12/12 Flowering Light Schedule

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What 12/12 actually is

A 12/12 light schedule means cannabis plants receive 12 hours of light and 12 hours of uninterrupted darkness per 24-hour cycle. In photoperiod cannabis (Cannabis sativa L.), flowering is triggered by the length of the dark period, not the length of the day — the plant measures darkness using the phytochrome pigment system ^[1][2] Strong evidence.

Cannabis is a short-day (long-night) plant. Under long days (typically 18+ hours of light), it stays in vegetative growth. Once the dark period reaches a critical threshold — approximately 11 hours for most photoperiod cultivars — the plant shifts hormonally and begins producing flowers ^[2][3] Strong evidence.

12/12 is simply a convenient, reliable schedule that sits comfortably past that threshold. It isn't the only option; 13/11 or 14/10 also flower most cultivars. 12/12 is the convention because it's symmetrical, easy to time, and well-established as the industry standard.

Why growers use it

1. It triggers flowering on demand. Indoors, there is no natural seasonal change, so the grower simulates autumn by shortening the day. This extended dark period signals the plant to begin reproductive growth Strong evidence.

2. It's predictable. Most cultivar breeders document flowering times based on a 12/12 flip — for example, "8 weeks flowering" or "10 weeks." Using the same schedule lets you compare strain data meaningfully and plan harvests ^[4] Strong evidence.

3. It balances energy use and yield. Shorter schedules (e.g., 10/14) save electricity but reduce photosynthesis time and can weaken yields. Longer schedules (e.g., 13/11) risk reverting some cultivars or diluting the flowering signal. 12/12 is the well-tested middle ground.

Note on autoflowers: Ruderalis hybrid autoflowers flower based on age, not photoperiod, and are usually kept on 18/6 or 20/4 from seed to harvest ^[5] Strong evidence. Putting autoflowers on 12/12 cuts their photosynthesis by nearly half for no biological reason.

When to start the flip

The most common rule of thumb: flip to 12/12 when plants are roughly half the final height you want. Photoperiod cannabis typically increases in height during the first 2–4 weeks of flowering — a phase called the stretch — though the magnitude varies by cultivar and growing conditions.

Practical timing factors:

• Tent or room height: In a 5-foot tent with light fixture occupying roughly a foot, flip when plants are around 18–24 inches tall.
• Cultivar characteristics: Plants with more sativa genetics tend to stretch more; those with more indica genetics tend to stretch less. Note that indica/sativa labels are a poor predictor of chemistry but remain useful shorthand for growth patterns among experienced growers.
• Training method: If using topping, low-stress training (LST), or ScrOG, you may veg longer to fill the canopy before flipping.
• From seed: Most photoperiod plants need at least 3–4 weeks of vegetative growth before they're mature enough to flower well, though they can technically be flipped earlier at reduced yields.

How to do it, step by step

1. Set up a reliable timer. Use a mechanical or digital timer on your light fixture, set to 12 hours on, 12 hours off. Do not manually switch — consistency is critical, and human error is inevitable.

2. Choose your "lights on" time. Many indoor growers run lights at night (e.g., 8 PM to 8 AM) so heat output coincides with cooler outdoor hours. The plant does not care which 12 hours you choose, as long as the schedule is consistent.

3. Seal the room for the dark period. During the 12-hour darkness, the space must be light-tight. Even brief light interruptions during the dark cycle can stress the plant and, in some cases, trigger hermaphroditism or reversion to vegetative growth ^[2][7] Strong evidence. Check for LED indicator lights on equipment, gaps around tent zippers, and light leaks under doors and vents.

4. Flip the switch. Move from your veg schedule (commonly 18/6 or 20/4) directly to 12/12. There is no need to taper gradually — abrupt transition is standard practice and is what most commercial growers use.

5. Adjust environmental targets. Many growers lower temperatures slightly (to roughly 20–26 °C / 68–79 °F during the light period) and reduce humidity to 40–50% relative humidity to suppress bud mold and powdery mildew ^[8] Strong evidence. Use a thermometer and hygrometer to monitor.

6. Hold 12/12 until harvest. Do not change the schedule mid-flower. Continue until trichomes shift from clear to your preferred level of cloudiness or amber Weak / limited.

Common mistakes

• Light leaks during the dark period. This is the single most common cause of 12/12 failure. Even small leaks can cause stress, male flower formation ("nanners") on female plants, and seedy buds. Check your setup with lights off and eyes adjusted to darkness Strong evidence.
• Flipping too late. Plants outgrow the tent, get too close to the light, and develop light bleaching or airflow problems that encourage mold.
• Flipping too early. Tiny plants that flower early produce tiny yields. Once flowering has started, vertical growth opportunity is limited to the stretch phase.
• Putting autoflowers on 12/12. This cuts their photosynthesis by nearly half with no benefit.
• Interrupting the dark cycle to check plants. If you must enter the room, use a dim red or green headlamp — cannabis is less sensitive to these wavelengths during the critical dark period ^[1] Weak / limited.
• Believing exotic finishing schedule claims. Folklore circulates about 11/13, 10/14, or other non-standard schedules allegedly boosting resin or flavor. There is no controlled evidence supporting these claims No data.

Related techniques and schedules

• Light Cycle for Vegetative Growth (18/6): The standard vegetative counterpart to 12/12.
• ScrOG (Screen of Green): A training method in which the flip to 12/12 is timed to fill a horizontal canopy net.
• Light Deprivation: The outdoor or greenhouse equivalent — using tarps to force a 12/12 cycle during natural long days.
• Autoflower Cultivation: Skips the flip entirely; plants flower by age.
• Fast-Flowering Photoperiods: Alternative schedules (e.g., 11/13) marketed as faster, with limited evidence.
• Light Leaks During Flower: Detailed troubleshooting for light-leak problems.

Sources

How this page was made

Related

• Autoflowering Cannabis Cultivation — Age-based flowering cannabis for faster harvests and simplified growing.
• Light Bleaching — Pale, white-tipped buds caused by excessive light intensity; does not increase potency.
• Flowering Stage — The cannabis life phase when plants produce resinous flower buds and cannabinoids.
• Light Cycle for Vegetative Growth (18/6) — The most common photoperiod for keeping cannabis in vegetative growth indoors.
• Light Deprivation Automation — Automate blackout tarps to force flowering and harvest multiple crops per year.
• Light Leaks During Flower — Uninterrupted darkness during flower keeps plants on track; even small leaks can derail fl...