20/4 vs 18/6 in Veg

What it is

A vegetative light schedule defines how many hours per day a photoperiod cannabis plant receives light versus darkness. The two most common indoor veg schedules are 18 hours on / 6 hours off (18/6) and 20 hours on / 4 hours off (20/4). Some growers also run 24/0 (continuous light), but that is a separate debate covered elsewhere.

Cannabis is a short-day plant: it stays vegetative as long as the dark period is short enough. Any schedule with a dark period under roughly 10–11 hours will generally keep a photoperiod plant in veg ^[1][2]. Both 18/6 and 20/4 sit comfortably in that range. Autoflowering varieties do not depend on photoperiod and can be run on either schedule based on grower preference.

Why growers use it

The case for 20/4: More daily light integral (DLI) at the same fixture intensity means more total photons hitting the canopy, which generally correlates with faster vegetative growth in cannabis up to a saturation point ^[3]. Growers in a hurry to fill a canopy or finish a perpetual cycle sometimes prefer this.

The case for 18/6: A longer dark period gives plants time to translocate sugars, conduct dark respiration, and reset stress responses Weak / limited. Practically, 18/6 also:

• Cuts electricity use by ~10% versus 20/4.
• Cuts heat load, which matters in small tents or hot climates.
• Gives the grower a predictable lights-off window for inspecting for pests like spider mites and for IPM sprays that shouldn't hit lit foliage.

What the evidence says: Controlled studies directly comparing 18/6 and 20/4 in cannabis are sparse. Most cannabis lighting research focuses on DLI and intensity rather than photoperiod within the vegetative range ^[3][4]. Claims that 20/4 produces dramatically faster or larger plants are largely Anecdote. In horticulture more broadly, extending photoperiod beyond ~16–18 hours shows diminishing returns for many C3 plants Weak / limited.

When to start

Seedlings and fresh clones don't need a long photoperiod to root or establish. Many growers start them under 18/6 from day one because the dark period reduces transpiration stress on plants that don't yet have functional root systems.

A reasonable default:

• Seedlings (days 1–10): 18/6 under low intensity (200–400 µmol/m²/s PPFD).
• Established veg (week 2 to flip): Stay on 18/6, or switch to 20/4 if you want to push growth and your environment can handle the heat.
• Last 24–48 hours before flip: Some growers give 36–48 hours of continuous darkness before switching to 12/12 to sharpen the flowering signal. Evidence for this is Anecdote; it doesn't hurt but isn't necessary.

How to do it (step-by-step)

1. Pick a schedule and commit. Don't flip back and forth between 18/6 and 20/4 mid-veg. Consistency matters more than the specific number.
2. Set a reliable timer. A mechanical timer works; a digital or smart timer (e.g., a Wi-Fi smart plug with a schedule) is better because it survives brief power blips. Verify it's actually firing at the right times for the first 48 hours.
3. Align the dark period with your cooling strategy. If your grow room runs hot, schedule lights-off during the hottest part of the day so the AC isn't fighting both sun and lamps. If your room runs cold, do the opposite.
4. Match intensity to stage. Schedule alone doesn't grow plants — DLI does. Target roughly 20–40 mol/m²/day during veg ^[3]. Use a PAR meter or your fixture's manufacturer chart to dial PPFD.
5. Don't interrupt the dark period. Even brief light leaks under 12/12 can cause problems, and while veg is more forgiving, getting in the habit of a clean dark cycle now pays off later ^[1].
6. Track it. Note the schedule, room temps, and growth rate in a log. If you ever want to A/B test 18/6 vs 20/4 in your own room, this is how you'd actually know.

Common mistakes

• Assuming 20/4 will fix slow growth. If your plants are slow under 18/6, the problem is almost always nutrients, root health, VPD, or light intensity — not the schedule.
• Running 20/4 in a hot tent. The extra two hours of heat can push leaf surface temps into stress territory, undoing any growth benefit.
• Flipping clones straight to 20/4 under high PPFD. Fresh cuts have no roots; they want lower light and a real dark period.
• Ignoring the dark period for pest checks. Spider mites and powdery mildew often show first during the hours you're not in the room. 18/6 gives you a natural inspection window during lights-off (with a green headlamp).
• Treating schedule as a yield lever. Genetics, environment, and intensity are the levers. Photoperiod within veg is a minor knob.

Related techniques

• 24/0 Continuous Light in Veg — the more aggressive cousin of 20/4.
• DLI for Cannabis — the metric that actually predicts veg growth rate.
• Flipping to Flower (12/12) — what comes after veg.
• Autoflower Light Schedules — why photoperiod rules differ for autos.
• VPD in Veg — usually a bigger factor than light schedule.

Sources

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