How Does Far Red Light Affect Plant Growth?
Introduction to Far Red LED Grow Light and Plant Growth
How does far red light affect plants?
Far red grow light technology impacts plants in many ways — sometimes confusing, misunderstood, and even controversial. Yet it remains a crucial subject for indoor growers seeking to optimize photosynthesis and flowering performance. Understanding how far red wavelengths interact with red light also helps answer a common question among growers: why is red light good for plants and how does far red light extend those effects?
Recently, we reviewed several studies and articles about far red light and decided to summarize the most practical findings for growers and lighting designers.
- What Is Far Red Light and How It Relates to the Emerson Effect
- Ｗhy Is Red Light Good for Plants?
- How Does Red Light Affect Plant Growth — and Where Far Red Fits In
- Far Red Light and the Bottleneck (PSII, PSI)
- What Do Real Experiments Show about Far Red Light?
- Far-red and Stem Elongation
- Far Red Light and Seed Dormancy
- Far-Red and Seasonal Growth
- Far-Red Doesn’t Always Mean Increasing Yield
- Far-Red Can Help, But Needs to be Understood
- Far Red LED Grow Light: From Research to Practical Application
Fig 1 Plant and Light
What Is Far Red Light and How It Relates to the Emerson Effect
Where do all these ideas of the benefits of far-red light come from? The Emerson Effect, discovered by Robert Emerson in the 1950s, describes how photosynthesis can be boosted in plants when exposed to light of two different wavelengths of red light simultaneously.
In short, Emerson found that adding far-red (~700nm) to red light (~653nm) produced a far better result than the combined results of testing each color separately.
- Red light (653nm) yield = 53
- Far-red Light (700nm) yield = 10
- Red + Far-red Light yield =72
(results in oxygen per quantum of light absorbed)
Fig 2 Robert Emerson
The fact that the red+far-red test results were better than the combined results from the separate tests (red and far-red) revolutionized our understanding of the mechanisms of photosynthesis.
Emerson surmised that instead of one system adding up the effects of two wavelengths of light, there must be two distinct systems working together, one system boosting the other – these later became known as PSII and PSI.
Ｗhy Is Red Light Good for Plants?
Red light plays a fundamental role in photosynthesis, driving energy production in plants more efficiently than most other wavelengths. In Emerson’s experiments, red light around 653 nm showed the highest yield when absorbed by photosystem II (PSII), the primary reaction center that initiates photosynthetic activity. This explains why red light is good for plants—it excites chlorophyll molecules responsible for converting light into chemical energy.
However, when far red light (~700 nm) was added to red light, the overall photosynthetic efficiency increased dramatically, revealing that far red light complements red light rather than replacing it. This discovery led to the identification of two light systems, PSII and PSI, which work together to maximize energy conversion in plants.
How Does Red Light Affect Plant Growth — and Where Far Red Fits In
Red light plays a dominant role in plant development because it drives photosynthesis and regulates critical processes such as flowering, seed germination, and stem elongation. In horticulture, it is well known that red light at around 660 nm activates chlorophyll absorption, which helps convert light energy into sugars that fuel plant growth. This explains how red light affects plant growth and why red LEDs have become a foundational component of most grow light systems.
However, research has shown that plants respond even more efficiently when far red light is introduced alongside red light. The combination of both wavelengths enhances the photosynthetic rate through a mechanism first demonstrated by the Emerson Effect. Far red light (~700 nm) complements red light by stimulating Photosystem I (PSI), which works in tandem with Photosystem II (PSII) — the red-light-sensitive system — to balance the entire energy conversion process.
Modern far red grow light designs leverage this synergy to promote more uniform canopy lighting, faster flowering, and improved biomass production. Instead of replacing red light, far red wavelengths extend its benefits, creating a complete spectral strategy that mirrors the balance found in natural sunlight.
Far Red Light and the Bottleneck (PSII, PSI)
PSII and PSI are subsystems located in the Thylakoid Membrane, buried in plant cells (you can read about it here). You can think of them as stations in a factory that produces energy stores called ATP and NADPH, eventually to be used to make sugar.
