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 band (usually 725–735 nm). Plants sense it through phytochrome, 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, taller plants, thinner leaves, more apical dominance. 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.
Most white LEDs without a far-red channel have a high R:FR, giving compact plants. HPS 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 strongly; it is only one half of the ratio.
