20/4 vs 18/6 in Veg
Two common vegetative light schedules: what works, what doesn't, and why the difference is smaller than growers think.
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Both 18/6 and 20/4 work fine for veg cannabis. 20/4 gives you ~11% more light but also more heat; 18/6 saves electricity and heat. The real growth driver is daily light integral (DLI) and fixture intensity, not the ratio itself. Claims that one schedule produces dramatically faster plants are mostly anecdote. Pick one, stick with it, and focus on DLI, nutrients, and environment instead.
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 topic.
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 Strong evidence[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 Strong evidence[3]. Growers in a hurry to fill a canopy or finish a perpetual cycle sometimes prefer this.
The case for 18/6: An 18-hour schedule:
- Cuts electricity use by approximately 10% versus 20/4.
- Reduces heat load, which matters in small tents or hot climates.
- Provides a predictable lights-off window for visual inspections and integrated pest management (IPM) sprays that shouldn't hit lit foliage.
- Gives the grower time to observe the plants under stable, dark conditions — important for detecting early pest damage like spider mite webbing.
What the research actually says: Controlled studies directly comparing 18/6 and 20/4 in cannabis are sparse Weak / limited[3][4]. Most cannabis lighting research focuses on DLI and light intensity rather than photoperiod within the vegetative range. In horticulture more broadly, many C3 plants show diminishing growth returns beyond 16–18 hours of daily light, though this threshold varies by species Weak / limited.
Claims that 20/4 produces dramatically faster or larger plants than 18/6 are largely Anecdote. The practical difference is usually marginal when DLI is held constant.
When to start
Seedlings and fresh clones do not require 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 do not yet have functional root systems.
A practical approach:
- Seedlings (days 1–10): 18/6 under low intensity (200–400 µmol/m²/s PPFD).
- Established veg (week 2 until flower switch): Stay on 18/6, or switch to 20/4 if you want to accelerate growth and your environment can handle the extra heat.
- Optional: dark period before flip: Some growers run 36–48 hours of continuous darkness before switching to 12/12 to sharpen the flowering signal. Evidence for this practice is Anecdote; it does not harm the plant but is not necessary.
How to do it (step-by-step)
- Pick a schedule and commit. Do not flip between 18/6 and 20/4 mid-veg. Consistency in photoperiod is more important than the specific ratio.
- Set a reliable timer. A mechanical timer works adequately; a digital or smart timer (e.g., a Wi-Fi smart plug with a repeating schedule) is preferable because it survives brief power interruptions. Verify the timer is firing at the correct times for the first 48 hours.
- Align the dark period with your cooling and heating strategy. If your grow room runs hot, schedule lights-off during the hottest part of the day so your AC is not competing against both ambient heat and lamp heat. If your room runs cold, schedule lights-off during the coldest part of the day.
- Match light intensity to plant stage. Photoperiod alone does not determine growth — daily light integral (DLI) does. Target approximately 20–40 mol/m²/day during veg Strong evidence[3]. Use a PAR meter or your fixture manufacturer's specifications to dial in photosynthetic photon flux density (PPFD).
- Keep the dark period uninterrupted. Even brief light leaks during the dark period are less critical during veg than during flower, but establishing a clean dark cycle early is good practice Strong evidence[1].
- Track your results. Log the schedule, room temperature, humidity, and relative growth rate. If you later want to A/B test 18/6 versus 20/4 in your own environment, baseline data will tell you whether the change actually made a difference.
Common mistakes
- Assuming 20/4 will fix slow growth. If plants are slow under 18/6, the cause is usually low nutrient availability, root problems, poor vapor pressure deficit (VPD), or insufficient light intensity — not the dark period. See VPD (Vapor Pressure Deficit) Basics for more detail.
- Running 20/4 in a hot tent. The extra two hours of lamp heat can push leaf surface temperatures into stress range, eliminating any potential growth benefit and sometimes causing heat damage.
- Switching clones to 20/4 under high PPFD immediately. Fresh cuttings have no roots and cannot uptake water efficiently. They need lower light intensity and a proper dark period until they establish roots (typically 7–14 days).
- Ignoring the dark period for pest and disease monitoring. Spider mites, Powdery Mildew Prevention, and other pests often appear during unobserved hours. 18/6 gives you a natural lights-off window to inspect under a green headlamp without disturbing the photoperiod.
- Treating photoperiod as a major yield lever. Genetics, environment (temperature, humidity, CO₂), light intensity, and DLI are the primary growth drivers. Photoperiod within the veg range is a secondary variable.
Related techniques
- Continuous light (24/0) — a more aggressive extension of 20/4, with no dark period at all.
- Autoflowering Cannabis Cultivation — why autoflowers do not respond to photoperiod.
- Daily light integral (DLI) — the metric that actually predicts veg growth rate and yield potential.
- Transitioning to flower (12/12) — the photoperiod change that triggers flowering.
- VPD (Vapor Pressure Deficit) Basics — often a bigger lever on veg growth than light schedule.
Sources
- Peer-reviewed Moher, M., Jones, M., & Zheng, Y. (2021). Photoperiodic response of in vitro Cannabis sativa plants. HortScience, 56(1), 108–113.
- Peer-reviewed Zhang, M., Anderson, S. L., Brym, Z. T., & Pearson, B. J. (2021). Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science, 12, 694153.
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D., & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science, 12, 646020.
- Peer-reviewed Eaves, J., Eaves, S., Morphy, C., & Murray, C. (2020). The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal, 112(2), 1466–1470.
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