Far-Red Light and the Emerson Effect
Balancing photosystems with far-red wavelengths for modest photosynthetic gains in flower.
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The Emerson effect is real physics, but cannabis growers often overstate the practical yield bump. Adding 10–15% far-red photons during mid-to-late flower can drive a measurable photosynthetic boost under controlled conditions. Real-world gains are typically low single digits to modest double digits—not the '30% more yield' vendors market. The effect also triggers stretch via phytochrome; too much far-red backfires.
What It Is
The Emerson effect is a photosynthesis phenomenon: when plants receive far-red light (700–740 nm) simultaneously with shorter-wavelength red light (~660 nm), photosynthetic rate is higher than the sum of each wavelength applied separately Strong evidence[1]. This occurs because the two photosystems in chloroplasts—PSII and PSI—absorb light differently. PSII is driven mostly by red and blue wavelengths; PSI absorbs across the far-red region. When only red light is supplied, PSII operates faster than PSI, and the electron transport chain becomes saturated at PSII. Adding far-red energizes PSI, rebalancing the two systems and allowing faster electron flow and greater overall photosynthetic output [1].
In modern horticulture, far-red is defined as 700–780 nm, just beyond the traditional PAR boundary of 400–700 nm. The McCree action spectrum, developed in the 1970s and long used to define photosynthetically active radiation, underestimates far-red's contribution to photosynthesis. Recent work proposes an extended PAR (ePAR) range of 400–750 nm to account for far-red's real photosynthetic efficiency Strong evidence[2].
Far-red also triggers phytochrome-mediated responses—shade-avoidance stem elongation, flowering time shifts, and other developmental cues Strong evidence[6]. This is mechanistically separate from the Emerson photosynthetic effect, though growers and vendors often conflate the two.
Why Growers Use It
Three main reasons drive far-red adoption in cannabis cultivation:
1. Photosynthetic efficiency boost. When far-red photons (typically 10–15% of total photon flux) are added to a balanced red-blue-white spectrum, photosynthetic rate in some crops increases measurably Weak / limited[2][3]. The exact gain depends on grow environment, baseline spectrum, and plant developmental stage. Controlled trials in lettuce and tomato show gains of 5–15%; peer-reviewed cannabis trials are scarce, and published results show smaller, condition-dependent improvements Disputed.
2. Possible flowering acceleration and bud enlargement. Cannabis is a short-day (long-night) plant, and phytochrome state—regulated by the red-to-far-red light ratio—influences reproductive timing. Some practitioner reports and limited research suggest far-red supplementation during early-to-mid flower can trigger faster flower initiation and marginally larger individual floral bracts Weak / limited[4]. No large, replicated trials confirm this in cannabis.
3. End-of-day (EOD) far-red pulse. A brief exposure to far-red light at lights-off converts the active phytochrome form (Pfr) back to the inactive form (Pr), mimicking the natural red-shift of sunlight at dusk. In some short-day species, this strengthens the flowering signal Weak / limited[6]. Application to cannabis is theoretical.
A pervasive marketing claim—that far-red 'unlocks 30% more yield'—is overstated. Peer-reviewed cannabis trials are limited, and controlled studies document smaller, context-dependent gains Disputed.
When to Start
Most growers introduce far-red at the lights-on start of the 12/12 flower photoperiod (day 1 of flip). Some experiment with late-veg introduction to encourage stretch before flip, though this can result in overly leggy plants and structural weakness.
Vegetative stage: Generally avoid or use sparingly. Excess far-red during veg causes shade-avoidance stem elongation and weak, thin stems—undesirable traits for robust clones or seedlings Strong evidence.
Early flower (days 1–21): Far-red is useful here if you want to accelerate flower site transition and manage pre-stretch. Monitor internode spacing; if plants stretch beyond 30–50% of their pre-flip height, reduce far-red dose.
Mid-to-late flower (days 22–49): This is the window where the photosynthetic Emerson benefit matters most. Buds are bulking and demand high assimilate flux. Co-illuminating with far-red supports carbohydrate production.
Final 1–2 weeks: Optional. Some growers discontinue far-red; others run it through harvest. No controlled data supports either approach No data.
How to Do It (Step-by-Step)
Step 1: Choose a delivery method.
- Integrated fixture: Many modern horticultural LED bars include built-in 730 nm diodes. Easiest and most controllable.
- Supplemental far-red bar: A dedicated 730 nm LED bar added alongside your main light fixture.
- EOD far-red lamp: A separate, timer-controlled 730 nm light for end-of-day pulses only (optional advanced technique).
Step 2: Target the right dose. Research in non-cannabis crops suggests adding far-red at roughly 10–15% of total photon flux in the 400–750 nm range produces the Emerson photosynthetic boost without triggering excessive shade-avoidance stretch Weak / limited[2][3]. Beyond ~25% far-red, plants respond primarily to phytochrome cues (stretching, altered flowering), and the photosynthetic benefit plateaus or reverses. Start at 10% and adjust based on canopy response.
