Does Far-Red Light Help Indoor Crops? The R:FR Ratio Explained
Far-red light sits just beyond the waveband that most light meters and most definitions of PAR cover, which makes it easy to dismiss as irrelevant. It is not. Photons between roughly 700 and 750 nm are read by phytochrome, the pigment system that tells a plant whether it is in open ground or under a canopy, and the balance between red and far-red — the R:FR ratio — decides whether a plant grows compact or stretches. This guide separates what the evidence supports from what the marketing implies.
- What far-red light is, and why PAR stops at 700 nm
- Phytochrome: a reversible switch between two forms
- The R:FR ratio and what a low ratio signals
- Where far-red actually helps
- Light environments compared by R:FR
- How to add far-red without fooling your measurements
- Risks, and when far-red is worth the watts
- Frequently asked questions
- About the publisher
- References
What far-red light is, and why PAR stops at 700 nm
Photosynthetically active radiation is conventionally defined as 400 to 700 nm. That boundary is a convention inherited from early measurements with filters and thermopiles, not a hard physical edge: chlorophyll still absorbs weakly beyond 700 nm, and the phytochrome system has a strong response there. Radiation from about 700 to 750 nm is what the horticultural literature usually calls far-red.
Two consequences follow. First, a standard PPFD meter stops counting at 700 nm, so adding far-red barely moves the number on the display. Second, the energy in those photons is real: it is emitted, it is paid for, and it can affect both morphology and, indirectly, photosynthesis.
Phytochrome: a reversible switch between two forms
Phytochrome exists in two interconvertible forms. The red-absorbing form, Pr, absorbs most strongly around 660 nm; when it captures a red photon it converts to the far-red-absorbing form, Pfr. Pfr absorbs most strongly around 730 nm, and capturing a far-red photon converts it back to Pr. The two forms therefore act as a reversible switch whose position depends on the spectrum falling on the plant, not on the total photon dose.
Because the conversion runs in both directions, a plant continuously samples the ratio between red and far-red rather than measuring either band alone. That ratio is what the literature calls R:FR, usually computed from narrow bands around 655 to 665 nm and 725 to 735 nm. Daylight is rich in red relative to far-red; light that has passed through leaves is not, because chlorophyll absorbs red strongly and transmits far-red.
The R:FR ratio and what a low ratio signals
A leaf canopy filters red light out of the spectrum while letting far-red through, so the light reaching a plant surrounded by neighbours has a lower R:FR than open sky. Plants read that change as a reliable indicator of competition, and respond with the shade avoidance syndrome: stem elongation, upward leaf movement, reduced branching, and in some species earlier flowering at the expense of vegetative bulk.
A review of the shade avoidance syndrome in Arabidopsis describes exactly this chain: plants detect neighbouring vegetation as a reduction in the red to far-red ratio of the incoming light, and the low R:FR signal then triggers the suite of elongation responses. The important point for indoor growing is that the signal is spectral, not energetic — a dim but red-poor light can produce more elongation than a bright, red-rich one.
The same signal is not purely morphological. A study indexed in PubMed Central reports that a low R:FR ratio acts as a signal that promotes carbon assimilation, which is a reminder that the phytochrome response and the photosynthetic response overlap rather than sitting in separate compartments.
Where far-red actually helps
- Reaching deeper leaves. Far-red passes through upper leaves that absorb red, so a canopy with some far-red in the incident spectrum delivers more usable light to the middle and lower layers. This is the argument for far-red in dense canopies and in interlighting.
- Photosynthetic interaction. Far-red photons are used poorly on their own, but when they arrive together with shorter-wavelength light they can raise the quantum yield of photosynthesis above what the shorter-wavelength light alone would give. This interaction, rather than far-red as a standalone energy source, is the defensible reason to include it.
- Deliberate morphological control. Where extension is wanted — plug establishment, cutting rooting, some leafy crops grown for petiole length — a controlled amount of far-red is a tool. Where compactness is wanted, the same photons are a liability.
- Photoperiod signalling in some crops. Because phytochrome participates in daylength perception, the red and far-red content of end-of-day light can shift flowering time in photoperiod-sensitive species. The direction of the effect is species-specific.
