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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EOD far-red genuinely accelerates flower initiation by 2–7 days in cannabis, replicating effects well-established in ornamentals. However, claims of 5–7 day reductions in total flowering time and 5–20% yield bumps are largely vendor marketing without peer-reviewed cannabis-specific support. The real value is faster transition to flower and cheap, low-risk implementation. Don't expect dramatic yield gains.
What It Is
Far-red flower initiation is a lighting technique in which photoperiod cannabis plants receive a short pulse of far-red light (peak ~730 nm) immediately after the main grow lights turn off during the 12/12 flowering cycle.
The technique exploits the phytochrome system, a family of plant photoreceptors that exists in two interconvertible forms: Pr (absorbs red light ~660 nm) and Pfr (absorbs far-red light ~730 nm). Pfr is the biologically active form. Strong evidence[1][2]
When standard white or red-heavy grow lights turn off, the plant retains a high Pfr:Pr ratio. During the dark period, Pfr slowly converts back to Pr (a process called dark reversion). A 10–15 minute pulse of 730 nm light at lights-off rapidly drives Pfr → Pr, effectively extending the plant's perception of the dark period before its circadian system registers 'long night.' This accelerates the flowering signal. Strong evidence[3]
Important distinction: This is not the same as using far-red light during the photoperiod to exploit the Emerson enhancement effect (which boosts photosynthesis efficiency). This article is specifically about end-of-day (EOD) far-red for flowering signaling.
Why Growers Use It
Three commonly cited reasons:
1. Faster transition into flower Plants exposed to EOD far-red typically show pistils and flower initiation 2–7 days earlier than controls under the same 12/12 schedule. Strong evidence[4] This effect is well-established in short-day ornamental crops like chrysanthemum and transferable to cannabis.
2. Shorter total flowering time Vendor marketing frequently claims 5–7 day reductions in total flower-to-harvest time. However, peer-reviewed cannabis-specific evidence for this claim is weak. Most data comes from in-house manufacturer trials without independent replication or published controls. Weak / limited
3. Yield increase Lighting companies often advertise 5–20% yield gains. The peer-reviewed cannabis literature does not support a specific yield benefit. No data Treat these claims as marketing folklore until a controlled study is published.
Legitimate side benefit: The technique is inexpensive, low-risk when dosed correctly, and easy to retrofit into an existing room without major infrastructure changes.
When to Start
Begin on day 1 of the 12/12 flip. The phytochrome-mediated decision to commit to flowering occurs during the first few long dark periods, so this is when EOD far-red has the most signaling leverage.
Most growers run far-red for the first 2–3 weeks of flower (through the stretch phase) and then stop once flowering is clearly established. Some continue throughout the entire flower cycle. There is no strong evidence that continuation past initial stretch provides additional benefit for initiation specifically, though anecdotal reports suggest subjective improvements in flower size from ongoing use. Anecdote Without side-by-side comparisons, these claims are confounded with other variables.
Critical: Do not run EOD far-red during vegetative growth on photoperiod plants. Far-red light signals 'longer night' to the plant, which can trigger premature flowering or excessive stem elongation (shade-avoidance response). Strong evidence[2]
How to Do It (Step-by-Step)
Step 1: Source the correct hardware Use a dedicated far-red bar or strip with peak emission at 730 nm. Many fixtures marketed as 'deep red' or 'far-red' actually peak at 660 nm, which is the wrong waveband. Red light (660 nm) activates Pfr and will work against your goal. Confirm the spectrum chart from the manufacturer before purchase.
Step 2: Size the fixture to your canopy Target approximately 10–20 µmol/m²/s of far-red photon flux density (PFD) at the canopy level. This is a signaling dose, not a photosynthesis dose, so high intensity is unnecessary and risks excessive stretch. One small far-red bar typically covers a 4×4 ft growing area. Weak / limited[3]
Step 3: Wire it to a timer Connect the far-red fixture to a separate timer set to turn on at the exact moment your main lights turn off. Run the far-red for 10–15 minutes, then switch off. The plant then enters true darkness for the remainder of the 12-hour night.
Step 4: Verify complete darkness after far-red shuts off After the pulse ends, the room must be completely dark. Light leaks from LED indicators, exhaust fans, or door gaps will reconvert Pr back to Pfr and undermine the technique. Use blackout tape or relocate leak sources.
Step 5: Run daily for the first 2–3 weeks of flower Consistency matters. Same dose, same 10–15 minute window, every single night during this critical initiation window.
Step 6: (Optional) Run a control for comparison If you have multiple tents or rooms, leave one tent without far-red and document the onset of visible flowering, stretch height, and final yield. This is the only reliable way to determine whether the technique works for your specific genetics and environment.
Common Mistakes
Using 660 nm instead of 730 nm These wavelengths trigger opposite signals. Red light (660 nm) accumulates Pfr (the flowering inhibitor); far-red (730 nm) depletes it. Verify your fixture's peak wavelength before deployment.
Running the pulse too long A 15-minute pulse is sufficient for the signaling effect. Running far-red for hours triggers excessive shade-avoidance response, leading to tall, leggy plants with weak stems and structural problems. Strong evidence[2]
Using EOD far-red during vegetative growth Will cause unwanted stretch and may trigger premature flowering signals in photoperiod plants.
Light leaks during the dark period Any stray light after the far-red pulse shuts off will undo the effect by reconverting Pr back to Pfr. Seal the room thoroughly.
Expecting 20%+ yield gains Marketing claims often vastly exceed the published evidence. The realistic outcome is 2–7 day faster initiation and tighter control of flowering timing. If a vendor promises dramatic yield bumps without publishing controlled data, remain skeptical.
Changing variables simultaneously If you add far-red the same week you change nutrients, CO₂, or genetics, you cannot isolate what caused any observed difference. Test one variable at a time.
Related Techniques
12/12 Flowering Light Schedule — the foundational photoperiod timing that far-red works alongside.
Light Leaks During Flower — explains why total darkness during the dark period is critical for any photoperiod-based signaling.
Flowering Stage — general overview of the flowering phase and its stages.
Fast-Flowering Photoperiods — alternative methods to accelerate flowering onset without spectrum manipulation.
Why Plants Stretch in Flower — explains shade-avoidance response, which excessive far-red can trigger.
Sources
- Peer-reviewed Smith, H. (2000). Phytochromes and light signal perception by plants—an emerging synthesis. Nature, 407(6804), 585–591.
- Peer-reviewed Franklin, K. A., & Quail, P. H. (2010). Phytochrome functions in Arabidopsis development. Journal of Experimental Botany, 61(1), 11–24.
- Peer-reviewed Craig, D. S., & Runkle, E. S. (2013). A moderate to high red to far-red light ratio from light-emitting diodes controls flowering of short-day plants. Journal of the American Society for Horticultural Science, 138(3), 167–172.
- Peer-reviewed Runkle, E. S., & Heins, R. D. (2001). Specific functions of red, far red, and blue light in flowering and stem extension of long-day plants. Journal of the American Society for Horticultural Science, 126(3), 275–282.
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