# What light schedule should seedlings get?

> Photoperiod and autoflower seedlings both do well on 18/6: 18 hours of light at about 150–300 µmol/m²/s, a DLI of 10–19 mol/m²/day. 24/0 is fine too, because cannabis needs no sleep period in veg; it just costs more power, so drop the intensity to keep the same DLI. Never put photoperiod seedlings on 12/12 unless you want them to flower small.

- Canonical: https://genetixbay.com/knowledge/answers/light-schedule-for-seedlings.html
- Topic: Germination · Type: Numbers
- Evidence: Grower practice · Status: Draft: awaiting expert review
- Full article: [Seedling care: the first three weeks](https://genetixbay.com/knowledge/germination/seedling-care.md)
- Also asked as: 18/6 or 24/0 for seedlings? | How many hours of light do seedlings need? | Seedling light cycle | What light schedule for autoflower seedlings?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Give cannabis [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md) [18/6](https://genetixbay.com/knowledge/glossary/18-6.md) at about 150–300 [µmol/m²/s](https://genetixbay.com/knowledge/glossary/ppfd.md), a [DLI](https://genetixbay.com/knowledge/glossary/dli.md) of 10–19 mol/m²/day. [24/0](https://genetixbay.com/knowledge/glossary/24-0.md) 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](https://genetixbay.com/knowledge/glossary/germination.md) protocol grew its seedlings on at 24 °C under 18/6 and 200 µmol/m²/s [2].

[Photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) cannabis is a [short-day plant](https://genetixbay.com/knowledge/glossary/short-day-plant.md): long days keep it vegetative and long nights trigger flowering [3]. A seedling on [12/12](https://genetixbay.com/knowledge/glossary/12-12.md) 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](https://genetixbay.com/knowledge/glossary/autoflower.md) |
| 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

1. Pick 18/6 unless you have a reason not to.
2. Start at 100–200 µmol/m²/s as the [cotyledons](https://genetixbay.com/knowledge/glossary/cotyledon.md) open.
3. Raise towards 300 as [true leaves](https://genetixbay.com/knowledge/glossary/true-leaves.md) form, watching for [bleaching](https://genetixbay.com/knowledge/glossary/light-stress.md) or hard praying.
4. 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](https://genetixbay.com/knowledge/glossary/light-leak.md) or timer drift on a mother or nursery shelf near a flower room.

## Related

- Full article: [Seedling care: the first three weeks](https://genetixbay.com/knowledge/germination/seedling-care.md)
- [Photoperiod and light leaks](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)
- [How long is the seedling stage?](https://genetixbay.com/knowledge/answers/how-long-is-the-seedling-stage.md)
- Terms: [24/0](https://genetixbay.com/knowledge/glossary/24-0.md) · [18/6](https://genetixbay.com/knowledge/glossary/18-6.md) · [DLI](https://genetixbay.com/knowledge/glossary/dli.md)

## Follow-up questions

**Is 24/0 bad for seedlings?** No. 24/0 is fine for cannabis seedlings; GenetixBay endorses it, because cannabis does not need a sleep period in vegetative growth. The plant only measures night length to decide when to flower. The catch is cost and dose: 24 hours at 300 µmol/m²/s is a DLI of about 26 mol/m²/day, so run around 150–220 µmol/m²/s to stay in the seedling band.

**How do I convert hours and PPFD to DLI?** Daily light integral (DLI) in mol/m²/day equals PPFD in µmol/m²/s times hours of light times 0.0036. For cannabis seedlings, 200 µmol/m²/s for 18 hours gives about 13 mol/m²/day, inside the 10–19 target. The same 200 µmol/m²/s for 24 hours gives about 17. Longer days let you use a dimmer light for the same total dose.

**Do autoflower seedlings need a different schedule?** No. Autoflower seedlings use the same schedule as photoperiod seedlings, usually 18/6 or 20/4, and many growers run them that way to harvest. Autoflowers are day-neutral, so the light schedule doesn't trigger their flowering, but any setback costs them time they can't recover. Keep intensity gentle at 150–300 µmol/m²/s while they are seedlings.

## Sources

1. Runkle E. The benefits of lighted plugs. Michigan State University Extension. https://www.canr.msu.edu/resources/the_benefits_of_lighted_plugs
2. Sorokin A, Yadav NS, Gaudet D, Kovalchuk I, 2021. Development and standardization of rapid and efficient seed germination protocol for Cannabis sativa. Bio-protocol 11(1): e3875. https://doi.org/10.21769/BioProtoc.3875
3. Zhang M, Anderson SL, Brym ZT, Pearson BJ, 2021. Photoperiodic flowering response of essential oil, grain, and fiber hemp (Cannabis sativa L.) cultivars. Frontiers in Plant Science 12:694153. https://doi.org/10.3389/fpls.2021.694153