The Edge of Visible Light: Harnessing Far-Red Light in Cannabis Cultivation

The relationship between light and plant life is complex, extending far beyond the visible spectrum that powers photosynthesis. While the bulk of a plant’s energy is derived from Photosynthetically Active Radiation (PAR), which spans wavelengths from 300 to 700 nm, the energy just beyond that range—specifically Far-Red (FR) light—holds the key to a suite of developmental and architectural changes.

FR lighting is defined by wavelengths above 700 nm, with the most physiologically relevant portion for many plant responses clustering around 730 nm. Unlike the visible light that drives the creation of sugars, FR light is primarily involved in photomorphogenesis, which is the process of light-induced shape and form development in plants. Simply put, while PAR dictates how much a plant can grow, the FR light ratio determines how it grows.

To understand its role, it is essential to place FR light in the broader context of the electromagnetic spectrum. Red light, crucial for photosynthesis and other signalling pathways, typically falls between 650–680 nm. The FR spectrum immediately follows, spanning roughly 710–740 nm.

When Far-Red Light is Most Prevalent

The presence and intensity of FR light in nature are critical environmental cues for a plant. Its prevalence is not constant, which is why plants evolved to sense it.

1. Canopy Penetration: The most significant source of FR prevalence is the shade cast by other plants. Chlorophyll absorbs red (R) and blue light very efficiently for photosynthesis, but transmits or reflects a substantial amount of FR light. Consequently, light filtering through a dense canopy has a dramatically reduced Red:Far-Red (R:FR) ratio, becoming rich in FR light. This low R:FR ratio signals that a plant is in the undergrowth, prompting a set of survival responses known as shade avoidance.
2. Time of Day: As the sun sets, the filtering effect of the atmosphere causes a natural shift towards a higher FR proportion, making FR light a signal for the end of the day and the onset of night.
3. Soil Depth: Even seed germination is governed by this ratio. In some plants, a high R:FR ratio is required to break dormancy, assuring the seed it is not buried too deeply under the soil or dense vegetation.

Photomorphogenesis and the Phytochrome System

The mechanisms by which a plant perceives and responds to FR light are centered around a protein known as phytochrome—the core component of the plant’s light-sensing system. This system regulates a wide range of temporary as well as permanent adaptations to the environment. It is part of how the plant receives clues about its surroundings, especially regarding seed germination, shade avoidance, and flower induction.

The Role of Photoreceptors

The entire process of light-regulated development, or photomorphogenesis (literally, light form begins), relies on specialized photoreceptors. While blue light photoreceptors are primarily involved in responses like phototropism (bending towards light) and stomatal opening, the red and far-red photoreceptors are dominated by the Phytochrome family of proteins.

Phytochrome is a light-absorbing protein that is sensitive across the spectrum, absorbing R, FR, and even a degree of blue light. Its power lies in its photoreversibility—its ability to exist in two distinct, interconvertible forms depending on the quality of light it absorbs.

The Phytochrome Switch: Pr and Pfr

The two stable forms of the Phytochrome protein act as a molecular switch:

1. Pr (Phytochrome red-light absorbing): This is the biologically inactive form of the protein. It appears blue to the eye because it absorbs red light. When Pr absorbs a photon of red light (650–680 nm), it rapidly undergoes a conformational change.
2. Pfr (Phytochrome far-red-light absorbing): This is the biologically active form. It is created from Pr after absorbing red light. It appears blue-green to the eye because it now strongly absorbs far-red light. When Pfr absorbs a photon of far-red light (710–740 nm) or is left in the dark for a period, it reverts to the inactive Pr form.

This constant conversion—Pr↔Pfr—is called photoreversibility. Under natural or white light (which contains both R and FR), the phytochrome pool is in a state of continuous transformation, existing as a dynamic mixture and intermediate states. The ratio of Pfr to total phytochrome (Ptotal) acts as the plant’s ultimate sensor for environmental light quality.

Phytochrome Response Groups

Phytochromes are classified into different types (A, B, C, etc.) that regulate various developmental processes. These effects can be grouped by the light required to trigger them:

• Quantity (Fluence): The total amount of photons required.
• Quality (Wavelength): The specific ratio of R to FR light.

The speed of the plant’s response to the phytochrome switch can vary widely, grouped by the lag time—the time between light absorption and the first noticeable effect:

For LFRs, it is important to note that the response depends on the total amount of photons received, meaning a short, bright flash of light can have the same effect as a longer, dimmer exposure.

Evolutionary Rationale and Plant Responses

The phytochrome system is present in virtually all green plants, from algae to massive sequoias, and of course, our beloved Cannabis plants. This ubiquity highlights its fundamental role as an evolutionary advantage, primarily to assess the competitive environment.

Pfr: The Active Inhibitor

The Pfr form is the physiologically active version. Its primary, evolutionarily-conserved functions are to:

1. Inhibit Flowering
2. Inhibit Stem Elongation

In simple terms, under full sun, the high concentration of R light keeps the Phytochrome pool mostly in the inhibitory Pfr state. When a plant is shaded, the ratio shifts dramatically towards Pr (due to FR light converting Pfr back to Pr). When this ratio is moved, the inhibition stops, and the plant responds by initiating the suppressed growth response, like removing a wedge under a car parked on a hill.

