PPFD for Lettuce and Herbs Indoors
Leafy greens and soft herbs are low-light crops compared with fruiting plants. They do not need the intensity a tomato or a flowering canopy wants, and pushing them too hard wastes energy and can cause tipburn on lettuce. These are the PPFD (µmol/m²/s) and DLI (mol/m²/day) ranges published horticulture guides and research converge on for indoor lettuce and herbs. Sources differ at the edges, so the numbers below are the span across several references rather than one fixed figure. Every range is sourced.
Target PPFD and DLI
| Crop | Typical PPFD (µmol/m²/s) | Typical DLI (mol/m²/day) |
|---|---|---|
| Lettuce / leafy greens | ~150–300 (up to ~400) | ~12–17 |
| Basil, mint, coriander | ~100–300 | ~10–15 |
| Microgreens | lower, short duration | ~6–12 |
For lettuce, guides commonly land around 200–300 PPFD at the canopy, with seedlings at the lower end and vegetative growth toward the top; some push to ~400 for faster growth. Research on indoor lettuce points to a DLI in the mid-teens (roughly 14–17 mol/m²/day) as a practical target, and greenhouse leafy-green guidance often cites a minimum near 17. Basil and other soft herbs sit in a similar-to-slightly-lower band.
Why leafy greens want less light
- They are harvested young and leafy, not fruited. There is no bud or fruit set to fuel, so the light demand is modest compared with a flowering crop.
- Too much light causes problems. Excess intensity on lettuce can trigger tipburn and does not buy proportional growth. More light is not always better for greens.
- DLI matters more than raw intensity. A moderate PPFD over a long photoperiod reaches the same daily total as a high PPFD over a short one, and greens tolerate long day lengths well.
Photoperiod and DLI
DLI is the total light delivered per day and is what actually drives leaf growth:
DLI = PPFD × (seconds of light per day) ÷ 1,000,000
Lettuce is usually run on a long photoperiod (often 14–18 hours), which lets a gentle PPFD reach a mid-teens DLI without high intensity. Set the DLI target first, then pick an intensity and light-hours combination that reaches it. Note that the right photoperiod differs by crop: a day length that suits basil can be excessive for lettuce at the same PPFD, so match hours to each crop’s DLI rather than copying one schedule across the shelf.
Measuring your own numbers
If you do not own a quantum sensor, a phone lux app plus a conversion factor gives a rough PPFD estimate, enough to confirm you are in the greens band and not far over. See how to measure PPFD without a meter for the method and its limits, and the lux to PPFD converter for the conversion. For how these targets change across a plant’s life, see PPFD for seedlings, veg and flowering. Treat these as guidance, not law. Variety, temperature, and nutrients shift the optimum, so watch the crop.
Sources
- Typical DLI and PPFD for leafy greens, herbs, and microgreens: https://www.nexsel.tech/typical-dli-and-ppfd-values-for-crops.php
- Optimal DLI for indoor lettuce (hydroponic study): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10322904/
- Indoor lettuce grow-light guide (PPFD and DLI ranges): https://www.gorillagrowtent.com/blogs/news/lettuce-grow-light
- Grow lights for lettuce (yield and light levels): https://indoorfarmsys.com/grow-lights-for-lettuce/