# UV-B for THC: marketing, not a potency tool. More light is all you need

> Adding UV-B lights to raise THC doesn't work in controlled cannabis trials. Three studies in two labs found no commercially relevant gain in THC or yield from UV-B or UV-A plus UV-B; the strongest doses cut yield and terpenes. The same lab found yield rose about 1.6 times when ordinary light went from 600 to 1,000 µmol/m²/s. UV photons are also far more expensive to make than plant light. GenetixBay's position: UV-B for THC is marketing; a good light, proper temperature and a dialled-in room win every time.

- Canonical: https://genetixbay.com/knowledge/environment/uv-b-for-thc.html
- Category: Environment · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Flower · Systems: Any system
- Updated: 2026-11-02 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**Does [UV-B](https://genetixbay.com/knowledge/glossary/uv-b.md) increase [THC](https://genetixbay.com/knowledge/glossary/thc.md) in cannabis?** No, not in a commercially useful way. In a controlled trial with 287 nm UV-B [LEDs](https://genetixbay.com/knowledge/glossary/led.md), total THC and [CBD](https://genetixbay.com/knowledge/glossary/cbd.md) did not increase in either cultivar and fell in one; a second trial found [UV-A](https://genetixbay.com/knowledge/glossary/uv-a.md) plus UV-B didn't change total THC; a third found a 3–13% rise in a high-CBD cultivar that wasn't statistically significant. GenetixBay's position: UV-B for THC is marketing.

**Does UV light hurt cannabis yield?** At high doses, yes. Under 287 nm UV-B at up to 13 kJ/m²/day, apical inflorescence weight fell 69–78% and total flower weight fell about 32% in one cultivar, with curled, smaller and [necrotic](https://genetixbay.com/knowledge/glossary/necrosis.md) leaves and early [pistil](https://genetixbay.com/knowledge/glossary/pistil.md) browning. In another trial the highest UV dose cut flower yield about 12%. Lower doses had little effect either way.

**Is UV or more light better for cannabis yield?** More light, by a wide margin. In the same lab that tested UV, raising canopy light from 600 to 1,000 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md) increased dry flower yield about 1.6 times with no change in [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) concentration, while adding UV-A or UV-A plus UV-B gave no commercially relevant benefit. A photon of plant light is also much cheaper to make than a UV photon.

**Where did the UV-B THC claim come from?** The UV-B THC idea traces to a 1987 study that reported a rise in flower THC from about 25 to 32 mg/g under UV-B, and to observations that cannabis from high-UV regions tended to be more potent. Modern trials with today's high-THC cultivars, which exceed 200 mg/g, found no commercially relevant benefit, and suggest regional patterns reflect genetics rather than short-term UV exposure.

**Is UV light dangerous for growers?** Yes. UV-B and UV-C damage eyes and skin, and UV-A at high intensity can harm eyes too. Lamps are classified for photobiological risk under IEC 62471. If you use UV, wear rated eye and skin protection, never look at the source, and switch UV off before working in the canopy.

**Short answer, in plain words.** Some grow-light sellers add UV-B bars and promise more THC. In proper side-by-side trials it didn't work: three studies in two labs found **no useful rise in THC or yield**, and the strongest UV doses **burned leaves, cut yield and lowered [terpenes](https://genetixbay.com/knowledge/glossary/terpene.md)** [1][2][3]. Meanwhile the same lab got about **1.6 times the yield** just by turning ordinary light up from 600 to 1,000 µmol/m²/s [2]. GenetixBay's position, in the founder's words: UV-B for THC is "marketing BS". The chips are expensive, and "a good light and a proper temp and dial-in will always outperform some made-up 0.00000001% THC production claim. **More light is all you need.**" (founder's field experience) [8].

> **Greenleaf says:** Want more THC? Turn up the light you already paid for, fix your temperature and [VPD](https://genetixbay.com/knowledge/glossary/vpd.md), and grow good genetics. A sunburn lamp isn't the shortcut.

## What did the controlled trials find?

Three controlled cannabis trials, two labs, three kinds of UV source, one answer: **no commercially relevant benefit.** *Supported.*

| Study | UV tested | Result |
|---|---|---|
| Rodriguez-Morrison, Llewellyn and Zheng 2021 [1] | 287 nm UV-B LEDs, 0.16–13 kJ/m²/day for the last 3.5 h of each day, 9 weeks of flower, [PAR](https://genetixbay.com/knowledge/glossary/par.md) about 408 µmol/m²/s; two cultivars | [Total THC](https://genetixbay.com/knowledge/glossary/total-thc.md) and CBD didn't rise in either cultivar and **fell** in one. Apical bud weight fell 69–78% from lowest to highest dose; total flower weight fell about 32% in one cultivar. Terpenes fell about 41% and 24%. Curled, smaller and necrotic leaves; early pistil browning |
| Llewellyn et al. 2022 [2] | UV-A (385 nm, 50 µmol/m²/s, 12 h) or UV-A plus UV-B ([fluorescent](https://genetixbay.com/knowledge/glossary/t5-fluorescent.md), 3 µmol/m²/s, 5 h, last 20 days); high-THC cultivar | Neither changed biomass or flower cannabinoids. "No commercially relevant benefits of adding UV" |
| Westmoreland, Kusuma and Bugbee 2023 [3] | Fluorescent UV (55% UV-B), four daily doses up to 218% of summer sunlight, from week 3 of flower; high-CBD cultivar | Cannabinoids 3–13% higher but **not statistically significant**; the highest dose cut flower yield about 12% (304 to 268 [g/m²](https://genetixbay.com/knowledge/glossary/yield.md)) |

The authors' own words: UV "as a production tool did not lead to any commercially relevant benefits" [1]; "an elevated flux of UV photons is not an effective approach to increase cannabinoid concentration" [3].

