DLI = PPFD × light hours × 0.0036
DLI — Daily Light Integral — is the cumulative photon dose a plant receives over 24 hours. Unit: mol/m²/day. Not your lamp's wattage, not lux, not colour temperature. DLI is the only number that describes how much light your plant actually receives per day.
800 µmol/m²/s at 18h light gives DLI 51.8. The same lamp at 12h: DLI 34.6. Same intensity, 33% less light — purely from the photoperiod. This is the variable most growers never actively control.
DLI Target Values by Growth Stage
| Stage | DLI Target | Typical PPFD | Light Period |
|---|---|---|---|
| Germination / Clone | 6–12 mol/m²/day | 150–300 µmol/m²/s | 18–20 h |
| Vegetative (early) | 20–28 mol/m²/day | 400–600 µmol/m²/s | 18–20 h |
| Vegetative (full) | 28–35 mol/m²/day | 600–800 µmol/m²/s | 18–20 h |
| Early Flower | 35–42 mol/m²/day | 800–1000 µmol/m²/s | 12 h |
| Full Flower | 40–50 mol/m²/day | 900–1200 µmol/m²/s | 12 h |
| Autoflowering | 25–40 mol/m²/day | 400–700 µmol/m²/s | 20 h |
The exact PPFD target values per phase — minimum, optimum, maximum and lamp distance: PPFD Table — Target Values for Veg, Flower and Seedling.
PPFD × Hours — Combination Table
| PPFD (µmol/m²/s) | 12 h | 16 h | 18 h | 20 h | 24 h |
|---|---|---|---|---|---|
| 200 | 8.6 | 11.5 | 13.0 | 14.4 | 17.3 |
| 400 | 17.3 | 23.0 | 25.9 | 28.8 | 34.6 |
| 600 | 25.9 | 34.6 | 38.9 | 43.2 | 51.8 |
| 800 | 34.6 | 46.1 | 51.8 | 57.6 | 69.1 |
| 900 | 38.9 | 51.8 | 58.3 | 64.8 | 77.8 |
| 1000 | 43.2 | 57.6 | 64.8 | 72.0 | 86.4 |
| 1100 | 47.5 | 63.4 | 71.3 | 79.2 | 95.0 |
Practical rule: 800 µmol/m²/s at 12/12 gives DLI 34.6 — the lower edge of the flower optimum. Increase to 900 µmol or extend to 13h to reach 40+ mol.
Too Low DLI
- Etiolation — stems stretch excessively towards the light source
- Reduced photosynthesis rate — growth slows measurably
- Small, loose buds with low resin density
- Extended flowering time — plant needs longer to reach the same ripeness
Too High DLI
- Light bleaching of top buds — white or yellowed tips above 1300 µmol/m²/s
- Leaf curl as the first stress response
- Stomatal closure → transpiration stops → VPD becomes ineffective
- Without CO₂ supplementation, photosynthesis is already CO₂-limited above 800–1000 µmol
Three Levers for DLI Optimisation
| Lever | Effect | When to use |
|---|---|---|
| Increase PPFD (move lamp closer / full power) | DLI rises proportionally | When lamp has headroom and VPD is stable |
| Extend light period | DLI rises linearly | During veg — not during 12/12 photoperiod |
| Reduce lamp distance | PPFD rises quadratically | Caution: check for heat stress, measure PPFD |