Figure 3a Photosynthesis takes place in the Thylakoid membrane, buried deep in a plant cell.
Figure 3b The Thylakoid membrane and PSII, PSI stations
Blue and Red light are absorbed in both PSII and PSI and are important for photosynthesis to take place.
Additionally, PSII is particularly efficient at absorbing red light at 680 nm.
Also, PSI is particularly efficient at absorbing far-red light at 700 nm
If you have plenty Blue and Red light from your Grow Lights, photosynthesis takes place, but there will be a bottleneck at PSI. Why?
There is a natural backup at PSI because the processing there is inherently slower. Since PSI is better at absorbing far-red, adding far-red to your grow lights should help things to move along.
In short far-red light IS NOT a magic growth elixir, as some people might first think from experiments showing miraculous results – it simply helps the process flow from PSII and PSI.
You won’t have a bottleneck problem with outdoor farming because the sun has plenty of far-red light (Figure 4).
Figure 4 Spectrum of the Sun shows plenty of far-red light (>700nm)
What Do Real Experiments Show about Far Red Light?
We looked at three studies that researched supplemental far-red light and discovered these outcomes.
- Study 1: 39.4%, 19.0%, and 0.0% increase in dry weights for three lettuce specimens.
- Study 2: 46-77% increase in dry weight and 58-75% increase in leaf area for lettuce.
- Study 3: 31.72% increase in leaf photosynthesis with far-red light.
(see references at the end.)
Even though these studies show significant results, we must reiterate, these findings do not say supplemental far-red light is a magic growth opportunity—they are simply validating that grow light farming could benefit from supplemental far-red light.
Far-red and Stem Elongation
Under the canopy in a forest, blue and red light diminish but there is an abundance of far-red light that transmits through the leaves (see Figure 5 ). This signals sprouting plants below to extend their stems and reach for better sunlight, otherwise known as “shade avoidance”.
In an indoor farming environment, you can use far-red light and stem elongation as advantages.
Fig 5 – This spectrometer measurement taken under a leaf during the noon day sun.
A positive aspect of stem elongation is in strawberry farming. Growers may use far-red light to help increase the stems, which allows for better ventilation and prevents mold and fungus. Additionally, with longer stems, the fruit is more easily visible and harvested.
Fig 6 – Far-red light encourages stem growth for better ventilation
However, in some cases far-red light to increase stem growth may not be desirable – you may want to keep your produce short and stocky for visual appeal (e.g. lettuce).
Also, some articles suggest that the sugars used for stem growth will affect the yield and quality of the fruit on fruit bearing plants.
Figure 7 – The color in light and plant size – Photo courtesy of Al Gracian at Albopepper.com
Far Red Light and Seed Dormancy
Seeds often are found on the ground under the canopy. They can also sense their presence in shaded areas reacting to far-red light, but unlike a sprout, the seed will go dormant, waiting for a more appropriate opportunity to sprout.
Figure 8 – Seeds react to far-red light by going dormant
Far-Red and Seasonal Growth
If you have plants in your yard or on your balcony, you probably notice a collective burgeoning of growth, especially in the spring/summer: new stems, fresh leaves, and flowers. Far-red and red light influence this magical transformation by influencing the structure of a phytochrome molecule.
Day hours and night hours considered equal, the phytochrome reaches a sort of stasis. But as spring/summer arrives, the day hours are longer, causing a shift in far-red and red light influence on the phytochrome state – and this triggers seasonal growth.
You can read all about seasonal growth here.
Fig 9 – The Phytochrome Molecule, the key to seasonal growth.
Figure 10 – Seasonal growth is determined by the daylight hours
Far-Red Doesn’t Always Mean Increasing Yield
The end product of photosynthesis is sugar. But just because you have an abundance of sugar from supplemental far-red light, doesn’t mean an increase in plant yields.
