Light Cycle for Vegetative Growth (18/6)

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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 is the most common photoperiod for keeping cannabis in vegetative growth indoors.

Cannabis is a short-day plant: photoperiod-sensitive cultivars initiate flowering when the dark period exceeds a critical threshold. For most photoperiod cultivars, this threshold is roughly 11–13 hours of continuous darkness Strong evidence^[1][2]. Any schedule well short of that—18/6, 20/4, or 24/0—keeps the plant in vegetative mode. 18/6 sits comfortably below the flowering trigger with margin to spare.

Note: This applies only to photoperiod cultivars. Autoflowering Cannabis Cultivation flower based on age regardless of light schedule.

Why growers use it

Three practical reasons:

1. Adequate light for vigorous growth — 18 hours of light is sufficient daily light integral (DLI) for vigorous vegetative growth in most indoor setups, assuming adequate light intensity Strong evidence^[3]. See DLI Targets by Growth Stage for stage-specific guidance.

2. Lower power and heat than 24/0 — Six hours of darkness cuts electricity use by 25% versus continuous light and gives the grow room time to cool. This matters for heat management, especially in summer or tightly sealed spaces.

3. Consistency and convention — 18/6 is reliable and well-tested across thousands of home and commercial grows. It is not a magic number, but a proven default that requires no unusual equipment or expertise.

Whether plants benefit from a "dark rest period" is overstated in grower folklore. Plants respire continuously in both light and darkness as part of normal metabolism; neither light nor darkness is metabolically special. Strong evidence^[4] The real question is whether 24/0 produces measurably better growth to justify the extra electricity cost—controlled studies comparing 18/6 to 24/0 have reported similar outcomes Weak / limited^[3], with no clear yield advantage to continuous light. Most growers choose 18/6 for cost and simplicity, not because it produces superior results.

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 approach works; choose based on power cost and room temperature.

Stop: When you are ready to induce flowering. The standard trigger is switching to 12/12 Flowering Light Schedule for Cannabis when plants have reached roughly 50–66% of the desired harvest height. Most cultivars will roughly double in height during the flowering stretch, though this varies with genetics, training, and nutrient regime.

Typical veg durations on 18/6:

• Rooted clones: 2–4 weeks
• Seedlings: 4–8 weeks
• SOG During Vegetative Stage: 7–14 days (minimal veg before flip)
• ScrOG (screen of green): 4–8+ weeks while filling the screen

These are guidelines, not prescriptions. Cultivar vigor, light intensity, and your target plant size will all shift these timelines.

How to set up 18/6 (step by step)

1. Choose your 'lights on' time Many growers schedule lights to turn on in the evening and off mid-afternoon (e.g., 8 PM to 2 PM) so the dark period falls during the hottest part of the day, reducing cooling load. Others prefer daytime-aligned schedules. Choose based on your room temperature and electricity rate (if time-of-use billing applies).

2. Install and set a timer Use a mechanical or digital timer rated for your grow light's electrical load. For high-powered HID or LED systems, verify the timer's maximum wattage rating to avoid fire hazard. Set the timer to 18 hours on, 6 hours off. If using a digital timer, double-check AM/PM settings and verify the timer activates at your intended on-time.

3. Ensure the dark period is truly dark Light-seal your tent or room. Small light leaks during veg on 18/6 are generally not problematic—the 6-hour dark period is well below the flowering trigger. However, establishing good light discipline now will reduce headaches when you flip to 12/12 Flowering Light Schedule for Cannabis, where light leaks can disrupt flowering Strong evidence^[1].

4. Set light height and intensity for the stage For veg, aim for a photosynthetic photon flux density (PPFD) of roughly 300–600 µmol/m²/s at the canopy, increasing as plants grow Strong evidence^[3]. See Light Height by Stage for fixture-specific guidance and DLI Targets by Growth Stage for stage-specific targets.

5. Maintain growing environment Target temperatures of ~22–28°C (72–82°F) during the lights-on period, with relative humidity 55–70% for young plants tapering to 40–60% as plants mature. See VPD by Growth Stage for humidity and temperature targets that optimize gas exchange.

6. Do not change the schedule mid-veg Consistency matters more than the exact hours. Pick 18/6, 20/4, or 24/0 and stick with it until you flip to flower. Frequent photoperiod changes can stress plants and disrupt growth.

Common mistakes

Timer set incorrectly — Digital timers with AM/PM are a frequent source of error. Verify the timer by checking lights at the intended on-time, and confirm the off-time is 18 hours later. A common mistake is setting 18 hours on and forgetting to verify AM/PM, resulting in an unintended 24-hour or 12-hour cycle.

Light leaks overlooked — An 18/6 schedule is forgiving, but new growers who become lax about light-sealing during veg often find Light Leaks During Flower derail flowering later. Get in the habit now of checking for and sealing small gaps.

Confusing 18/6 with a yield technique — 18/6 is a baseline schedule, not a yield optimizer. Yield is driven by genetics, light intensity, VPD (Vapor Pressure Deficit) Basics, nutrients, and training. Tweaking veg hours from 18 to 20 will not increase final yield if other factors are suboptimal.

Running 24/0 unnecessarily — Some growers assume "more light equals more growth" and run 24/0 despite higher electricity costs. Controlled studies report similar or slightly better growth on 18/6, with no yield advantage to justify 33% higher power consumption Weak / limited^[3].

Vegging too long — Plants on 18/6 continue growing indefinitely. New growers often veg longer than planned, resulting in plants that outgrow the tent or canopy during the flowering stretch. Start with a veg target (e.g., 4 weeks for seedlings) and flip on schedule.

Ignoring light intensity — Running 18/6 on insufficient light (e.g., <200 µmol/m²/s) will not support vigorous growth. 18 hours of dim light is worse than 12 hours of bright light. Prioritize adequate intensity for your stage before optimizing photoperiod.

Related techniques and alternatives

12/12 Flowering Light Schedule for Cannabis — The photoperiod you switch to when you want to induce flowering in photoperiod cultivars.

20/4 and 24/0 — Alternative veg schedules. Studies report roughly equivalent results to 18/6 in most reports Weak / limited^[3]; choose based on power costs, room temperature, and personal preference.

Autoflowering Cannabis Cultivation — Many auto growers run 18/6 or 20/4 from seed to harvest, since autoflowers flower on age, not photoperiod.

Gas lantern routine (12/5.5) — A veg schedule of 12 hours continuous light, then dark with a short light interruption mid-darkness. Claimed to save power while maintaining vegetative growth; evidence for equivalence to 18/6 is mixed and inconsistent Weak / limited.

Outdoor seasonal photoperiod — Outdoors, daylight length naturally keeps plants in veg through spring and summer and triggers flowering as nights lengthen in late summer and fall.

Sources

How this page was made

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• 12/12 Flowering Light Schedule — The standard light schedule to trigger flowering in photoperiod cannabis indoors.
• 20/4 Vegetative Light Schedule — Running lights 20 hours on, 4 hours off during veg to prevent flowering and manage heat an...
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• DLI Targets by Growth Stage — Light dose targets by stage, from clones through harvest maturity.
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• Light Leaks During Flower — Uninterrupted darkness during flower keeps plants on track; even small leaks can derail fl...
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