The Shade Avoidance Syndrome

The most critical function of FR detection is shade avoidance. By sensing the low R:FR ratio under a canopy, a plant correctly assumes it is being out-competed for light. To win the race for the sun, it launches a response characterized by:

• Rapid Internode Elongation (Stretching): An attempt to grow taller than the competitor.
• Reduced Leaf Size and Thickness: Less investment in leaves that aren’t receiving enough light.
• Early Flowering: A desperate attempt to reproduce before being completely shaded out.

Shade-adapted plants (“shade plants”) and sun-adapted plants (“sun plants”) have different sensitivity levels to the R:FR ratio, but the mechanism is universal.

Phytochrome in Growth Regulation and Photoperiodism

The effects of FR light are often described as hypersensitive reactions because they can be produced with very little wattage, achieving pronounced effects on plant architecture and flowering. In cannabis cultivation, we manipulate these reactions to optimize yield and structure.

Manipulating Plant Architecture

By strategically introducing FR light, growers aim to balance two key desired reactions:

1. Longer Internodes: A controlled amount of Pr (by adding FR light) during the vegetative phase can reduce the Pfr-mediated inhibition of stem elongation. This leads to longer internodes, which can be beneficial for canopy management and spacing out flower sites for better light penetration.
2. Bigger Leaves: FR light can also promote leaf expansion, leading to bigger leaves that can capture more PAR light during the main daylight hours.

The Two Strategies: Mixing vs. End-of-Day

In horticulture, FR light is utilized in two primary ways, each with a different goal:

1. Mixing into the Spectrum (Enhancing Vegetative Growth): Incorporating a steady, low level of FR light alongside PAR throughout the photoperiod keeps the R:FR ratio lower than in typical sun/red-only LED setups. This sustains a higher Pr concentration, leading to continuous, subtle shade avoidance effects—specifically the longer internodes and bigger leaves. This helps create a more open, productive canopy.
2. End-of-Day (EOD) Treatment (Flower Induction / Phase Signaling): A brief, intense burst of FR light (710–740 nm) delivered just as the PAR light is turned off. The goal is to drive the final, complete conversion of all remaining Pfr (active form) to Pr (inactive form). This crucial action—creating a clean slate of Pr at the start of the dark period—has profound implications for the plant’s perception of night length, especially in relation to flower induction.

Photoperiodism: Measuring the Night

The plant uses the circadian clock to determine the time of day and photoperiodism to detect the length of the day (or year). This latter mechanism is crucial for flowering.

We know that cannabis, a short-day plant, needs 12 hours of light and 12 hours of uninterrupted darkness (12/12) to flower reliably. The fundamental discovery from plant experiments is that plants measure the length of the night, not the day. If the plant achieves a sufficiently long night, it can flower even if the light period is extended. As growers also know, interrupting the night, even by just a few minutes, completely inhibits flowering, whereas short periods of darkness during the day are irrelevant.

Far-Red for Bud Set and Beyond

The application of EOD FR treatment is a direct manipulation of the phytochrome system to ensure the plant registers the night as being long enough for the flowering signal to proceed.

The EOD Treatment and Flower Induction

When the lights go out naturally (or are simply turned off), the Pfr (active, flowering-inhibiting) form slowly reverts to Pr in a process called thermal reversion. However, a brief EOD FR treatment forces this conversion to completion instantaneously.

• By immediately converting all Pfr to Pr, the grower ensures the plant begins its dark cycle with the lowest possible level of the active flowering-inhibitor.
• This is especially helpful in encouraging a robust transition from stretch to bud set and can often shave several days off the time it takes for a strain to visibly begin flowering and up to a week off of the whole flowering cycle. This effect is especially pronounced in strains with heavy equatorial influences that otherwise take weeks to start producing buds.

While FR light primarily influences form, its inclusion in the spectrum may also have secondary, less-studied effects on budset density and potentially other physiological functions. However, the exact relationship with nutrient cycling remains a speculative area. It is important to note how small a percentage of FR is already creating all the desired effects. If you’re using a 300 W lamp, a 30 W FR lamp running for 8 minutes after lights-out will be sufficient to produce the desired flowering acceleration. If you are mixing spectra for structural enhancements, you need even less.

How to Apply This at Home

For cannabis growers, the knowledge of Pr and Pfr simply confirms the mechanism behind the well-known light deprivation (light dep) schedule. By understanding that Pfr is the chemical brake on the flowering process, growers can leverage FR light to ensure that brake is fully released at the start of every dark cycle, leading to quicker and more uniform flowering initiation. FR strips are widely available nowadays, providing a cheap and effective way to manipulate plant structure and flowering time at home. Who doesn’t want their 9-week strains to finish around day 56? The wavelength of whispers is simply the key to unlocking the plant’s full developmental potential.