Does Far-Red Light Increase Plant Growth? R:FR Ratio and the Emerson Effect

Technical guide · Zhongshan City Ruixian Electronics Factory (XineLam) · Published 11 September 2026

Far-red is the awkward part of a grow-light spectrum. It sits outside the waveband that PAR and PPFD are defined over, so it does not appear in the headline output figure of most fixtures, yet plants read it, respond to it, and in some trials grow larger because of it. This article separates the two mechanisms behind that response — a signalling effect through phytochrome and a photochemical effect on photosystem I — and sets out the measured dose ranges, the trade-offs, and the points on which the evidence is still mixed.

What far-red is, and why it is outside PAR

The wavebands used in horticultural lighting are defined by wavelength, not by manufacturer convention, and far-red is the band immediately beyond the photosynthetically active range. Kelly and Runkle set this out explicitly in a 2024 PLoS ONE study: photosynthetically active radiation (PAR) is traditionally defined as 400 to 700 nanometres, far-red (FR) is defined as 700 to 799 nanometres, and far-red therefore lies outside PAR even though it can increase photosynthesis and induce shade-avoidance responses.^[1]

Two mechanisms: phytochrome signalling and photosystem I

Far-red changes plants in two ways that are easy to conflate, and the distinction decides what a grower should expect from it.

1. A signalling effect through phytochrome

Phytochrome is a family of photoreceptors that primarily absorbs red and far-red light. Each phytochrome exists in two interconvertible forms, Pr and Pfr, and the balance between them depends on the proportion of red and far-red in the spectrum. That balance — the phytochrome photoequilibrium, defined as the ratio of Pfr to Pr plus Pfr — can be estimated for a given spectrum. Far-red converts phytochrome B to its inactive Pr form, and the Pr form then releases phytochrome-interacting factors that promote the expression of shade-avoidance genes, including the genes behind cell elongation.^[1]

The practical result is a shape change rather than a photosynthesis change. Shade-avoidance responses triggered through phytochrome can raise leaf area and canopy size, which increases light capture and therefore whole-plant photosynthesis indirectly. The size of that response is not fixed: it depends on the other wavebands present, the total photon flux density and plant density.^[1]

2. A photochemical effect: the Emerson enhancement effect

Far-red also acts directly on the light reactions. When it is supplied together with PAR wavebands such as red, it preferentially excites photosystem I and restores the excitation balance between the two photosystems.^[1] This was first described as the Emerson enhancement effect: photosynthetic rates are greater when light of 670–680 nm and light above 680 nm are applied together than when the two wavebands are applied separately.^[1]

More recent work cited in the same study measured where the effect is strongest. Adding red and far-red light at wavelengths from 686 to 703 nm to a red plus blue spectrum, or to a simulated solar spectrum, progressively increased the quantum yield of photosystem II and the photosynthetic rate. Far-red at 721 to 731 nm increased the quantum yield of photosystem II by a similar amount to 703 nm light.^[1]

PPE is the quantity a spectrum designer uses to summarise the red and far-red balance. The same study notes that PPE can be refined to an internal value, iPPE, that accounts for spectral distortions caused by photon scattering and absorption inside the leaf, which change the light the photoreceptors actually see.^[1]

How much far-red? The fraction that maximised lettuce mass

Because both mechanisms are dose dependent, the useful question is not whether to include far-red but how much. The 2024 PLoS ONE study was designed to answer that for lettuce, and its method matters as much as its result.

Red- and green-leaf lettuce were grown at a fixed total photon flux density with twelve different fractions of red, green and far-red light, while the photon flux density of blue light was held constant. The plants were grown at 23 °C under a 20-hour photoperiod, with canopy temperature averaging 24.5 °C, relative humidity 39 percent and carbon dioxide at 427 µmol/mol; the nutrient solution was held near pH 5.7 and 1.9 dS/m.^[1] Holding total photon flux constant is what makes the trial informative: it isolates the effect of spectrum from the effect of light quantity.

Two secondary findings in the same study are worth carrying into a spectrum decision. First, substituting far-red for red increased the leaf area of both cultivars, but substituting green for red had a smaller effect on leaf expansion and fresh mass than far-red did. Second, far-red is not simply additive to quality: adding far-red to a red plus blue spectrum increased total plant phenolic content and soluble sugar content while decreasing chlorophyll content as leaf area increased, and adding far-red to a white spectrum decreased anthocyanin concentration while increasing shoot fresh mass, stem length, leaf length and leaf width relative to the white control.^[1]

End-of-day far-red: elongation without leaf expansion

A second way to use far-red is to deliver it briefly at the end of the photoperiod rather than across the whole day. This is a lower-dose strategy, and the results differ from the whole-day trials in an important way.

