PPFD Targets by Stage

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 — the wavelengths plants use most ^[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 seconds, converted to mol/m²/day. DLI is arguably the more honest number 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 climb with PPFD until they plateau. Controlled studies on cannabis show net photosynthesis increasing roughly linearly up to ~1500 µmol/m²/s when CO2 is elevated 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, generates heat, can cause photobleaching and leaf necrosis, and degrades cannabinoids — particularly THC, which converts to CBN under excess light and heat ^[4] Weak / limited. Going lower leaves yield on the table and produces stretchy, low-density flower.

Dialing PPFD to stage is the cheapest, most repeatable yield lever after genetics and basic environmental control.

Stage-by-stage targets

These ranges assume ambient CO2 (400–500 ppm), healthy VPD (0.8–1.5 kPa), and leaf surface temperature around 25–28°C. Push higher only if CO2, temp, and nutrition are also dialed in ^[3].

Seedling / clone (week 1–2): 100–300 µmol/m²/s. Roots are minimal; high light scorches cotyledons and stalls 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 handle 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 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 pays off most.

Late flower / ripening (last 1–2 weeks): Some growers drop to 700–900 µmol/m²/s to reduce heat stress and preserve terpenes Anecdote. There's no strong controlled data showing this improves quality, but it's common practice and unlikely to hurt yield meaningfully at that stage.

How to dial it in, step by step

1. Get a real quantum sensor. Apogee MQ-500 or similar full-spectrum quantum meters are the standard. Phone apps using the camera are unreliable for LED spectra Weak / limited. If you can't afford one, borrow or rent.

1. Measure at canopy height, not floor. Hold the sensor parallel to the top of the plants, facing the light. Take readings at 9 points in a 3×3 grid across the canopy.

1. Average the grid. The average is your working PPFD. The spread (max minus min) tells you how uniform your light is — aim for less than 20% variation across the canopy.

1. Adjust by dimming first, height second. Modern LEDs dim cleanly. Raise the fixture only if dimming alone can't get you uniform coverage.

1. Re-measure weekly. The canopy rises as plants grow, so PPFD at the tops climbs even if the fixture doesn't move. Many growers over-light flower by accident this way.

1. Watch the plants. Tacoing leaves, bleached colas, or pale tops mean too much light regardless of what the meter says. Back off 100–200 µmol/m²/s and observe.

Common mistakes

• Trusting lux meters or phone apps. They under-read red-heavy LED spectra by 30–50% Strong evidence. You'll think you're at 600 when you're actually at 900.
• Measuring at one spot. Center-of-canopy readings are almost always the brightest. Edges may be at 50% of center.
• Pushing 1000+ µmol/m²/s without CO2. Above ambient CO2's photosynthetic ceiling, extra photons mostly become heat and stress ^[3].
• Ignoring leaf temperature. Leaf surface temp matters more than air temp for photosynthesis. An IR thermometer is cheap and useful.
• Copying a famous grower's numbers. Their room has different CO2, airflow, VPD, and genetics. Their 1200 µmol/m²/s may be your plant's 1400.
• Confusing PPFD with PPF or PPE. PPF is total fixture output (µmol/s). PPE is efficacy (µmol/J). Neither tells you what hits your canopy.

Related techniques

PPFD targeting works hand-in-hand with VPD Management — high light requires tight humidity and temperature control to avoid stressing stomata. CO2 Supplementation is the prerequisite for pushing past ~1000 µmol/m²/s. Canopy training methods like SCROG and Topping flatten the canopy so a single PPFD target actually applies to the whole plant rather than just the tallest cola. For tracking cumulative light exposure, see DLI (Daily Light Integral).

Sources

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