12/12 Flowering Light Schedule

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 long nights, not short days — the plant measures the length of the dark period 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 crosses a critical threshold — around 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 magic; 13/11 or 14/10 also flower most cultivars. 12/12 is the convention because it's symmetrical, easy to time, and well past the trigger point.

Why growers use it

Three reasons:

1. It triggers flowering on demand. Indoors, there's no natural seasonal change, so the grower has to simulate autumn. Flipping to 12/12 tells the plant winter is coming and it needs to reproduce Strong evidence.

1. It's predictable. Most cultivar breeders document flowering times based on a 12/12 flip (e.g., "9 weeks flowering"). Using the same schedule lets you compare strain data meaningfully ^[4] Strong evidence.

1. It balances energy use and yield. Going shorter (e.g., 10/14) saves electricity but gives the plant less photosynthesis time. Going longer (e.g., 13/11) risks reverting some cultivars or weakening the flowering signal. 12/12 is the well-tested middle.

Note: autoflowering cannabis (ruderalis hybrids) flowers based on age, not photoperiod, and is usually kept on 18/6 or 20/4 from seed to harvest ^[5] Strong evidence. Don't put autoflowers on 12/12 unless you're trying to save power.

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 doubles in size — sometimes triples — during the first 2–4 weeks of flowering, a phase called the stretch [evidence:weak, widely reported by growers but not rigorously quantified in literature].

Practical timing factors:

• Tent height: In a 5-foot tent with a light fixture taking up a foot, flip when plants are around 18–24 inches.
• Cultivar: Sativa-leaning plants stretch more (sometimes 3×); indica-leaning plants stretch less (1.3–1.8×). The indica/sativa labels are a poor predictor of chemistry ^[6] Disputed, but they remain useful shorthand for stretch behavior among experienced growers.
• Training: If you're using topping, LST, or ScrOG, you may veg longer to fill the canopy first.
• From seed: Most photoperiod plants need at least 3–4 weeks of veg before they're mature enough to flower well, though they can be flipped earlier.

How to do it, step by step

1. Set up a reliable timer. A mechanical or digital timer on your light fixture, set to 12 hours on, 12 hours off. Don't manually switch — you will forget.

1. Pick your "lights on" time. Many indoor growers run lights at night (e.g., 8 PM to 8 AM) so the heat output coincides with cooler outdoor temperatures. The plant doesn't care which 12 hours.

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

1. Flip the switch. Move from your veg schedule (commonly 18/6) directly to 12/12. There's no need to taper — abrupt is fine and is what most commercial growers do.

1. Adjust environmental targets. Many growers lower temps slightly (to roughly 20–26 °C / 68–79 °F) and reduce humidity into flower (40–50% RH) to suppress bud mold ^[8] Strong evidence.

1. Hold 12/12 until harvest. Don't change the schedule mid-flower. Harvest when trichomes shift from clear to cloudy/amber per your preference ^[9] [evidence:weak — trichome color is correlated with cannabinoid maturity but is a rough proxy].

Common mistakes

• Light leaks during the dark period. The single most common 12/12 failure. Even a small leak can cause "nanners" (male flowers on female plants) and seedy buds Strong evidence.
• Flipping too late. Plants outgrow the tent, get too close to the light, and develop light burn or airflow problems.
• Flipping too early. Tiny plants flower into tiny yields. There's no rescue — once flowering starts, vertical growth is limited to the stretch.
• Putting autoflowers on 12/12. You'll cut their photosynthesis nearly in half for no biological reason.
• Interrupting the dark cycle to "check on" plants. Use a dim green headlamp if you must — cannabis is much less sensitive to green wavelengths for photoperiod signaling ^[1] [evidence:weak, frequently repeated; based on phytochrome absorption spectra rather than cannabis-specific studies].
• Believing "11/13 boosts resin" claims. There's no good controlled evidence for the various exotic finishing schedules circulating in grow forums No data.

Related techniques

• 18/6 Veg Schedule: The standard vegetative counterpart to 12/12.
• ScrOG (Screen of Green): Training method designed around timing the flip to fill a horizontal screen.
• Sea of Green (SoG): Flipping small plants to 12/12 almost immediately to maximize plants per square foot.
• Light Deprivation: The outdoor/greenhouse equivalent — using tarps to force a 12/12 cycle in summer.
• Autoflower Cultivation: Skips the flip entirely.
• Finishing schedule experiments (10/14, 11/13): Folklore-tier; not supported by controlled data No data.

Sources

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