A 2023 HortScience study grew petunia ‘Wave Purple’ and ‘Dreams Midnight’ seedlings in a common greenhouse environment at a simulated winter daily light integral of approximately 5.3 mol/m²/day, applying end-of-day far-red for three weeks from cotyledon emergence. A supplemental-lighting control was included at a target PPFD of 70 µmol/m²/s over a 14-hour photoperiod.^[2]

Three results from that trial are directly useful. Stem length generally increased under end-of-day far-red compared with both the control and the supplemental-lighting treatments; the elongation was greater when the R:FR fell from about 0.8 to about 0.15, and greater again when the duration rose from 30 minutes to 240 minutes. But raising far-red intensity from 10 to 20 µmol/m²/s at an R:FR of 0.15 for 30 minutes did not promote further elongation — the two treatments produced similar stem lengths. And, unlike the whole-day lettuce trials, none of the end-of-day treatments increased leaf area relative to the controls; the authors concluded that under low daily light integrals, end-of-day far-red applied in the first three weeks of plug production does not promote early leaf expansion and generally decreases seedling quality for petunia.^[2]

The study also reports a contrasting whole-day result from earlier work: adding 54 µmol/m²/s of far-red to a background PPFD of 149 µmol/m²/s increased the leaf area of tomato ‘Komeett’ by 3 percent and raised whole-plant radiation absorption by 10 percent compared with plants given no added far-red.^[2] Read together with the lettuce fraction trial, the pattern is that far-red's leaf-area benefit shows up most clearly when it is part of the main spectrum at a controlled fraction, while a short end-of-day pulse mostly buys elongation.

How to apply far-red in a real installation

Four decisions carry most of the outcome. They are worth settling before a spectrum is specified, because they point in different directions.

Start from the trait, not the wavelength

Far-red is a morphology lever and an efficiency lever at the same time, and the two can pull in opposite directions. If the goal is biomass in a leafy crop and compact form is not critical, a moderate far-red fraction is the relevant setting, with the measured optimum near 0.25 for lettuce fresh mass.^[1] If the goal is compact, transplant-ready plugs, elongation is the risk to manage, and a low R:FR is precisely what produces it.^[2]

Control the fraction and the ratio, not the micromoles alone

Because far-red lies outside PAR, a PPFD figure reported over 400–700 nm will not describe it. The quantities that do describe it are the far-red fraction FR/(R+FR) used in the lettuce trial and the red to far-red ratio R:FR used in the seedling trial, with R:FR defined there over 600–700 nm divided by 700–780 nm.^[1][2] Those are the numbers to request from a supplier alongside the conventional PPF and efficacy figures.

Dose by duration before intensity

The seedling trial is explicit on this point. Going from an R:FR of about 0.8 to about 0.15 produced more elongation, and lengthening the end-of-day treatment from 30 minutes to 240 minutes produced more again — but doubling far-red intensity from 10 to 20 µmol/m²/s at a fixed 30-minute duration did not.^[2] Where far-red is used as a timed morphological signal, duration is the variable with the demonstrated dose response.

Check the interaction with the rest of the spectrum and with density

Far-red responses are not independent of the background. Adding far-red to a red plus blue spectrum increased lettuce shoot fresh mass and leaf length, and the effect was more pronounced under a high blue-to-red ratio or a low photon flux density than under the reverse. Substituting blue light with far-red also increased leaf expansion and fresh mass of lettuce.^[1] Because the magnitude of the response depends on the other wavebands present, the total photon flux density and plant density, a far-red setting validated in one room does not transfer unchanged to another.^[1]

What is still uncertain

Three limits on the evidence should be stated plainly rather than smoothed over.

