UV Light for Cannabis: Does UV-B Raise THC?
Adding UV-B light to cannabis did not raise yield or THC in a University of Guelph study of two indoor cultivars. Over nine weeks of flowering, higher UV doses caused smaller, malformed leaves, lower photosynthesis and browned pistils, and terpene content fell. The authors found no commercially relevant benefit from using UV as a production tool.
Does UV Light Increase THC in Cannabis?
Not in this study. With rising UV exposure, total THC and CBD concentrations fell in one cultivar, and total terpene content fell in both. The authors concluded that using UV as a production tool gave no commercially relevant benefit to yield or flower chemistry (Rodriguez-Morrison et al.). For how other wavelengths are thought to work, see light wavelengths.
What Did the UV-B Study Test?
Plants grew for two weeks in vegetative growth, then nine weeks under a 12-hour flowering photoperiod at about 400 µmol/m²/s from the main lights. UV LEDs with a 287 nm peak added UV for 3.5 hours a day at 0.01 to 0.8 µmol/m²/s, a biologically effective dose of 0.16 to 13 kJ/m²/d (Rodriguez-Morrison et al.). The main lights were LED grow lights, with UV as a supplement.
UV-B study design and results (two indoor cultivars)
Source: Rodriguez-Morrison, Llewellyn and Zheng 2021, Frontiers in Plant Science 12:725078. Cultivars are labeled A and B.
| Item | Detail | Source |
|---|---|---|
| Main light | About 400 µmol/m²/s, 12-hour photoperiod, 9 weeks | Rodriguez-Morrison et al. 2021 |
| Added UV | 287 nm peak, 3.5 h a day, 0.01 to 0.8 µmol/m²/s | Rodriguez-Morrison et al. 2021 |
| UV dose | 0.16 to 13 kJ/m²/d (biologically effective) | Rodriguez-Morrison et al. 2021 |
| Yield | Fell in cultivar A only | Rodriguez-Morrison et al. 2021 |
| THC and CBD | Fell in cultivar A only | Rodriguez-Morrison et al. 2021 |
| Total terpenes | Fell in both cultivars | Rodriguez-Morrison et al. 2021 |
What Damage Can UV Cause to Cannabis?
As UV increased, the study saw smaller plants and leaves, leaf malformations, browning of the stigmas and lower leaf photosynthesis. Total dry flower yield fell in one cultivar, and the share of yield from top flowers fell in both (Rodriguez-Morrison et al.). These were indoor results under LED lights, not field findings.
Sources
Only what the paper above reports appears here, set out by Kind Seed Co as general growing information.
FAQ
Do cannabis plants need UV light?
The Guelph study grew both cultivars under its main LED lights, and adding UV-B did not improve yield or flower chemistry. The authors concluded UV as a production tool gave no commercially relevant benefit. The study did not test whether other UV types or schedules would help.
Could a different UV setup work better?
The authors suggest future research could try less energetic UV, or shorter exposure close to harvest, to see whether benefits appear without yield losses. Those approaches were not tested in this study, so this work has no result to report for them yet.
What wavelength was used in the study?
The study used LEDs with a peak wavelength of 287 nm, in the UV-B range. UV was added for 3.5 hours a day at photon flux densities from 0.01 to 0.8 µmol/m²/s, equal to biologically effective doses of 0.16 to 13 kJ/m²/d.
Did UV change yield in both cultivars?
No. Total dry flower yield fell with more UV in only one of the two cultivars. In both, the share of yield from the top flowers fell, total terpene content dropped, and leaves and plants got smaller as UV exposure rose.
