# Far-red

> Far-red is light from about 700 to 750 nm, just beyond visible red; it adds to canopy photosynthesis when mixed with other light and triggers shade responses like stretch.

- Canonical: https://genetixbay.com/knowledge/glossary/far-red.html
- Also written: far red, FR, far-red light, 730 nm, 730nm
- Glossary category: Environment and light · Evidence: Supported
- 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"

Far-red is the waveband from roughly 700 to 750 nm (some definitions run to 800 nm). It sits outside classical [PAR](https://genetixbay.com/knowledge/glossary/par.md), and the eye barely sees it. On its own it drives little [photosynthesis](https://genetixbay.com/knowledge/glossary/photosynthesis.md), but mixed with shorter wavelengths it boosts photosynthesis through the [Emerson enhancement effect](https://genetixbay.com/knowledge/glossary/emerson-effect.md). Zhen and Bugbee (2020) showed far-red photons up to 750 nm were about as effective as PAR photons for whole-canopy photosynthesis in several crops. *Established* outside cannabis.

## In practice

Far-red is also a signal. A low [red:far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md) reads as shade and promotes stem elongation and leaf expansion. Some growers use a short burst of far-red at lights-off (end-of-day far-red) to speed the switch to darkness signalling; cannabis-specific evidence for meaningful gains is limited. Peterswald and colleagues (2025) reviewed and tested far-red in [medicinal cannabis](https://genetixbay.com/knowledge/glossary/medical-vs-adult-use.md) with mixed outcomes on yield and quality.

Add far-red if you want faster canopy closure and can tolerate [stretch](https://genetixbay.com/knowledge/glossary/stretch.md). Skip it if you are short on height. Fixtures with far-red are often rated in ePAR (400–750 nm), which inflates the headline number compared with PAR-only ratings.

## Don't confuse with

- **Infrared (heat)**: thermal infrared is far longer wavelength (above about 3,000 nm); 730 nm far-red [LEDs](https://genetixbay.com/knowledge/glossary/led.md) do not heat plants meaningfully.
- **Red (660 nm)**: the most efficient photosynthetic band; far-red balances it.

## Related terms

[Red:far-red ratio](https://genetixbay.com/knowledge/glossary/red-far-red-ratio.md), [Emerson effect](https://genetixbay.com/knowledge/glossary/emerson-effect.md), [PAR](https://genetixbay.com/knowledge/glossary/par.md), [Spectrum](https://genetixbay.com/knowledge/glossary/spectrum.md)

## Read more

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

## Sources

1. Zhen S, Bugbee B, 2020. Far-red photons have equivalent efficiency to traditional photosynthetic photons: implications for redefining photosynthetically active radiation. Plant, Cell & Environment 43(5):1259–1272. https://doi.org/10.1111/pce.13730
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/
3. 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