Far-Red Light and the Emerson Effect
Adding a small dose of 700-740 nm light at the right time can speed photosynthesis and flowering — within limits.
Far-red light is one of the few cannabis cultivation tweaks with solid plant-science backing — but the marketing has run ahead of the data. The Emerson effect is real in lab conditions, and modest yield bumps from adding far-red to white LEDs have been measured in horticultural crops. For cannabis specifically, the strongest demonstrated effects are faster flower initiation and slight bud size increases, not the doubled yields some fixture vendors imply. Use it as a small efficiency gain, not a magic bullet.
What it is
The Emerson effect is a photosynthesis phenomenon discovered by Robert Emerson in the 1950s: when you illuminate a plant with far-red light (around 700-740 nm) together with shorter-wavelength red light (~660 nm), photosynthetic rate is higher than the sum of each wavelength applied alone [1]. The reason is that the two photosystems in chloroplasts — PSI and PSII — absorb light differently. PSII is driven mostly by red and blue; PSI absorbs further into the far-red. When only red light is present, PSII runs faster than PSI and the electron transport chain bottlenecks. Adding far-red balances the two photosystems Strong evidence.
In modern horticulture, 'far-red' is usually defined as 700-780 nm, just past the traditional PAR range (400-700 nm). The McCree action spectrum, long used to define PAR, undercounts far-red's photosynthetic contribution; newer work proposes an 'extended PAR' or 'ePAR' range of 400-750 nm [2].
Far-red also triggers phytochrome-mediated responses: shade avoidance, stem elongation, and shifts in flowering time. That's a separate effect from the Emerson photosynthetic enhancement, and growers often conflate the two.
Why growers use it
Three main reasons:
- Photosynthetic efficiency. Adding ~10-15% far-red photons to a white/red LED spectrum can increase canopy photosynthesis by a similar fraction in some crops [3] [evidence:weak for cannabis specifically].
- Faster flowering / larger buds. Cannabis is a short-day plant, and phytochrome state influences flowering. Some practitioner reports and limited research suggest far-red supplementation during flower can accelerate flower onset and increase flower size modestly Weak / limited.
- End-of-day (EOD) far-red pulse. A short far-red exposure at lights-off converts active phytochrome (Pfr) back to inactive (Pr), mimicking the natural dusk signal. In some short-day species this strengthens the flowering signal Weak / limited.
A common marketing claim is that far-red 'unlocks 30% more yield.' That's overstated. Peer-reviewed cannabis trials are scarce, and the ones that exist show smaller, condition-dependent gains Disputed.
When to start
Most growers add far-red at the flip to 12/12 (start of flowering photoperiod). A few use it in late veg to encourage stretch before flip, though this can produce overly leggy plants.
- Veg: Generally skip, or use sparingly. Too much far-red causes shade-avoidance stretch and weak stems Strong evidence.
- Early flower (weeks 1-3): Useful if you want faster flower initiation and a controlled stretch.
- Mid-late flower (weeks 4-7): This is where the photosynthetic Emerson benefit matters most — buds are bulking and any extra assimilate helps.
- Last 1-2 weeks: Optional. Some growers cut it; others run it through harvest. No strong data either way No data.
How to do it (step-by-step)
Step 1: Pick your delivery method.
- Integrated fixture with built-in 730 nm diodes (easiest, e.g. many modern horticultural LED bars).
- Supplemental far-red bar added alongside your main light.
- EOD far-red lamp on a separate timer for end-of-day pulses.
Step 2: Target the right ratio. Research suggests adding far-red at roughly 10-15% of total photon flux in the 400-750 nm range produces the Emerson benefit without excessive stretch [3] Weak / limited. More than ~25% tips you firmly into shade-avoidance territory.
Step 3: Run far-red simultaneously with your main light. The Emerson effect requires co-illumination — far-red alone does very little photosynthetically. Wire your far-red bar on the same schedule as your primary light.
Step 4: (Optional) Add an EOD pulse. Set a separate timer to run far-red for 10-15 minutes immediately after main lights-off. This drives the phytochrome conversion and may accelerate flowering Weak / limited.
Step 5: Watch for stretch. During the first two weeks of flower, monitor internode spacing. If plants are stretching more than 30-50% of their pre-flip height, dial far-red back.
Step 6: Measure, don't guess. A quantum sensor that reads ePAR (400-750 nm) gives you real numbers. If you only have a PAR meter, know that it undercounts far-red — your effective photon delivery is higher than it reads.
Common mistakes
- Treating far-red as a replacement for red. It isn't. PSII still needs 660 nm. Far-red supplements; it doesn't substitute.
- Running far-red alone during the photoperiod. Without simultaneous shorter-wavelength light, you get phytochrome effects (stretch) but not photosynthetic enhancement.
- Overdoing the dose. Excess far-red causes thin stems, larfy buds, and reduced cannabinoid density. Shade-avoidance is a real response and not a desirable one in flower Strong evidence.
- Believing vendor yield claims uncritically. A fixture marketed as 'Emerson-optimized' is not a guarantee of better results. Independent peer-reviewed cannabis data is limited Disputed.
- Forgetting heat and DLI. Adding any extra light increases your daily light integral and may push canopy temps up. Recalculate accordingly.
- Using incandescent bulbs as a 'far-red' source. Incandescents output broad-spectrum heat with some far-red, but the dose and spectral purity are poor compared to dedicated 730 nm LEDs.
Related techniques
- UV-B supplementation: Another spectral tweak, this one aimed at trichome and cannabinoid response. Evidence is similarly mixed.
- DLI management: The bigger lever. Getting your total daily light right matters more than spectral fine-tuning.
- Light deprivation: Manipulating photoperiod directly to trigger or extend flowering.
- Spectrum tuning: The broader category — adjusting red:blue:green:far-red ratios across the grow cycle.
Far-red is a real tool with a real mechanism. Just keep its scale honest: it's a single-digit-to-low-double-digit improvement under good conditions, 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 Kusuma, P., Pattison, P. M., & Bugbee, B. (2020). From physics to fixtures to food: current and potential LED efficacy. Horticulture Research, 7, 56.
- Peer-reviewed Smith, H. (2000). Phytochromes and light signal perception by plants — an emerging synthesis. Nature, 407(6804), 585-591.
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