Short answer
Give cannabis seedlings 18/6 at about 150–300 µmol/m²/s, a DLI of 10–19 mol/m²/day. 24/0 also works (house position: no sleep period needed in veg), as long as you lower intensity to keep the same daily dose. Grower practice.
Why
What the seedling actually counts is the total daily light, the DLI. Young plants of other crops grown below about 8 mol/m²/day were inferior, while 10–14 gave better roots and mass [1].
18 hours is a proven default. A cannabis germination protocol grew its seedlings on at 24 °C under 18/6 and 200 µmol/m²/s [2].
Photoperiod cannabis is a short-day plant: long days keep it vegetative and long nights trigger flowering [3]. A seedling on 12/12 can flower while tiny. 24/0 simply removes the night, which keeps it firmly in veg.
Hours and intensity trade off. A longer day at lower PPFD delivers the same DLI with less heat and stress at the canopy.
The numbers
| Schedule |
PPFD for seedlings (µmol/m²/s) |
DLI (mol/m²/day) |
Notes |
| 18/6 |
150–300 |
10–19 |
Default |
| 20/4 |
140–260 |
10–19 |
Common for autos |
| 24/0 |
115–220 |
10–19 |
House-endorsed; highest power use |
| 12/12 |
Not for photoperiod seedlings |
— |
Triggers early flowering |
DLI = PPFD × hours × 0.0036.
What to do
- Pick 18/6 unless you have a reason not to.
- Start at 100–200 µmol/m²/s as the cotyledons open.
- Raise towards 300 as true leaves form, watching for bleaching or hard praying.
- On 24/0, dim the light to keep DLI inside 10–19.
Common mistakes
- Running 24/0 at full veg intensity on new seedlings.
- Starting photoperiods on 12/12 to "save time".
- Choosing the schedule and ignoring the PPFD.
- Light leaks or timer drift on a mother or nursery shelf near a flower room.
A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.18/6 is the standard vegetative light schedule: 18 hours of light and 6 hours of darkness each day, keeping photoperiod cannabis from flowering.Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.Daily light integral: the total number of PAR photons a square metre of canopy receives in one day, in mol/m²/day; PPFD multiplied by hours of light.24/0 means lights on 24 hours a day with no dark period; GenetixBay endorses it for veg, as cannabis does not need a sleep period before flowering.Germination is the stage where a viable seed absorbs water, splits its shell and pushes out a root (radicle), usually within 1-7 days at 20-30 °C.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.A plant that flowers when the night exceeds a critical length (days are short enough); photoperiod cannabis is a short-day plant.12/12 is the classic flowering schedule for photoperiod cannabis: 12 hours of light and 12 hours of uninterrupted darkness, which triggers and sustains flowering.A day-neutral cannabis plant that starts flowering by age, usually a few weeks after germination, regardless of light schedule; the trait is mainly recessive.One of the two rounded seed leaves packed in the embryo that open first after germination and feed the seedling until the true leaves take over.The first leaves produced by the shoot tip after the cotyledons, with the plant's characteristic serrated leaflets.Light stress is damage when leaves or buds absorb more light than they can use or dissipate, showing as bleached white or yellow tops nearest the fixture.A light leak is unwanted light entering the grow space during the dark period; in flower it can delay or disrupt flowering in photoperiod cannabis.