Light environments compared by R:FR
| Light environment | R:FR (approx.) | Phytochrome balance | Typical plant response | Relevance indoors |
|---|---|---|---|---|
| Open sky, full sun | about 1.1–1.2 | Pfr favoured | Compact growth, branching, strong stem | Reference point for a "no shade" signal |
| White LED at close range | high, red-rich | Pfr favoured | Compact, sometimes excessively stocky | Common in propagation rooms |
| Red-blue LED, no far-red | very high | Strongly Pfr favoured | Short internodes, thick leaves, little extension | Classic plant-factory spectrum |
| Sunlight under one leaf layer | about 0.7–1.0 | Shift towards Pr | Mild elongation, leaf angle change | Why greenhouse crops stretch in dense rows |
| Deep canopy shade | below 0.5 | Pr favoured | Strong shade avoidance: rapid extension | What added far-red imitates |
| Fixture with added 730 nm | lowered by design | Tunable with channel ratio | Depends on dose and crop | The variable being controlled |
How to add far-red without fooling your measurements
The practical difficulty is bookkeeping. If a fixture delivers 300 µmol/m²/s in the 400–700 nm band and 8 percent of its total photon output in 700–750 nm, the far-red addition is about 24 µmol/m²/s. A standard PPFD meter will show 300, not 324, so the extra photons never appear in a DLI calculation even though the crop may respond to them.
Three habits keep the numbers honest. Ask for the spectrum as a table or file rather than a picture, so the far-red share is stated in numbers. Keep far-red on a separate channel or a separate fixture, so you can run a comparison rather than a permanent change. And when you report results, state which band you are describing — 400–700 or the extended 400–750 — because both are used in the literature and they are not interchangeable.
Risks, and when far-red is worth the watts
The main risk is unwanted extension. In propagation, a low R:FR can produce seedlings that are taller but weaker, with thinner stems and poorer transplant survival, and the effect appears at doses far below what a "supplemental" label suggests. The second risk is measurement drift: because far-red is invisible to a standard meter, a spectrum change can look like no change at all, and experiments that do not control for it produce contradictory results between rooms.
The third is cost. Far-red photons are photons, and if they are not contributing to assimilation in your crop they are reducing the efficacy of the fixture in micromol per joule. That is why far-red belongs on a dimmable channel rather than blended permanently into a broad spectrum: it lets a grower test the response, then keep the setting that paid for itself.
In practice, far-red is worth including where the canopy is dense enough that light penetration matters, where a specific crop responds to extension control, or where a research protocol requires a defined R:FR. It is not worth including as a default additive to a general-purpose fixture for compact leafy production.
Frequently asked questions
Is far-red light counted in PPFD?
Most horticultural meters report PPFD over 400 to 700 nm, so photons between 700 and 750 nm are not counted. That matters because those photons still reach phytochrome and can contribute to photosynthesis when shorter-wavelength light is present at the same time. If a fixture adds far-red, its PPFD reading may stay almost unchanged while the crop responds, and some researchers therefore describe an extended PAR band of 400 to 750 nm.
What red to far-red ratio should an indoor grow light have?
There is no single correct ratio, because the ratio is a signal rather than a nutrient. Daylight sits close to 1, while light filtered through a dense leaf canopy falls well below 1, and that reduction is what triggers shade avoidance. A fixture intended to keep plants compact should therefore not deliver a low R:FR during propagation, while a fixture used to reach deeper leaves or to promote extension in a specific crop may deliberately include far-red.
Does far-red light make seedlings stretch?
Yes, that is its best-documented morphological effect. A low red to far-red ratio shifts the phytochrome balance towards the far-red-absorbing form and triggers the shade avoidance syndrome, which includes stem elongation, upward leaf movement and reduced branching. In propagation, where compact growth is the goal, added far-red usually works against you unless it is balanced by a higher blue share or a higher overall PPFD.
Can far-red light increase yield by itself?
Not by itself. Far-red photons are used poorly when they arrive alone; their value comes from interaction with shorter-wavelength light, where they can raise the quantum yield of photosynthesis and reach leaves deeper in the canopy. Reported responses are strongly crop-specific, so far-red should be treated as a control variable to be tested on your own crop rather than as an additive that raises yield per photon.
About the publisher
Zhongshan City Ruixian Electronics Factory (XineLam) designs and manufactures LED lighting and LED grow light modules, with 17 years of experience in LED lighting and 300+ patents in China and internationally. Its grow-light range includes single-channel and multi-channel fixtures in which white, red and far-red outputs can be dimmed independently, so that a spectrum change can be tested on the crop instead of argued about on paper. Product information is on the product index; technical enquiries go through the contact page.
References
- The Shade Avoidance Syndrome in Arabidopsis. PubMed Central review. https://pmc.ncbi.nlm.nih.gov/articles/PMC4204825/ — used for the definition of the red to far-red ratio as a neighbour-detection signal and for the elongation responses that follow a low ratio.
- Low red/far-red ratio as a signal promotes carbon assimilation. PubMed Central. https://pmc.ncbi.nlm.nih.gov/articles/PMC7140557/ — used for the point that a low R:FR signal affects carbon assimilation as well as morphology.