UV-A is the longest-wavelength ultraviolet band, 315–400 nm. It is mostly sensed by the same blue-light receptors (cryptochromes and phototropins) that respond to 400–500 nm, so plants treat it much like extra-deep blue. It is the safest UV band, but still harmful to eyes and skin with long exposure. Many fixtures marketed with "UV" actually emit 385–405 nm, at or just inside the edge of this band.
In practice
Growers add UV-A hoping for more resin or terpenes. Controlled cannabis trials have not shown consistent gains. Llewellyn and colleagues (2022) found UV treatments did not affect yield or cannabinoid content, and Westmoreland and colleagues (2023) found elevated UV photon fluxes changed cannabinoid concentration only minimally. Small amounts of UV-A are harmless and may help compactness slightly. They are not a substitute for intensity.
Wear UV-rated eye protection when working under any fixture with UV channels, and switch them off during maintenance.
Don't confuse with
- UV-B: shorter (280–315 nm), more biologically active and more damaging.
- UV-C: 100–280 nm, germicidal; never use on living plants or people.
- Violet (400–420 nm): visible light; often sold as "UV".
Myth check
"UV-A boosts THC" is not supported by controlled trials. Myth as a reliable potency claim.
