# Blue light

> Blue light is the 400–500 nm band; it keeps plants compact with shorter internodes and thicker leaves, but a high blue fraction slightly lowers cannabis yield.

- Canonical: https://genetixbay.com/knowledge/glossary/blue-light.html
- Also written: blue photons, blue spectrum, 400-500 nm, blue fraction, blue LEDs
- Glossary category: Environment and light · Evidence: Established
- Status: Draft: awaiting expert review · Updated: 2026-10-31
- Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay cannabis glossary"

Blue light covers roughly 400–500 nm. Plants detect it with cryptochromes and phototropins, which control stomatal opening, phototropism (bending towards light) and stem elongation. More blue generally means shorter [internodes](https://genetixbay.com/knowledge/glossary/internode.md), smaller and thicker leaves and more compact plants. Blue photons drive [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) a little less efficiently than red ones per photon, and blue diodes are typically less efficient to produce.

## In practice

Westmoreland, Kusuma and Bugbee (2021) grew cannabis under fixtures with blue fractions from about 4% to 20% of photons. Lower blue gave slightly higher yield and slightly taller plants, while [cannabinoid](https://genetixbay.com/knowledge/glossary/cannabinoid.md) concentration barely changed. They concluded fixture [efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) matters more than the blue fraction. *Established*.

Use more blue (around 15–20% of photons) when height is your problem, for example in a short tent or with stretchy cultivars, and in veg if you want tight plants. Most white [LEDs](https://genetixbay.com/knowledge/glossary/led.md) already give 10–20% blue, which is fine for the whole run.

## Units and formula

Blue fraction (%) = photons 400–500 nm ÷ total [PAR](https://genetixbay.com/knowledge/glossary/par.md) photons × 100. A spectroradiometer or the fixture's spectral datasheet gives this.

## Don't confuse with

- **Colour temperature**: "cool" 5,000–6,500 K white has more blue than "warm" 3,000 K white, but kelvin is not a blue percentage.
- **UV-A**: 315–400 nm, sensed partly by the same receptors.

## Myth check

"Veg needs a blue lamp and flower needs a red lamp" is outdated HPS-era advice. A single broad white spectrum works for both. *Supported*.

## Related terms

[Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md), [UV-A](https://genetixbay.com/knowledge/glossary/uv-a.md), [Red:far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md), [Photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)

## Sources

1. Westmoreland FM, Kusuma P, Bugbee B, 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE 16(3): e0248988. https://doi.org/10.1371/journal.pone.0248988