The DNA in the nucleus of a plant goes through a checklist of items (checkpoint control) before it deems appropriate to trigger growth. It evaluates temperature, humidity, hormone availability, water availability, plant stress etc. In effect, just because you have blue-light, red-light and far-red light to adequately encourage photosynthesis, you still may not achieve the results you want.
Fig 11 – The Nucleus of the Plant Cell controls all the functions of cell division.
Figure 12 – The 4th stage of cell cycle is cell division (Mitosis) – by Ali Zifan (wikipedia-CC BY-SA 4.0)
And Remember, Every Plant is Different
Each plant species has its own quirks. What works perfectly for one might not be ideal for another. Their peculiarities lie deep in the DNA of the nucleus as part of the master plan of evolution, survival of the fittest.
Fig 13 – Far-red, red and uv light measurements from a Spectral PAR meter.
Far-Red Can Help, But Needs to be Understood
In this article, we tried to cover all the aspects of far-red light
But we found it’s not as easy as turning on the far-red light and increasing yield, as you have to consider many aspects of far-red light: PSI bottlenecks, stem elongation, seasonal growth, and nucleus / DNA considerations.
Also, all plants have peculiarities—if you’ve ever tried growing plants, you quickly learn that what’s good for one plant may not work for another. Nevertheless, understanding the nuances of far-red light gives growers a better chance of achieving far-red results, through experimentation and tweaking the parameters.
All measurement data in this article was taken by a UPRtek Spectral PAR meter.
Far Red LED Grow Light: From Research to Practical Application
While research helps us understand the complex behavior of far red light, growers rely on far red LED grow light systems to translate those scientific insights into measurable results.
Modern LED grow light technologies are now designed with precise spectral tuning, allowing users to balance red and far red wavelengths for specific crops or growth stages.
By using a far red LED grow light, growers can replicate the natural light conditions that drive photosynthesis efficiency, flower initiation, and biomass accumulation. However, it’s not a one-size-fits-all formula—each species responds differently to far red radiation, and fine-tuning spectral ratios remains essential.
This is why accurate spectrum measurement tools, such as the UPRtek PG200N Spectral PAR Meter, are indispensable. They enable researchers and growers to analyze and optimize far red performance, ensuring that lighting strategies are data-driven rather than based on trial and error.
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Others
- Emerson Effect & Red Drop | Neela Bakore Tutorials | 2015 | |https://www.youtube.com/watch?v=yqJBdNOHY5E
- Light Reactions and Emerson Enhancement Effect | Fluence | 2023 | https://www.youtube.com/watch?v=oIf_XwWVIq8
- A Closer Look at Far-red Radiation | Erik Runkle | Michigan State U | chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.canr.msu.edu/uploads/resources/pdfs/fr-radiation.pdf
- Far-Red Light Effects on Lettuce Growth and Morphology in Indoor Production Are Cultivar Specific | Jun Liu1,* and Marc W. van Iersel2 | NIH |2022 Oct 14 | https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9611250/
- Adding Far-Red to Red-Blue Light-Emitting Diode Light Promotes Yield of Lettuce at Different Planting Densities | Wenqing Jin, Jorge Leigh Urbina, Ep Heuvelink, Leo F.M. Marcelis | Frontiersin.org | Jan 15, 2021
- Far-red light modulates grapevine growth by increasing leaf photosynthesis efficiency and triggering organ-specific transcriptome remodelling | 6. Junhua Kong, 6. Yan Zhao, 6. Peige Fan, 6. Yongjian Wang, 6. Xiaobo Xu, 6. Lijun Wang, 6. Shaohua Li, 6. Wei Duan, 6. Zhenchang Liang, 6. Zhanwu Dai | BMC | March 2024 | https://bmcplantbiol.biomedcentral.com/articles/10.1186/s12870-024-04870-7
- How Does The Far Red Light Spectrum Affect Plants? } California Lightworks | August 2019 } https://californialightworks.com/blog/how-does-the-far-red-light-spectrum-affect-plants/
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UPRtek (est. 2010) is a manufacturer of portable, high-precision light measurement instruments; Handheld Spectrometers, PAR meters, Spectroradiometers, Light Calibration Solutions.
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