PPFD Targets by Stage
Optimize light intensity at each stage to maximize photosynthesis and yield.
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PPFD is the only light metric that actually tells you how many photons hit your plants each second. Lux meters and phone apps won't work on LEDs. Real quantum sensors cost ~$400, but you can borrow one. The targets here assume ambient CO2; if you're enriching to 1200 ppm, you can push higher. Watch your plants—if leaves taco or bleach, back off regardless of what the meter says.
What PPFD Actually Is
PPFD stands for Photosynthetic Photon Flux Density. It measures the number of photons in the 400–700 nm range (photosynthetically active radiation, or PAR) hitting a square meter of canopy each second, expressed in µmol/m²/s [1].
PPFD is not the same as lux, foot-candles, or wattage. Lux is weighted to human eye sensitivity and undervalues red and blue wavelengths—the colours plants use most for photosynthesis [1]. Watts measure electricity going in, not light coming out. Only a quantum sensor calibrated for PAR gives you a real PPFD reading Strong evidence.
A related metric, DLI (Daily Light Integral), is PPFD multiplied by photoperiod length in seconds, then converted to mol/m²/day. DLI Targets by Growth Stage is arguably more honest because it accounts for how long the lights are on [2].
Why Growers Target Specific PPFD
Cannabis photosynthesis follows a light response curve: yield and photosynthetic rate increase with PPFD until they plateau. Controlled studies on cannabis show net photosynthesis increasing roughly linearly up to approximately 1500 µmol/m²/s when CO2 is enriched to ~1200 ppm, with diminishing returns above that [3]. At ambient CO2 (~420 ppm), the plateau is closer to 900–1000 µmol/m²/s [3] Strong evidence.
Going higher than the plant can use wastes electricity and generates heat. Excessive PPFD can cause photobleaching, leaf necrosis, and stress responses. Going lower leaves yield on the table and produces stretchy, low-density flower.
Dialing PPFD to stage is the most cost-effective yield lever after genetics and basic environmental control (temperature, humidity, CO2).
Stage-by-Stage PPFD Targets
These ranges assume ambient CO2 (400–500 ppm), healthy VPD (0.8–1.5 kPa), and leaf surface temperature around 25–28°C. Do not push higher unless CO2, temperature, and nutrition are also optimized [3].
Seedling / clone (week 1–2): 100–300 µmol/m²/s. Roots are minimal; high light scorches cotyledons and delays rooting Strong evidence. Keep humidity high (70%+).
Early veg (week 2–4): 300–600 µmol/m²/s. Plants are building leaf area; this range maximizes growth without stressing young tissue.
Late veg (week 4+): 600–900 µmol/m²/s. Plants can now tolerate near-flowering intensity. Use this stage to acclimate before the photoperiod flip.
Early flower (week 1–3 of 12/12): 700–900 µmol/m²/s. Stretch phase; plants are still building vertical structure.
Peak flower (week 4–6): 900–1100 µmol/m²/s at ambient CO2; up to 1500 µmol/m²/s with CO2 enrichment [3] Strong evidence. This is the bud-bulking window where light investment returns the most yield.
Late flower / ripening (final 1–2 weeks): Some growers reduce to 700–900 µmol/m²/s to minimize heat stress and preserve terpenes Anecdote. Controlled data on this practice is sparse, but it is common in the industry and unlikely to harm yield meaningfully at this final stage.
How to Dial It In, Step by Step
- Get a real quantum sensor. An Apogee MQ-500 or equivalent full-spectrum quantum meter is the standard. Phone camera apps are unreliable for LED spectra because they cannot distinguish red and blue wavelengths accurately Weak / limited. If a quantum sensor is out of budget, borrow or rent one.
- Measure at canopy height, not the floor. Hold the sensor parallel to the plant canopy tops, facing the light. Take readings at nine points in a 3×3 grid across the canopy.
- Average the grid. The average is your working PPFD. The spread (maximum minus minimum) tells you uniformity—aim for less than 20% variation across the canopy.
- Adjust by dimming first, height second. Modern LED fixtures dim cleanly. Raise the fixture only if dimming alone cannot achieve uniform coverage.
- Re-measure weekly. The canopy rises as plants grow, so PPFD at the tops climbs even if the fixture stays still. Many growers accidentally over-light flower this way.
- Watch the plants. Tacoing leaves, bleached colas, or pale upper growth mean too much light regardless of meter readings. Back off 100–200 µmol/m²/s and observe for 3–5 days.
Common Mistakes
- Trusting lux meters or phone apps. These tools were not designed for LEDs and cannot accurately measure PAR. Your actual PPFD may be significantly higher than what they read Weak / limited.
- Measuring at one spot only. Center-of-canopy readings are almost always the brightest. Edges may be at 50% of center intensity.
- Pushing 1000+ µmol/m²/s without CO2 enrichment. Above ambient CO2's photosynthetic ceiling (~900–1000 µmol/m²/s), extra photons mostly become heat and plant stress [3].
- Ignoring leaf surface temperature. Leaf temperature, not air temperature, determines photosynthetic efficiency. An inexpensive IR thermometer is useful for tracking this.
- Copying a famous grower's numbers. Their room has different CO2, airflow, VPD, and genetics. Their 1200 µmol/m²/s may be your plants' 1400.
- Confusing PPFD with PPF or PPE. PPF is the fixture's total photon output (µmol/s). PPE is efficacy (µmol/J). Neither tells you what hits your actual canopy.
Related Techniques
PPFD targeting works hand-in-hand with VPD (Vapor Pressure Deficit) Basics—high light requires tight humidity and temperature control to avoid stressing stomata. CO₂ enrichment is the main prerequisite for pushing past ~1000 µmol/m²/s. Canopy training methods flatten the plant so a single PPFD target applies to the whole canopy rather than just the tallest cola. Light Height by Stage complements PPFD targeting by helping you dial in vertical distance for even coverage.
Sources
- Peer-reviewed McCree, K.J. (1972). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agricultural Meteorology, 9, 191–216.
- Peer-reviewed Faust, J.E. & Logan, J. (2018). Daily Light Integral: A Research Review and High-resolution Maps of the United States. HortScience, 53(9), 1250–1257.
- Peer-reviewed Chandra, S., Lata, H., Khan, I.A. & ElSohly, M.A. (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO₂ conditions. Physiology and Molecular Biology of Plants, 14(4), 299–306.
- Peer-reviewed Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021). Cannabis Yield, Potency, and Leaf Photosynthesis Respond Differently to Increasing Light Levels in an Indoor Environment. Frontiers in Plant Science, 12, 646020.
- Peer-reviewed Eaves, J., Eaves, S., Morphy, C. & Murray, C. (2020). The relationship between light intensity, cannabis yields, and profitability. Agronomy Journal, 112(2), 1466–1470.
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