PPFD — Photosynthetic Photon Flux Density — measures how many photons per second and square metre arrive in the photosynthetically active range (400–700 nm). Unit: µmol/m²/s. This is the only measurement that directly describes what your plant actually receives at the point where it stands.
Why it matters: watts describe power consumption. Lumens describe brightness for the human eye. Kelvin describes colour temperature. None of these tell you whether your plant is receiving too much, too little, or exactly the right amount of light. PPFD does.
PPFD Target Values by Growth Stage
| Stage | PPFD Minimum | PPFD Optimum | PPFD Maximum | Typical Distance |
|---|---|---|---|---|
| Germination (0–7 days) | 50 µmol/m²/s | 150–200 µmol/m²/s | 250 µmol/m²/s | 40–60 cm |
| Rooted clone | 100 µmol/m²/s | 200–350 µmol/m²/s | 400 µmol/m²/s | 35–50 cm |
| Vegetative (early, 1–2 weeks) | 300 µmol/m²/s | 400–600 µmol/m²/s | 700 µmol/m²/s | 25–40 cm |
| Vegetative (full, 3+ weeks) | 500 µmol/m²/s | 600–900 µmol/m²/s | 1000 µmol/m²/s | 20–35 cm |
| Early Flower (stretch) | 600 µmol/m²/s | 700–1000 µmol/m²/s | 1100 µmol/m²/s | 18–30 cm |
| Full Flower | 700 µmol/m²/s | 900–1200 µmol/m²/s | 1400 µmol/m²/s* | 15–25 cm |
| Late Flower / Ripening | 500 µmol/m²/s | 600–900 µmol/m²/s | 1000 µmol/m²/s | 20–30 cm |
| Autoflowering (full cycle) | 300 µmol/m²/s | 500–800 µmol/m²/s | 900 µmol/m²/s | 20–35 cm |
* Maximum above 1200 µmol/m²/s only makes sense with CO₂ supplementation at 1200–1500 ppm.
How PPFD and light hours together determine your daily light dose — with interactive calculator: DLI Calculator — Daily Light Integral for Every Growth Stage.
Measuring PPFD Correctly
- Measure at canopy height — not on the floor, not at the lamp
- Take 9 measurement points: centre, 4 corners, 4 edge midpoints
- Average all 9 = real grow PPFD
- Centre is typically 20–30% above the edge — single-point measurement overestimates
Light Stress Symptoms by PPFD Range
| Symptom | Cause | PPFD Range |
|---|---|---|
| Etiolation, stretching | Too little light | < 300 µmol/m²/s in veg |
| Leaf curl (upwards) | Too much light or heat | > 1000–1200 µmol/m²/s |
| Bleaching of top buds | Direct light stress | > 1300 µmol/m²/s at close range |
| Chlorosis despite good nutrients | Light stress → chloroplast damage | Too high for current stage |
CO₂ and PPFD Interaction
At normal ambient CO₂ (400 ppm), photosynthesis plateaus at approximately 800–1000 µmol/m²/s. Any further PPFD increase produces no yield gain — it only creates light stress and heat load.
| CO₂ Level | Useful PPFD Ceiling |
|---|---|
| 400 ppm (ambient) | 800–1000 µmol/m²/s |
| 800 ppm (enriched) | 1100–1300 µmol/m²/s |
| 1200 ppm (enriched) | 1400–1600 µmol/m²/s |
Light Acclimation — never from 0 to 1000
One of the most common mistakes: applying full PPFD target values from day one. Jorge Cervantes explicitly warns against sudden PPFD increases — especially for plants that have been growing under lower intensity.
Plants develop protective pigments (carotenoids, xanthophylls) and enzyme systems in response to high light intensity. This adaptation takes time. Moving a plant from 400 to 1000 µmol/m²/s overnight risks light bleaching and chloroplast damage — even if 1000 µmol/m²/s is technically correct for the phase.