Grow science · 09
PPFD guide
Watts tell you what the lamp uses. PPFD tells you what the plant gets.
What PPFD measures
Photosynthetic photon flux density is the number of light particles in the photosynthetic range (400–700 nm) that hit one square metre every second, in µmol/m²/s. It’s measured at the canopy, so the same lamp gives a different PPFD at different heights.
Lux doesn’t work for plants: lux meters are tuned to the human eye, which barely sees deep red and blue, exactly the light plants use most.
Targets by stage
| Stage | PPFD | Typical schedule |
|---|---|---|
| Seedlings & clones | 150–300 | 18 h |
| Veg | 400–600 | 18 h |
| Flower | 700–1000 | 12 h |
µmol/m²/s at canopy height, for high-light crops without added CO₂. With CO₂ enrichment, flower can go higher (around 1200 and more). Ramp up over a week instead of jumping.
PPFD and DLI belong together
PPFD is the brightness right now, DLI is the dose over the whole day. The fewer hours your lights run, the higher the PPFD needs to be for the same daily dose:
| Target DLI | PPFD at 18 h | PPFD at 12 h |
|---|---|---|
| 20 mol/m²/day | 309 | 463 |
| 30 | 463 | 694 |
| 40 | 617 | 926 |
| 45 | 694 | 1042 |
How to measure
- Use a quantum (PAR) sensor at canopy height. Phone apps with a diffuser can give a rough idea, nothing more.
- Measure a grid of 9 points across the tent, not just the center. Edges often get 30–50 % less.
- Lights on full for a few minutes, reflective walls closed, sensor pointing straight up.
Too much or too little?
- Too much: bleached or yellowing top leaves, leaves folding up at the edges, slow growth at the top despite good climate.
- Too little: long gaps between nodes, plants stretching toward the lamp, thin stems.
More light also means more transpiration, so check your VPD after turning the lamp up.
Questions growers ask
What PPFD do I need for flowering?
Around 700–1000 µmol/m²/s at canopy height on a 12-hour schedule without added CO₂. That gives a DLI of about 30–43 mol/m²/day.
What PPFD is good for veg?
Around 400–600 µmol/m²/s on an 18-hour schedule, which gives a DLI of about 26–39 mol/m²/day.
What PPFD do seedlings need?
Around 150–300 µmol/m²/s. Too much light on young plants causes stress, so ramp up slowly as they grow.
Can I measure PPFD with a lux meter?
Not reliably. Lux meters are tuned to the human eye and miss much of the red and blue light plants use. Conversion factors depend on the light spectrum, so use a quantum (PAR) sensor if you can.