Step 3: Co-illuminate with main light. The Emerson effect requires simultaneous far-red and shorter-wavelength illumination. Far-red alone produces phytochrome effects but minimal photosynthetic gain. Wire your far-red bar on the same timer and schedule as your primary light source.
Step 4: (Optional) Add an EOD pulse. Set a separate timer to deliver 10–15 minutes of far-red immediately after main lights-off. This drives phytochrome conversion and may accelerate flowering Weak / limited. Most growers skip this; data in cannabis are absent.
Step 5: Monitor stretch. During the first two weeks of flower, track internode length week-to-week. If plants stretch more than your historical baseline by 30–50%, reduce far-red intensity or duration.
Step 6: Measure your delivery. A quantum sensor capable of reading ePAR (400–750 nm) gives you real-world photon counts. Standard PAR meters (400–700 nm) undercount far-red—your actual photosynthetic photon delivery is higher than the meter reads. This matters for reproducibility across grows.
Common Mistakes
- Treating far-red as a substitute for red. It isn't. PSII still requires 660 nm red light. Far-red supplements red; it does not replace it.
- Running far-red alone during the photoperiod. Without simultaneous shorter-wavelength light, you trigger phytochrome effects (stretch, thin stems, reduced vigor) but gain no photosynthetic enhancement. Always co-illuminate.
- Overdosing far-red. Excess far-red (>25% of photon flux) amplifies shade-avoidance responses: thin stems, larfy (airy) buds, and reduced resin density. Phytochrome-driven stretch is real and undesirable in flower Strong evidence.
- Accepting vendor yield claims uncritically. Fixtures marketed as 'Emerson-optimized' or 'far-red enhanced' do not guarantee yield gains. Independent peer-reviewed cannabis trials are limited, and real-world improvements are modest and context-dependent Disputed.
- Ignoring heat and daily light integral (DLI) changes. Adding any light increases your canopy temperature and cumulative photon dose. Recalculate your DLI and monitor temperature accordingly.
- Using incandescent bulbs as a far-red source. Incandescents emit broad-spectrum heat with some far-red output, but spectral purity and photon dose are poor compared to dedicated 730 nm LEDs. Inefficient and risky for heat buildup.
- Forgetting to account for existing far-red in your baseline spectrum. Many full-spectrum LED fixtures already include some 730 nm diodes. Check your fixture's spectrum chart before adding supplemental far-red; you may already be in the 10–15% range.
Related Techniques
- Light Height by Stage: Proper fixture distance ensures even, safe light delivery. Far-red supplementation increases photon density; monitor for light stress.
- Daily Light Integral (DLI): The bigger lever. Total daily photons matter more than spectral fine-tuning. Optimize DLI for your strain and stage before worrying about far-red ratios.
- Light Cycle for Vegetative Growth (18/6): Far-red is generally avoided in veg to prevent stretch. Stick to balanced red-blue spectra during this stage.
- 12/12 Flowering Light Schedule: Far-red supplementation fits into a standard 12-hour bloom photoperiod. No modifications needed.
- Light Deprivation Automation: Light-dep growers can add far-red to their supplemental lighting during shorter natural days.
- Spectrum tuning (broader category): Adjusting red:blue:green:far-red ratios across the grow cycle is an advanced tool. Far-red is one dial; it is not the lever for most yield gains.
Far-red is a real tool backed by sound physics. Under optimized conditions, it delivers modest, measurable gains—typically low single digits to low double digits in yield or photosynthetic rate. Market it as a refinement, not a transformation.
Sources
- Peer-reviewed Emerson, R., Chalmers, R., & Cederstrand, C. (1957). Some factors influencing the long-wave limit of photosynthesis. Proceedings of the National Academy of Sciences, 43(1), 133–143.
- Peer-reviewed Zhen, S., & Bugbee, B. (2020). Far-red photons have equivalent efficiency to traditional photosynthetic photons: Implications for redefining photosynthetically active radiation. Plant, Cell & Environment, 43(5), 1259–1272.
- Peer-reviewed Zhen, S., & van Iersel, M. W. (2017). Far-red light is needed for efficient photochemistry and photosynthesis. Journal of Plant Physiology, 209, 115–122.
- Peer-reviewed Magagnini, G., Grassi, G., & Kotiranta, S. (2018). The effect of light spectrum on the morphology and cannabinoid content of Cannabis sativa L. Medical Cannabis and Cannabinoids, 1(1), 19–27.
- Peer-reviewed Smith, H. (2000). Phytochromes and light signal perception by plants—an emerging synthesis. Nature, 407(6804), 585–591.
- Peer-reviewed Kusuma, P., Pattison, P. M., & Bugbee, B. (2020). From physics to fixtures to food: current and potential LED efficacy. Horticulture Research, 7, 56.
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- Adding Far-Red at Flower Initiation — Use end-of-day far-red light pulses to accelerate flowering onset in photoperiod cannabis.
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