13/11 means 13 hours of light and 11 hours of darkness. For most drug-type cultivars, 11 hours of uninterrupted darkness is still long enough to trigger and hold flowering, and the extra hour adds about 8% more daily light than 12/12 at the same PPFD.
In practice
At 900 µmol/m²/s, 13/11 gives about 42 mol/m²/day against 39 on 12/12. Many growers run it as roughly 12 hours at full intensity plus ramps at each end: a 30-minute sunrise and 30-minute sunset add time without adding much DLI, but ease the transition and match the founder's practice.
Peterswald and colleagues (2023) tested photoperiods longer than 12 hours in medicinal cannabis and found that some cultivars yielded well on them; Zhang and colleagues (2021) showed critical day length differs between cultivars. Long-flowering, equatorial-type lines may delay or stall on longer days, so watch the first two weeks: if pistils are slow to appear, drop to 12/12.
Units and formula
DLI = PPFD × 13 × 0.0036.
Don't confuse with
- 12/12: the conservative default.
- Light leak: unplanned light in the dark period; 13/11 is planned and dark periods must stay dark.
Myth check
GenetixBay house position: flower does not have to be exactly 12/12. 13/11 is endorsed, and some cultivars flower at 14–15 hours. Supported by cultivar-variation studies; the house says "test your cultivar" and members can log results through Mythbusters.
