The Cannabis Light Spectrum Illuminated - Part 2: Far-Red

4 minute read

As a master cannabis cultivator, you know the importance of the horticultural lighting spectrum. You understand how cannabis uses green light, and why blue light and red light are critical for robust growth.

But how does cannabis use far-red light?  Moreover, why is far-red present in natural sunlight and how do plants respond? And, how does the ratio of red to far-red light affect plant growth and morphology?  

Knowing how to use far-red light will help you grow bigger plants with more reproductive weight, but beware: misunderstanding the role of far-red light can stunt your plants’ growth and limit your returns.

Download our Ebook on Cannabis >>



What is Far-Red Light?

Far-red (FR) light is light at the furthest edge of the light spectrum, beyond red light, near-infrared. Our human eyes can barely perceive its long wavelength of 700-750 nm. But for plants, FR is very noticeable and quite important.  

When you use it right, far-red light can reduce the flowering period by up to 15% and enable quicker crop turnover.

An increasing number of horticultural lights incorporate FR, so it may come as a surprise that FR is not photosynthetically active; it does not drive photosynthesis directly. It does, however, have important effects as a biological signal and as a support for photosynthesis in other ways. In fact, many plant researchers are advocating to change the definition of PAR to include far-red due to the direct impact on crop performance.  


Far Red and Photosynthesis

While FR is not photosynthetically active, it plays a synergistic role with other colors of light. When plants are exposed to FR, the broad white spectrum of light as a whole becomes more efficient. (1)  

Robert Emerson discovered this synergistic benefit in the 1950s and is appropriately named the Emerson Effect. The exact mechanisms of the Emerson Effect are unclear, but we know that plants have two photosystems that work in synergy to “pump” electrons and transport energy.  

These photosystems work best when both are functioning well. The first is driven by red light and produces the electrons needed for photosynthesis. The second is driven by FR light, and it further aids the transport of electrons. So, when both work together, plants photosynthesize more biomass.


The Science Behind FR Light

Let’s get scientific for a moment and take a look at how plants really use FR. Cannabis — like all flowering plants — has phytochromes. Phytochromes are photoreceptors that perform biological functions based on their exposure to red light and FR light.  

When a phytochrome absorbs a red photon, it switches into a biologically active state; when it receives FR photons, it switches back. Depending on their state, phytochromes inhibit or enable biological processes.

The red-to-far-red ratio (R: FR) that your plants receive is critical. Based on the red-to-FR ratio, phytochromes help plants know when they’re being shaded by other plants. That’s because plants higher in the canopy absorb red light and mostly pass the FR through.

When plants since they’re in the shade of other plants, they stretch to compete by elongating their stems and widening their leaves. Botanists call this phenomenon the Shade Avoidance Syndrome (SAS). It’s one of the many ways plants use spectral ratios to mediate how they grow. This is noticeable even within a single plant. For instance, when a top is shaded by a large fan leaf, that top will grow more rapidly to avoid the shade.

Shading events change the R: FR ratio but so does the daily shift in the outdoor light spectrum. At the end of the day, there’s proportionately more far-red during twilight, and the ratio goes down.  

The increase in FR during twilight is an important tool that plants use to synchronize their internal clocks. The phytochromes receive a dose of FR photons, and that triggers important gene expressions as the plant enters the nocturnal period. During twilight, the R: FR ratio swings from a red-dominant 1.3 to an FR-dominant 0.6. (2)

Which Grow Lights Have Far-Red Light?

The R: FR ratio of a typical grow light is strongly red-dominant, with ratios ranging from  2.6 to 13, depending on the lighting technology. That’s very different from the R: FR ratio of sunlight, which is 1.3. Most white LED fixtures also feature red-dominant spectrums, too, with ratios ranging from 3.6 to 4.0.  

In today’s horticulture, supplement LEDs can provide the FR light that incandescent lights cannot. LEDs can recreate a wide variety of spectra depending on their design, and some allow growers to modify the spectrum throughout the photoperiod, achieving the end-of-day FR that plants expect from nature.


How to Use Far-Red Light for Growing Cannabis

FR affects cultivation outcomes in two important ways:

  1. FR light impacts stretch and internodal spacing.
  2. FR shortens the length of the flowering period.

When applied correctly — at the correct stage of growth and the right moment of the daily light cycle — FR can increase your annual production by speeding up your crop cycle. If it’s overused during the vegetative stage, it can cause lanky plants that will struggle to support heavy flowers.

Plants thrive when they receive FR just before and just after the photoperiod. Applying FR for ten minutes at these times — before the lights go on — can help level the canopy after the lights go off. The lower colas will stretch to avoid shade and produce a flat canopy top where plants receive better-distributed light. This builds biomass while also expediting flowering, as explained below.

Growers have found that a simple end-of-day FR application can decrease the flowering cycle by up to 15%. That’s because FR helps signal the shortening day length that plants would naturally experience outdoors. As the angle of the sun lowers, less red light makes it through the atmosphere, and the R: FR ratio drops.

Plants bred in Humboldt, California, do especially well with FR light. Over the past 60 years of cultivation in the area, growers have crossed plants that do well in the shade. The area's large pine trees — and the need for pre-legalization secrecy — made indica-hybrids perform better than their Sativa cousins. Indicas naturally evolved with more FR based on their northern origins, so they’re perfectly suited for NorCal cultivation.  

As leaves do not easily absorb FR, the far-red light is also able to penetrate deeper into the canopy similar to green light.


Research-Backed FR Lighting Technologies

At Heliospectra, we believe in moving horticulture forward with better spectral engineering and more efficient products. With all the benefits of FR light, our team recently conducted studies to test the effect of FR in three scenarios.

Our researchers grew tomatoes in a climate-controlled environment using 1) full-spectrum light without FR, (2) full-spectrum light with FR enrichment, and (3) full-spectrum light with FR applied only at the end of the day.  

After 15-18 days of treatment, the end-of-day far-red group was the largest. It was 15 percent taller than the control and experienced a 3.7-percent increase in stem thickness. These results suggest that an end-of-day FR treatment can build biomass during the flowering phase while saving energy when compared to continuous FR exposure.


Want to learn more about how advanced LED technologies can benefit your crops? 

Download the Grower-to-Grower Guide to Cannabis Lighting. You’ll find growing tips from real-world commercial cultivators and lots more information about cannabis photobiology.

(1) Zhen, S., Bugbee, B. Far-red photons have equivalent efficiency to traditional photosynthetic photons: Implications for redefining photosynthetically active radiation. Plant Cell Environ. 2020.

(2) Ashdown, I. Far-Red Lighting and the Phytochromes. Maximum Yield. 2018.


This is part one of a three-part series documenting the effects of the light spectrum on cannabis. For even more information about cannabis photobiology, make sure to check out Heliospectra's Cannabis eBook.