Light Cycle for Vegetative Growth (18/6)

What 18/6 means

An 18/6 light cycle means 18 hours of light followed by 6 hours of uninterrupted darkness every 24 hours. It's the most common photoperiod for keeping cannabis in vegetative growth indoors.

Cannabis is a short-day (long-night) plant. Photoperiod-sensitive cultivars initiate flowering when the dark period exceeds a critical length — for most cultivars, somewhere around 11-13 hours of continuous darkness Strong evidence^[1][2]. Anything well short of that — 18/6, 20/4, 24/0 — keeps the plant in vegetative mode. 18/6 sits comfortably below the flowering threshold with margin to spare.

Note: this only applies to photoperiod cultivars. Autoflowering plants flower based on age regardless of light schedule.

Why growers use it

Three practical reasons:

1. Reliable vegetative growth. 18 hours of light is more than enough daily light integral (DLI) for vigorous growth in most setups, assuming adequate light intensity Strong evidence^[3].
2. Lower power and heat than 24/0. Six hours off cuts electricity use by 25% versus continuous light, and gives the room time to cool. Studies comparing 18/6 to 24/0 generally find similar or slightly better growth on 18/6, with no clear yield advantage to running lights 24 hours Weak / limited^[4].
3. Plants respire and process in the dark. Dark-period respiration is part of normal plant metabolism. While the claim that cannabis specifically "needs" rest is overstated, there's no evidence 24/0 produces better outcomes worth the extra cost Disputed.

The "18/6 is optimal" claim is more convention than proven optimum. It's a good default, not a magic number.

When to start (and stop)

Start: As soon as seedlings have their first true leaves, or immediately for rooted clones. Some growers run seedlings on 18/6 from germination; others use 24/0 for the first week or two. Either works.

Stop: When you're ready to flower. The standard trigger is switching to 12/12 light cycle when plants have reached roughly 50-66% of the height you want at harvest, because most cultivars will roughly double in height during the flowering stretch.

Typical veg durations:

• Clones: 2-4 weeks on 18/6
• Seedlings: 4-8 weeks on 18/6
• Sea of Green (SOG): as little as 7-14 days
• ScrOG: 4-8+ weeks while you fill the screen

How to do it (step by step)

1. Pick your "lights on" time. Many growers run lights at night (e.g., 8 PM to 2 PM) so the dark period falls during the hottest part of the day, reducing cooling load.
2. Set a timer. Use a mechanical or digital timer rated for your light's wattage. Set it to 18 hours on, 6 hours off. Double-check AM/PM settings on digital timers.
3. Verify the dark period is truly dark. Light-tight your tent or room. Small leaks during the dark period generally won't cause issues at 18/6 (the dark gap is well below the flowering trigger), but it's a good habit before you move to 12/12 Strong evidence^[1].
4. Set light height and intensity for the stage. For veg, a PPFD of roughly 300-600 µmol/m²/s at the canopy is a reasonable target, increasing as plants grow Strong evidence^[3].
5. Maintain environment. Aim for ~22-28°C (72-82°F) lights-on, with relative humidity 55-70% for young plants tapering down as they mature.
6. Don't change the schedule mid-veg. Consistency matters more than the exact hours. Pick 18/6 (or 20/4, or 24/0) and stick with it until you flip to flower.

Common mistakes

• Light leaks once you flip to 12/12. Not an 18/6 problem per se, but growers used to a forgiving veg schedule sometimes don't tighten up their dark period before flowering.
• Timer set wrong. Digital timers with AM/PM are a frequent culprit. Verify by checking lights at the expected on/off time.
• Confusing 18/6 with a yield technique. It's a baseline schedule. Yield is driven by genetics, light intensity, VPD, nutrients, and training — not by tweaking veg hours from 18 to 20.
• Running 24/0 because "more light = more growth." Evidence for a meaningful advantage is thin, and you pay 33% more for electricity versus 18/6 Weak / limited.
• Flipping to flower too late. Plants on 18/6 keep growing indefinitely. New growers often veg too long, end up with plants that outgrow the tent during stretch.

Related techniques and alternatives

• 12/12 flowering cycle — the schedule you switch to when you want to induce flowering.
• 20/4 and 24/0 — alternative veg schedules. Roughly equivalent results to 18/6 in most reports; pick based on power costs and heat Weak / limited.
• Autoflowering cultivars — flower on age, not photoperiod. Many auto growers run 18/6 or 20/4 from seed to harvest.
• Gas lantern routine (12/5.5) — a veg schedule of 12 hours on, then dark with a short light interruption mid-night. Claimed power savings; mixed evidence on growth equivalence Weak / limited.
• Outdoor seasonal photoperiod — outdoors, daylight length naturally keeps plants in veg through summer and triggers flowering as nights lengthen in late summer.

Sources

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