Adding Far-Red at Flower Initiation

What It Is

Far-red flower initiation is the practice of exposing photoperiod cannabis to a short burst 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 exist in two interconvertible forms: Pr (absorbs red ~660 nm) and Pfr (absorbs far-red ~730 nm). Pfr is the biologically active form that suppresses flowering in short-day plants. Strong evidence^[1][2]

White/red-heavy grow light leaves the plant with a high Pfr:Pr ratio at lights-off. The plant then spends part of the dark period slowly converting Pfr back to Pr (a process called dark reversion) before the night-length signal can fully register as 'long night.' A 5-15 minute pulse of 730 nm light at lights-off rapidly drives Pfr → Pr, effectively making the dark period feel longer to the plant. Strong evidence^[1][3]

This is sometimes confused with using far-red during the photoperiod to exploit the Emerson enhancement effect (boosting photosystem efficiency). That is a separate, distinct application. This article is specifically about end-of-day (EOD) far-red for flowering signaling.

Why Growers Use It

Three commonly cited reasons, with varying levels of evidence:

1. Faster transition into flower. Plants exposed to EOD far-red typically show pistils and flower sites 2-7 days earlier than control plants under the same 12/12 schedule. This is well-documented in short-day ornamental crops like chrysanthemum and is plausible in cannabis. [evidence:strong in ornamentals; weak in cannabis]^[1][4]

1. Shorter total flowering time. If initiation is accelerated, the harvest window may move up. Vendor marketing often claims 5-7 days off finish time. Independent cannabis-specific peer-reviewed confirmation is sparse. Weak / limited^[5]

1. Yield increase. Lighting manufacturers frequently advertise 5-20% yield gains. The peer-reviewed cannabis literature does not currently support a specific number, and most claims trace back to in-house trials without controls or replication. Treat this as folklore until proven otherwise. Disputed

A legitimate side benefit: the technique is cheap, low-risk if dosed correctly, and easy to add to an existing room.

When to Start

Begin on day 1 of the 12/12 flip. The phytochrome signaling that determines whether the plant commits to flowering happens during the first few long nights, so this is when EOD far-red has the most leverage.

Many growers run far-red for the first 2-3 weeks of flower, then stop once flowering is clearly established. Others continue throughout flower. There is no strong evidence that continued use past stretch provides additional benefit for initiation — though some growers report subjective improvements in flower size from continued use, likely confounded with other variables. Anecdote

Do not run EOD far-red during vegetative growth on photoperiod plants — you risk triggering premature flowering or excessive stem elongation (shade-avoidance response). Strong evidence^[2]

How to Do It (Step-by-Step)

Step 1: Get the right hardware. Use a dedicated far-red bar or strip with peak emission at 730 nm. Avoid 'deep red' fixtures peaking at 660 nm — that's the wrong waveband and will prevent the effect you want. Confirm the spectrum chart from the manufacturer.

Step 2: Size the fixture to the canopy. Target roughly 10-20 µmol/m²/s of far-red PFD at the canopy. You don't need high intensity; this is a signaling dose, not a photosynthesis dose. One small bar typically covers a 4x4 ft tent. Weak / limited^[3]

Step 3: Wire it to a timer. Set the far-red fixture to turn on at the moment the main lights turn off, and run for 10-15 minutes. Then it shuts off and the plant enters true darkness for the remainder of the 12-hour night.

Step 4: Verify the dark period is truly dark. After the far-red shuts off, the room must be light-tight. Any light leaks (LED indicators, door cracks) can undo your effort by reconverting Pr back to Pfr.

Step 5: Run it daily for the first 14-21 days of flower. No need to ramp up or down. Same dose, same timing, every night.

Step 6: Compare against a control if you can. If you run multiple tents or rooms, leave one without far-red and document flowering onset, stretch, and final yield. This is the only way to know if it works for your genetics and setup.

Common Mistakes

• Using 660 nm instead of 730 nm. These are completely different signals. Red light activates Pfr (the flowering inhibitor); far-red deactivates it. Verify your fixture's peak wavelength.
• Running it too long. A 15-minute pulse is plenty. Hours of far-red exposure can trigger excessive stretch via shade-avoidance, leading to leggy plants and structural problems. Strong evidence^[2]
• Running it during the photoperiod with veg plants. Will cause stretch and may trigger unwanted flowering signals.
• Light leaks during the dark period. Defeats the entire mechanism.
• Expecting miracle yield gains. The realistic benefit is faster initiation and slightly tighter timing. If a vendor promises 25%+ yield bumps, ask for the controlled study. They usually don't have one.
• Stacking with unrelated changes. If you add far-red the same week you switch nutrients or change genetics, you'll never know what did what.

Related Techniques

• UV-B Supplementation in Late Flower — another spectrum-tweaking approach with mixed evidence.
• Light Deprivation Greenhouse Flowering — uses physical darkness rather than spectrum to trigger flower.
• The 12/12 Flip — the foundational photoperiod schedule far-red works alongside.
• Emerson Enhancement Effect — using far-red during the day to boost photosynthesis, a separate use case from EOD signaling.
• Phytochrome and Plant Photomorphogenesis — the underlying science.

Sources

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