Species and cultivar variation. The lettuce optimum of about 0.25 was measured on two lettuce cultivars in one controlled-environment study, and the end-of-day result was measured on two petunia cultivars. Whether other crops share those numbers is an open question — the seedling authors themselves note that responses to far-red radiation may vary with taxon, incident radiation and daily light integral, and that further work on end-of-day far-red is warranted.^[1][2]

Quality, not just quantity. Far-red shifts composition as well as size. In the lettuce work it was associated with higher phenolic and soluble sugar content but lower chlorophyll content, and with lower anthocyanin concentration in the white-spectrum comparison.^[1] For crops sold on colour or appearance, mass gain is not the whole outcome.

Light is not the only variable. Shade-avoidance behaviour mediated through phytochrome is a response to competition as well as to spectrum, and the study notes that the size of the far-red response depends on plant density.^[1] A spectrum decision made on a full bench may behave differently at a lower planting density.

Frequently asked questions

Does far-red light increase plant growth?

It can, but not in a straight line. In a 2024 PLoS ONE study on red- and green-leaf lettuce grown at a fixed total photon flux density, replacing red photons with far-red photons increased leaf area, and shoot fresh mass was greatest at a far-red fraction of about 0.25, calculated as FR/(R+FR). At higher far-red fractions leaf area kept rising while fresh mass did not, and plant quality began to deteriorate.^[1]

Is far-red light part of PAR?

No. The traditionally defined photosynthetically active radiation waveband is 400 to 700 nanometres, and far-red is defined as 700 to 799 nanometres, so it falls outside PAR.^[1] That matters for specifications: a fixture's PPF or PPFD figure in micromol per second, or per square metre per second, is normally quoted over 400 to 700 nanometres and therefore does not count its far-red output.

What is a safe red to far-red ratio for seedlings?

There is no single safe number, because the response is dose and species dependent. In a 2023 HortScience study, petunia seedlings under a simulated winter daily light integral of about 5.3 mol/m²/day showed more stem elongation as the red to far-red ratio fell from about 0.8 to about 0.15, and as the end-of-day far-red period lengthened from 30 to 240 minutes.^[2] That elongation is usually undesirable in plug production, so a low R:FR at the end of the day should be treated as a deliberate morphological tool rather than a default.

Why does adding far-red to red light raise photosynthesis?

This is the Emerson enhancement effect. Photosynthetic rates are higher when light around 670 to 680 nanometres and light above 680 nanometres are applied together than when the two wavebands are applied separately, because far-red preferentially excites photosystem I and rebalances excitation between the two photosystems. Work cited in the 2024 PLoS ONE study found that adding red and far-red light from 686 to 703 nanometres progressively increased the quantum yield of photosystem II, and that 721 to 731 nanometres increased it as well.^[1]

About the publisher

Zhongshan City Ruixian Electronics Factory (XineLam) designs and manufactures LED lighting products and LED grow light modules. The company has 17 years of experience in the LED lighting industry and holds 300+ patents in China and internationally.

Far-red emitters are one of the design choices available in the module platforms we build, alongside the white, blue and red channels described above. Where a project needs a specific far-red fraction, R:FR or channel-level dimming behaviour, that is set at the module and driver level rather than added after the fact — the discussion above describes the parameters to specify, and the appropriate values still depend on the crop, the daily light integral and the trait you are managing.

This page is a technical explainer assembled from the peer-reviewed sources listed below. The wavelength bands, fractions and treatment values quoted are the published figures from those studies, not measurements of any specific product, and nothing here is a performance claim for a particular fixture.

References

1. Kelly, N. and Runkle, E. S. Dependence of far-red light on red and green light at increasing growth of lettuce. PLoS ONE 19(11): e0313084, 2024. https://pmc.ncbi.nlm.nih.gov/articles/PMC11567594/ (accessed 11 September 2026).
2. Percival, A. C. and Craver, J. K. End-of-day Far-red Lighting with a Low Daily Light Integral Increases Stem Length But Does Not Promote Early Leaf Expansion for Petunia × hybrida Seedlings. HortScience 58(9): 1010–1017, 2023. https://journals.ashs.org/view/journals/hortsci/58/9/article-p1010.xml (accessed 11 September 2026).
3. Cabrera-Garcia, J. and Ernst, M. Controlled Environment Agriculture: Understanding Grow Lights (Publication No. G6987). University of Missouri Extension. https://extension.missouri.edu/publications/g6987 (accessed 11 September 2026).

Note on scope: this guide describes what published trials measured and how to state a far-red specification. It does not recommend a specific product, and the far-red fractions, ratios and durations quoted are the conditions used in those trials rather than a prescription for any particular crop.