**UV-B in the trials.** 0: Commercially relevant THC gains (Three trials, two labs); −69 to −78%: Apical bud weight at the highest 287 nm dose; −24 to −41%: Terpenes under strong UV-B; −12%: Flower yield at the highest dose in a third trial. Source: [1][3]

## Where did the UV-B THC claim come from?

From two older ideas. *Supported (history of the claim).*

- **A 1987 study** reported flower THC rising from about 25 to 32 mg/g under UV-B [4]. The modern authors call that rise inconsequential next to today's cultivars, which exceed 200 mg/g [1].
- **Geography:** cannabis from high-UV regions was observed to be more potent. The 2021 trial found no support for this as a short-term response to UV; any pattern more likely reflects genetics adapted over generations [1].

## Why more light beats UV

**More light is all you need** (house position; founder's field experience) [8]. *Supported.*

- **Yield follows light.** In the same lab's trial, dry flower yield rose about **1.6 times** from 600 to 1,000 µmol/m²/s, roughly **4.6 g per plant for each extra 100 µmol/m²/s**, with cannabinoid and terpene concentrations unchanged [2]. An earlier trial from the same group found yield rising in a straight line up to about 1,800 µmol/m²/s (see [more light, more yield](https://genetixbay.com/knowledge/environment/more-light-more-yield.md)).
- **UV photons are expensive.** Deep-UV and UV-B LEDs still have low efficiency compared with the blue and white LEDs in grow lights, because of the aluminium-rich materials they need [5]; white LEDs convert roughly three-quarters of their electricity to light [6]. Every watt spent on UV makes far fewer photons, and none of them grow buds.
- **Damage costs grams.** At the doses that might trigger a stress response, the trials saw leaf damage and lost yield [1][3].

**Spend the watts on UV-B, or on more light?**
- **Add UV-B** (The marketing pitch)
  - Con: No commercially relevant THC gain in three trials
  - Con: High doses cut yield and terpenes
  - Con: Expensive, inefficient photons
  - Con: Eye and skin hazard
- **Add more plant light** (600 → 1,000 µmol/m²/s)
  - Pro: About 1.6× the yield in the same lab
  - Pro: Cannabinoid concentration unchanged, so more total THC
  - Pro: Cheapest photons a fixture makes
  - More light is all you need.

*Sol & the Zips: Sol and the Zips: the blue, green and red photons that actually grow buds.*

## Is UV dangerous?

Yes, for people. UV-B and UV-C damage eyes and skin, and strong UV-A can harm eyes; lamps are classified for photobiological risk under IEC 62471 [7]. If you run UV anyway: rated eye and skin protection, never look at the source, and switch it off before working in the canopy. See also the safety note in [light spectrum](https://genetixbay.com/knowledge/environment/light-spectrum.md).

## What should I do instead?

1. Raise canopy PPFD toward your stage target and beyond, with the room to match (see [lighting sizing](https://genetixbay.com/knowledge/equipment/calc-lighting-sizing.md)).
2. Dial in temperature and VPD (see [VPD explained](https://genetixbay.com/knowledge/environment/vpd-explained.md)).
3. Start from good genetics; potency is mostly cultivar.
4. Want to test UV on your cultivar? Split identical clones, same light and climate, UV on half, then weigh and lab-test both. Post it as a GenetixBay grow journal.

## Common mistakes

- Buying a UV bar instead of more PAR light.
- Running UV at doses that burn leaves and pistils, then calling the brown "ripeness".
- Trusting a product's THC claim with no side-by-side control.
- Working in the canopy with UV on and no eye protection.

## 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
4. Lydon J, Teramura AH, Coffman CB. 1987. UV-B radiation effects on photosynthesis, growth and cannabinoid production of two Cannabis sativa chemotypes. Photochemistry and Photobiology 46(2):201-206. https://doi.org/10.1111/j.1751-1097.1987.tb04757.x
5. Kneissl M, Seong TY, Han J, Amano H. 2019. The emergence and prospects of deep-ultraviolet light-emitting diode technologies. Nature Photonics 13:233-244. https://doi.org/10.1038/s41566-019-0359-9
6. Kusuma P, Pattison PM, Bugbee B. 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7
7. International Electrotechnical Commission. 2006. IEC 62471: Photobiological safety of lamps and lamp systems. https://webstore.iec.ch/publication/7076
8. GenetixBay founder. Field experience (first-hand account; GenetixBay house position).