Growers who want to increase the potency of their cannabis may consider adding ultraviolet (UV) light to their grow setup. UV light is radiation with wavelengths between 100 – 400 nm and it’s not visible to the human eye. Outdoors, about 10% of the sun’s light is ultraviolet. Furthermore, UV light is broken down into several subtypes, including UV-A (315 – 400 nm), UV-B (280 – 315 nm), and UV-C (100 – 280 nm). Most lighting technology provides little UV light. So, to match the intensity of the sun, you would need a light that produces at least 32 W of UV radiation  A quick Google search shows that it’s not easy (or cheap) to find a light that can deliver this much light! And so this may make growers wonder: Are these lights worth the price? And — do cannabis plants even need UV light?
UV Light May Increase Trichome Density and THC Content
When UV light hits a leaf, the plant makes “sunscreen” to protect itself from damage . Plants produce many different types of sunscreen. Some of these sunscreens are physical, like trichomes, and some of them are chemical, like anthocyanins and beta-carotene. Trichomes are hair-like growths found on the skin (epidermis) of plants. They protect the plant by reflecting away the harmful UV rays. In cannabis, these trichomes are the site of THC and CBD storage. For this reason, scientists wondered whether adding UV rays could increase THC content in cannabis plants.
Scientists surveyed cannabis strains that originated from different places around the world. They looked in places that had high UV levels (like near the equator, or on tall mountains) as well as places with low UV levels (away from the equator and in shady spots). They found that strains from the equator and high-altitude regions had more THC .
UV Light May Decrease CBD Content
Unfortunately, UV exposure is also connected with lower CBD levels. This is likely because there is a tradeoff between THC production and CBD production. To increase THC in the flowers of a cannabis plant, at least 1.34 Watt-seconds/m2 of UV-B light is necessary. It should be given for 6 hours per day, for at least 40 days. Research shows that doing this increases Δ9-THC content from 2.5% to 3.2% .
Much of the scientific information we know about UV wavelengths and trichome development in cannabis was conducted more than 20 years ago! This is important to keep in mind because the technology for developing UV lights and for measuring cannabinoid content has drastically improved since then.
UV Light May Not be the Most Effective Way to Increase Trichomes
Although UV wavelengths (primarily UV-B) is effective for increasing THC content in cannabis, it may not be the most cost-effective method. A true UV-B LED light that delivers 1.3 W/ m2 can cost up to $2,000! Although cheaper “UV” lights exist on the market, watch out for a few tricks:
- Black lights are sometimes (incorrectly) marketed as ultraviolet lights
- UV-A lights are typically cheaper than UV-B (but less effective), and
- Most UV lights do not deliver a high enough wattage
What makes UV lights so expensive? To start, they are extremely inefficient compared to a standard grow light. They use huge amounts of energy and typically have a shorter lifespan than standard grow lights. Furthermore, they can reduce the lifespan of other equipment in your grow environment. This is because some materials, like plastic, degrade quickly when exposed to UV. Lastly – if you decide to proceed forward with installing UV lights, they must be turned off when people are in the grow room. Like plants, UV wavelengths cause cell damage in humans. Fortunately, there are other ways of increasing trichome number and density.
UV Light may Defend Against Insects
UV wavelengths can also help plants defend against insects. UV wavelengths, and to some extent, other wavelengths of light, can trigger plants to release defence compounds. Some examples of these defence compounds are volatiles (smells that deter insects), hormones (alert defence responses in other parts of the plant), secondary metabolites (change the colour and taste of a plant) and trichomes . These compounds make a plant more distasteful or toxic to insects. As a result, plants with increased levels of these compounds get eaten less !
Combining UV-B light with high amounts of PAR produces even more fighting power than either UV-B or PAR separately. When both high PAR and UV-B irradiation is applied to plants, a greater concentration of defence compounds is produced . This is especially true in older plant leaves. The fact that older leaves experience increased amounts of defensive compounds is particularly interesting. Some insects, like whitefly and thrips, show a preference for older leaves than young ones. Therefore, scientists are speculating that UV-B light might be especially effective for these types of pests.
Final Thoughts on UV Light
Ultraviolet light has the potential benefits of increasing THC content, reducing insect harm, and preventing the spread of fungal spores. However, buying a true UV light is expensive! As a grower, it’s important to weigh the benefits and costs of using UV light to decide if it will significantly improve your grow environment.
- Solar and Sustainable Energy. Institute of Agriculture at the University of Tennessee.
- Ulm, R., & Jenkins, G. I. (2015). Q&A: How do plants sense and respond to UV-B radiation? BMC Biology, 13 (1), 4–9.
- Pate, D. W. (1981). Possible Role of Ultraviolet Radiation in Evolution of Cannabis Chemotypes, 37 (4), 396–405.
- Lydon, J. et al (1987). UV‐B Radiation Effects on Photosynthesis, Growth, and Cannabinoid Production of Two Cannabis sativa Chemotypes. Photochemistry and Photobiology, 46 (2), 201–206.
- Escobar-Bravo et al (2017) Interactive Effects of UV-B light with abiotic factors on plant growth and chemistry, and their consequences for defense against arthropod herbivores. Front. Plant Sci. 8, 1-14
- Rechner, O., Neugart, S., Schreiner, M., Wu, S., & Poehling, H. (2017). Can narrow-bandwidth light from UV-A to green alter secondary plant metabolism and increase Brassica plant defenses against aphids ? 1–20.
- Palmer, J. M. et al (2018). Extreme sensitivity to ultraviolet light in the fungal pathogen causing white-nose syndrome of bats. Nature Communications, 9 (1).