Why does almost everyone flower at 12/12?
Photoperiod cannabis triggers flowering when nights get longer: it needs an uninterrupted dark period long enough to fire the hormonal signal that flips the plant from veg to bloom. The 12/12 schedule (12 hours light, 12 hours dark) is the historical standard because it reliably triggers that signal in virtually any strain. It's safe and consistent — but 'safe' doesn't always mean 'optimal.'
What did the 13/11 study actually find?
Ahrens, Llewellyn & Zheng (2024, Plants) ran two high-THC cultivars under both 12/12 and 13/11 (one extra hour of light, one less of darkness) throughout the entire flower cycle. The results were hard to ignore:
- 'Incredible Milk': +35% dry flower weight under 13/11.
- 'Gorilla Glue': +50% dry flower weight under 13/11.
- Cannabinoids: equal or higher (one cultivar gained +9% THC equivalent; the other held steady). No potency loss.
Why does one extra hour of light drive that much yield?
Because what really drives yield is DLI — Daily Light Integral: the total photon load the plant receives each day. Going from 12 to 13 hours of light adds roughly 8% more daily photons, and the plant converts them into extra flower biomass. An 11-hour dark period still sustains flowering in the strains tested: the bloom signal holds, but the plant gets more time to photosynthesize.
What should you consider before flipping your whole room?
The study is upfront about its limits, and honesty matters more than the headline here:
- Strain-dependent: both cultivars responded, but at very different magnitudes. Not every strain will hit +50%.
- Can delay ripening by a few days (one cultivar showed roughly a 1.5-day delay in flower initiation) and may slow trichome turnover.
- Density trade-off: in one cultivar, the main cola lost 36% density — more total weight, but slightly looser buds.
- This was small-scale research plots: the exact numbers may not scale the same way in a large commercial room.
- Highly light-sensitive strains may still need classic 12/12 to flower cleanly — never go below 11 hours of darkness.
How do I test it without risking my whole harvest?
The smart way to adopt a change like this is to run a side-by-side on part of your canopy — not flip the whole room at once. Keep everything else identical (feeding, VPD, watering) so the only variable is that extra hour of light.
