# UV-B

> UV-B is ultraviolet light from 280 to 315 nm; added to grow rooms in hopes of more resin, it gives little or no visible difference in controlled trials.

- Canonical: https://genetixbay.com/knowledge/glossary/uv-b.html
- Also written: UVB, UV-B light, ultraviolet B, UVB supplementation, UV-B lamps
- Glossary category: Environment and light · Evidence: Supported
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

UV-B is ultraviolet light from 280 to 315 nm. Plants sense it through the UVR8 photoreceptor and respond with protective pigments, thicker leaves and, at high doses, tissue damage. Sunlight at ground level contains a small amount; most indoor fixtures contain none unless UV-B lamps or diodes are added.

## In practice

The popular theory is that cannabis makes resin to shield itself from UV-B, so adding UV-B should raise [THC](https://genetixbay.com/knowledge/glossary/thc.md). Controlled trials do not bear this out. Rodriguez-Morrison, Llewellyn and Zheng (2021) found UV-B did not increase inflorescence yield or [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) concentration and reduced yield at higher doses. Llewellyn and colleagues (2022) and Westmoreland and colleagues (2023) reached similar conclusions for UV generally.

UV-B is also a real hazard: it burns skin and damages eyes (photokeratitis) quickly. Never look at UV-B sources and switch them off before working in the room.

## Don't confuse with

- **UV-A**: longer wavelength, gentler, acts more like deep blue.
- **UV-C**: germicidal; used for surface or air sterilisation, never on plants.

## Myth check

GenetixBay house position: UV-B produces **little or no visible difference** in the finished crop. That matches the controlled trials. *Supported*. Members who want to test their own cultivars can run a side-by-side trial; see [Mythbusters](https://genetixbay.com/knowledge/mythbusters.md).

## Related terms

[UV-A](https://genetixbay.com/knowledge/glossary/uv-a.md), [Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md), [Light stress](https://genetixbay.com/knowledge/glossary/light-stress.md), [Trichome](https://genetixbay.com/knowledge/glossary/trichome.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)
- [Heat and light stress: taco leaves, bleaching and foxtails](https://genetixbay.com/knowledge/environment/heat-light-stress.md)

## Sources

1. Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis inflorescence yield and cannabinoid concentration are not increased with exposure to short-wavelength ultraviolet-B radiation. Frontiers in Plant Science 12:725078. https://doi.org/10.3389/fpls.2021.725078
2. Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y, 2022. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science 13:974018. https://doi.org/10.3389/fpls.2022.974018
3. Westmoreland FM, Kusuma P, Bugbee B, 2023. Elevated UV photon fluxes minimally affected cannabinoid concentration in a high-CBD cultivar. Frontiers in Plant Science 14:1220585. https://doi.org/10.3389/fpls.2023.1220585