Cannabis produces trichomes partly as a UV shield. In nature, plants at higher elevations and lower latitudes — where UV radiation is stronger — tend to produce more resin. This observation has driven growers to add supplemental UVB light during flowering, aiming to trigger the plant's defensive resin response and increase trichome density.
The science supports the concept. The execution is where most growers get it wrong.
The UV spectrum
Ultraviolet light falls below the visible spectrum:
| Type | Wavelength | Source | Effect on cannabis |
|---|---|---|---|
| UVA | 320–400 nm | Sunlight, some LEDs, T5 bulbs | Mild stress, minor trichome stimulus |
| UVB | 280–320 nm | Sunlight, reptile bulbs, specialty fixtures | Significant trichome production trigger |
| UVC | 100–280 nm | Germicidal lamps | Damages/kills plant tissue — never use |
UVB is the target. UVA has some effect but is less potent as a trichome trigger. UVC is destructive — it's used to sterilize surfaces, not grow plants.
How UVB triggers trichome production
The defense mechanism
Trichomes contain UV-absorbing compounds — primarily flavonoids and cannabinoids (THC itself absorbs UV). When the plant senses UVB radiation:
- Photoreceptors in leaf tissue detect UV wavelengths
- The plant upregulates genes for secondary metabolite production (flavonoids, terpenoids, cannabinoids)
- Trichome density increases, especially on sun-facing surfaces
- Cannabinoid concentration within trichomes may increase
The plant is essentially building a layer of biological sunscreen. More UV stress = more sunscreen production = more trichomes and resin.
What the research shows
- Lydon et al. (1987): cannabis grown under supplemental UVB produced significantly more THC than plants grown without UV exposure. One of the earliest controlled studies confirming the UV-THC connection
- Magagnini et al. (2018): UV exposure during flowering increased THC concentration without significantly affecting CBD. The ratio shifted toward THC
- Altitude studies: cannabis cultivated at high elevations (stronger natural UVB) consistently shows higher cannabinoid content than genetically identical plants grown at lower elevations
- Practical grower reports: experienced growers adding UVB during the last 3–4 weeks of flowering report visually denser trichome coverage and increased resin stickiness. Quantitative measurements vary, but 10–30% increases in trichome density are commonly reported