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Glossary · Environment and light

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.

Also written UVBUV-B lightultraviolet BUVB supplementationUV-B lamps

SupportedDraft: awaiting expert reviewUpdated

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. Controlled trials do not bear this out. Rodriguez-Morrison, Llewellyn and Zheng (2021) found UV-B did not increase inflorescence yield or cannabinoid 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.

Related terms

Read more in the Knowledge Base

EnvironmentContestedIntermediate · Draft

Light spectrum: blue, red, far-red and UV, what's real

Photon quantity (PPFD/DLI) and fixture efficacy matter far more than spectrum. Less blue slightly increased cannabis yield (about 12 % from 4 % to 20 % blue). Far-red stretches plants; its yield effect is contested and seems to trade off against cannabinoid concentration. UV has repeatedly failed to raise cannabinoids in controlled trials and can cut yield at high doses.

EnvironmentSupportedBeginner · Draft

Heat and light stress: taco leaves, bleaching and foxtails

Heat and light stress show at the top of the plant, closest to the light: leaves curl up or taco, bracts bleach white or yellow, buds may foxtail. Measure leaf temperature and canopy PPFD before blaming nutrients. Typical fixes: dim or raise the light 10–20 %, bring leaf temperature under about 30 °C (86 °F), restore airflow and root-zone water uptake.

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. 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. 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. doi.org/10.3389/fpls.2023.1220585
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