18/6 means 18 hours of light followed by 6 hours of uninterrupted darkness every 24 hours. It is the most common vegetative schedule for photoperiod cannabis: the long day keeps plants in veg, and the dark period gives a natural break that some growers use to lower room temperature and electricity costs at off-peak times.
In practice
At 600 µmol/m²/s, 18/6 delivers about 39 mol/m²/day (600 × 18 × 0.0036). Many growers also run autoflowers on 18/6 or 20/4; autoflowers flower on age, not day length, so the schedule there is a trade-off between DLI and power cost.
18/6 is a convention, not a biological requirement. Cannabis grows fine on 24/0 in veg, and the GenetixBay house position endorses it. Some growers simply prefer 18/6 for cheaper power, cooler nights or a fixed maintenance window. Either works if the DLI is adequate.
Light leaks during the 6 dark hours do not matter much in veg. They matter in flower.
Units and formula
DLI = PPFD × 18 × 0.0036 on 18/6.
Don't confuse with
- 12/12: the flowering schedule.
- 20/4: a common alternative for autoflowers and veg, giving more DLI.
Myth check
"Plants need 6 hours of sleep in veg" is a Myth for cannabis; see the 24/0 entry and the photoperiod article.
