Short answer
No. Cannabis seeds germinate in the dark; they need warmth (22–27 °C / 72–81 °F), moisture and air. Light matters from the moment the seedling breaks the surface, starting at about 100–200 µmol/m²/s for 18 hours. Supported.
Why
A germinating seed runs on its own stored reserves, not photosynthesis. A standard cannabis protocol germinates seeds in closed tubes in the dark and only moves them under light once sprouted [1].
Temperature, not light, sets the pace. Hemp germinates fastest in roughly the 19–30 °C band and slows sharply in the cold [2].
Once the shoot emerges, the reserves run out fast. Young plants given enough daily light produce better roots and more mass; below a DLI of about 8 mol/m²/day plugs of other crops were inferior [3].
Later on, light is the biggest yield lever indoors: flower yield rose linearly with light from 120 to 1,800 µmol/m²/s in one trial [4].
The numbers
| Stage | PPFD (µmol/m²/s) | Hours | DLI (mol/m²/day) |
|---|---|---|---|
| Germinating seed | None needed | Dark is fine | 0 |
| Emerging seedling | 100–200 | 18 | 6–13 |
| Seedling, true leaves | 150–300 | 18 | 10–19 |
| Veg | 300–700 (up to ~900) | 18–24 | 19–58 |
| Flower | 800–1,200 | 12–13 | 35–56 |
What to do
- Germinate in a warm, dark place: a cupboard, or a tub on a gentle heat mat.
- Check daily for a taproot; plant at 0.5–2 cm.
- Put the planted seed under the light straight away, or within hours of emergence.
- Start at 100–200 µmol/m²/s, and raise intensity as true leaves appear.
Common mistakes
- Germinating on a sunny windowsill that cooks or dries the towel.
- Leaving emerged seedlings in a dark cupboard "one more day".
- Blasting new seedlings with full flowering intensity.
Related
- Full article: Germination methods compared
- PPFD and DLI by stage
- What light schedule for seedlings?
- Terms: PPFD · germination
