PPFD

PPFD (Photosynthetic Photon Flux Density) is the practical measurement of light intensity at the plant canopy — the metric that actually determines cannabis growth rates. PPFD measures how many micromoles of photosynthetically active photons (PAR light, 400-700nm) hit each square meter of canopy surface per second. Cannabis PPFD targets by lifecycle stage: Seedlings/clones — 100-300 µmol/m²/s (minimal light to prevent stretching while building roots); Early vegetative — 300-500 µmol/m²/s; Late vegetative — 500-700 µmol/m²/s (maximum vegetative growth without bleaching); Stretch/early flower — 600-800 µmol/m²/s (transitioning plants need consistent light); Mid-flower without CO2 — 800-1000 µmol/m²/s (peak flowering intensity); Mid-flower with CO2 supplementation — 1000-1500 µmol/m²/s (CO2 enrichment allows higher light tolerance and dramatically increased growth rates); Late flower — 700-1000 µmol/m²/s (slight reduction prevents leaf burn during ripening). Above 1500 µmol/m²/s, even CO2-supplemented plants experience light burn and bleaching with no additional yield benefit. Measuring PPFD: requires a quantum sensor — Apogee MQ-500 ($385) is the industry standard; cheaper alternatives include the Hipargero PAR meter ($150) and free smartphone apps using camera sensors (less accurate but useful for relative comparisons). Place the sensor parallel to the canopy at multiple positions and average the readings to determine average canopy PPFD; always measure with the grow light at intended operating height and full intensity. PPFD distribution: high-quality grow lights produce uniform PPFD across the entire intended canopy area with under 20% variance from edge to center; budget lights show massive hotspots (1500+ µmol/m²/s directly under the light) and shaded edges (400 µmol/m²/s at the corners); uniform PPFD distribution is critical because variable light intensity causes variable growth — plants in hot spots get bleached and burn while plants in shadows under-perform. Adjusting PPFD: raise light height to reduce PPFD across the entire canopy proportionally; dim the light if it has a controller; for fine control, hang multiple smaller lights and adjust their independent positions to fill in canopy edges. PPFD is half of the DLI (Daily Light Integral) calculation that determines total daily light exposure.