PPFD Targets for Lettuce at Different Photoperiods
| Photoperiod | PPFD for DLI 12 | PPFD for DLI 14 | PPFD for DLI 17 |
|---|---|---|---|
| 14h | 238 | 278 | 337 |
| 16h | 208 | 243 | 295 |
| 18h | 185 | 216 | 262 |
| 20h | 167 | 194 | 236 |
| 24h | 139 | 162 | 197 |
PPFD in μmol/m²/s. Measured at canopy level.
Why Lettuce is the Ideal Indoor Crop
Lettuce has the lowest DLI requirement of any commercial food crop, making it uniquely suited for vertical farming and low-power indoor setups. At 12–17 mol/m²/day, a modest LED panel can support multiple tiers of lettuce in a small space. Harvest cycle is 25–35 days from transplant, allowing 10–12 harvests per year.
The combination of low DLI requirement, no photoperiod sensitivity, and rapid turnover makes lettuce the benchmark crop for evaluating indoor lighting efficiency. If your system can hit DLI 14 economically, it is viable for commercial leafy green production.
Frequently Asked Questions
What DLI does lettuce need?
Lettuce grows well at 12–17 mol/m²/day DLI. Below 10, growth slows significantly. Above 20, some varieties develop tipburn from calcium deficiency caused by excessive transpiration and edge tissue drying.
What PPFD do I need for lettuce at 16 hours?
At a 16-hour photoperiod, you need 208 μmol/m²/s PPFD to reach DLI 12, and 295 μmol/m²/s to reach DLI 17. These are achievable with most modern LED grow panels at 30–50 cm hanging height above the canopy.
Can I grow lettuce with 18-hour photoperiod?
Yes. Lettuce is day-neutral (not photoperiod-sensitive) and grows under continuous light without bolting issues. An 18-hour photoperiod lets you use lower PPFD (185–265 μmol/m²/s) to hit the same DLI range, saving energy and reducing heat.
What causes tipburn in lettuce under high DLI?
Tipburn is a calcium deficiency that appears at leaf edges. High DLI increases transpiration everywhere except the very edge of leaves (which lack stomata). Under high DLI, these cells desiccate faster than calcium can be supplied. Keeping DLI below 18 and maintaining VPD at 0.6–1.0 kPa reduces tipburn risk.
Is red/blue LED spectrum better for lettuce than full-spectrum?
Red and blue LEDs are efficient for lettuce photosynthesis, but studies show full-spectrum white LEDs produce comparable or better biomass while also enabling better visual assessment of plant health. The difference is minimal at the same DLI — total photon delivery matters more than spectrum ratio for lettuce.