What does far red light do for plants?

by | May 29, 2024 | blogs, Grow Lights | 0 comments


What does far-red light do for plants?  Far-red light impacts plants in so many ways that it’s sometimes confusing, misunderstood, and even controversial.  Nevertheless, it’s an important topic for indoor growers to grasp in order to harness the strategies of far-red light in their businesses.

Recently, we’ve reviewed several articles about far-red Light and we decided to accumulate and summarize the information we found.

Fig 1 Plant and Light

The Emerson Effect and Far-red Light.

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)

Indoor Horticulture

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.

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.

Phytochromes, Pr and Pfr molecular

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.

Effects of Blue Red light on 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.

Effects of Blue Red light on Plants

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.

Phytochrome molecule

Fig 9 – The Phytochrome Molecule, the key to seasonal growth.

Circadian Sunlight

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.

Phytochromes and sprouting seeds

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 or Spectrophotometer.

  1. Emerson Effect & Red Drop | Neela Bakore Tutorials | 2015 | |https://www.youtube.com/watch?v=yqJBdNOHY5E
  2. Light Reactions and Emerson Enhancement Effect | Fluence | 2023 | https://www.youtube.com/watch?v=oIf_XwWVIq8
  3. 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
  4. 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/
  5. 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
  6. 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
  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/

PG200N Spectral PAR Meter

Spectral PAR  measurement allow farmers and researchers the advantages of expert lighting in tweaking the color parameters to improve quality, timing and quantity of their products.

PG200N Spectral PAR Meter

Handbook Series

The Flicker Handbook

Everything thing you need to know about Flicker, an insidious, potentially serious lighting artifact impacting visual safety for public places like hospitals, offices, libraries, and more...

▸ Get it!

About UPRtek

United Power Research and Technology

UPRtek (est. 2010) is a manufacturer of portable, high-precision light measurement instruments; Handheld Spectrometers, PAR meters, Spectroradiometers, Light Calibration Solutions.

UPRtek HQ, R&D and manufacturing are all based out of Taiwan, with Worldwide representation through our certified Global Resellers.

▸ Read on


Medical Setting using LED or Fluorescent Lights

What is a Spectrometer, Spectrophotometer, Spectroradiometer?

In the realm of spectral devices, three devices stand out: a Spectrometer, a Spectrophotometer, and a Spectroradiometer. However, these terms have been so interchangeably used that we felt obligated to clarify their differences in one short article.

  ▸ Spectrometer v Spectrophotometer v Spectroradiometer