Effects of UV and IR Light on Cannabis

UV Light (UV-A and UV-B)

UV-A (315–400 nm) promotes the formation of secondary metabolites such as terpenes and flavonoids. However, studies show that UV-A alone does not significantly increase THC content.
UV-B (280–315 nm) can trigger stress responses that lead to increased cannabinoid production. Research findings are inconsistent—some report positive effects, while others indicate growth suppression and reduced yields.

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Infrared Light (Far-Red, 730 nm)

Far-Red light (700–750 nm) affects photomorphogenesis via the phytochrome system. It can stimulate flowering by signaling the onset of night, thus inducing or accelerating bloom. Applying Far-Red after the main light cycle may help shorten the flowering period.

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Optimal Wavelengths and Application Phases

Spectrum	Wavelength (nm)	Recommended Phase	Effect
UV-B	280–315	Late flowering	Stimulates cannabinoid production
UV-A	315–400	Vegetative & flowering	Promotes secondary metabolite production
Far-Red	700–750	Transition to flowering	Supports bloom initiation

Duration and Light Cycles

UV-B: Max. 1–2 hours daily during late flowering. Excess causes stress, leaf burns, and yield loss.
UV-A: Up to 6 hours daily during veg and bloom.
Far-Red: 10–15 minutes right after the main light phase, optionally 15 minutes before lights on.

Sources:

https://migrolight.com/collections/uv-grow-lights

Recommended LED Chips

UV LEDs

Far-Red LEDs

Combined LED Modules

Ineffective or Problematic Chips

IR LEDs below 720 nm: ineffective for the phytochrome system.
UV LEDs above 400 nm: minimal impact on secondary metabolite or cannabinoid production.

Risks of Continuous UV/IR Exposure

UV Light: Growth disorders, leaf damage, reduced yields.
Far-Red Light: Excessive stretching, reduced flower mass, hormonal imbalance.

Hermaphroditism Triggered by Light Stress

Incorrect light scheduling can lead to hermaphroditism, especially in feminized genetics:

UV-A/B during the dark cycle
Continuous Far-Red without true night interruption
Missing dark phases entirely

Recommendation: Use timers for precise light control.

All Sources at a Glance: