# Red:far-red ratio

> The red:far-red ratio compares photons near 660 nm with those near 730 nm; plants read a low ratio as shade and respond by stretching.

- Canonical: https://genetixbay.com/knowledge/glossary/red-far-red-ratio.html
- Also written: R:FR, R/FR, red to far-red ratio, R:FR ratio, red/far-red
- 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"

The red:far-red ratio (R:FR) is the photon flux in a red band (usually 655–665 nm) divided by that in a [far-red](https://genetixbay.com/knowledge/glossary/far-red.md) band (usually 725–735 nm). Plants sense it through [phytochrome](https://genetixbay.com/knowledge/glossary/phytochrome.md), which flips between a red-absorbing and a far-red-absorbing form. Daylight has an R:FR of roughly 1.1–1.2; under a leafy canopy it drops well below 1, because leaves absorb red and pass far-red. A low ratio tells the plant it is shaded.

## In practice

Lower R:FR triggers shade-avoidance responses: longer [internodes](https://genetixbay.com/knowledge/glossary/internode.md), taller plants, thinner leaves, more [apical dominance](https://genetixbay.com/knowledge/glossary/apical-dominance.md). Kotiranta and colleagues (2024) found low R:FR increased cannabis height and reduced inflorescence yield in their setup, while a later trial by the same group (2025) reported higher yield but lower specialised-metabolite concentrations with a decreasing ratio. The direction depends on intensity, cultivar and how the far-red is added. *Contested* for yield; *Established* for [stretch](https://genetixbay.com/knowledge/glossary/stretch.md).

Most white [LEDs](https://genetixbay.com/knowledge/glossary/led.md) without a far-red channel have a high R:FR, giving compact plants. [HPS](https://genetixbay.com/knowledge/glossary/hps.md) has a moderate ratio. Growers who add far-red usually want faster canopy fill or a stronger end-of-day signal; those fighting stretch avoid it.

## Units and formula

R:FR = photon flux (655–665 nm) ÷ photon flux (725–735 nm). Band definitions vary between papers; check which one a datasheet uses.

## Don't confuse with

- **Far-red**: the waveband itself; the ratio is about balance.
- **Red light**: drives [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md) strongly; it is only one half of the ratio.

## Related terms

[Far-red](https://genetixbay.com/knowledge/glossary/far-red.md), [Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md), [Emerson effect](https://genetixbay.com/knowledge/glossary/emerson-effect.md), [Stretch](https://genetixbay.com/knowledge/glossary/stretch.md)

## Read more

- [Light spectrum: blue, red, far-red and UV, what's real](https://genetixbay.com/knowledge/environment/light-spectrum.md)
- [Stretchy, leggy cannabis: a differential](https://genetixbay.com/knowledge/diagnosis/stretching.md)

## Sources

1. Kotiranta S et al., 2024. (Low R:FR increased height and reduced inflorescence yield.) Industrial Crops and Products. https://doi.org/10.1016/j.indcrop.2023.117772
2. Peterswald TJ et al., 2025. The effects of far-red light on medicinal Cannabis. Scientific Reports. https://pmc.ncbi.nlm.nih.gov/articles/PMC12092679/