually destroy it.

It is thus important to get UV enriched spectra from companies that have a proven track record in photobiological research and who are willing to share their research data. In a market with over 100 LED providers, data should speak louder than any other kind of claim.

Valoya is a research driven horticultural LED lighting manufacturer. Our patented, wide spectra optimise the growth of a variety of plants in applications such as crop science, medicinal plants cultivation and vertical farming. With 600+ plant trials on 300+ plant species/varieties, we are the knowledge leader of the industry. To learn more, contact [email protected] or visit


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Article Sources

Maximum Yield uses high-quality sources to support the facts within our content including peer-reviewed studies, academic research institutions, professional organizations, and governmental organizations.

  • Magagnini G., Grassi G., Kotiranta S.. The Effect of Light Spectrum on the Morphology and Cannabinoid Content of Cannabis sativa L. Medical Cannabis and Cannabinoids, (2018).
  • Kim K., Kook H., Jang J., Lee W., Kamala-Kannan S., et al.. The Effect of Blue-light-emitting Diodes on Antioxidant Properties and Resistance to Botrytis cinerea in Tomato. Journal of Plant Pathology & Microbiology, (2013).

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