PPFD is the metric that tells you how much usable light is actually reaching your plants. Understanding it removes the guesswork from light placement, helps you choose the right fixture for your space, and explains why two lights with identical wattage can produce very different results. It also explains why the grow lights sold at hardware stores cannot support a productive hydroponic grow, something we cover in detail in the grow lights vs Bunnings guide.
This guide covers what PPFD measures, the target ranges for different crops and growth stages, and how to apply that information in a real Australian grow room.
What PPFD Measures
PPFD stands for Photosynthetic Photon Flux Density. It measures the number of photons in the photosynthetically active range (400 to 700 nanometres) that land on a given surface area per second. The unit is micromoles per square metre per second, written as µmol/m2/s. PPFD is measured at a specific point and height. The same fixture will produce different PPFD readings at different distances from the canopy, at different positions across the coverage area, and at different heights within the tent. This is why PPFD maps showing the distribution of light across an entire coverage area are more useful than a single peak number.
PPF, PPFD and Efficacy: Why the Difference Matters
PPF (Photosynthetic Photon Flux) measures the total photon output of a fixture in all directions. PPFD measures what actually reaches the plant canopy in a specific footprint. Efficacy measures how efficiently the fixture converts electricity into those photons, expressed in µmol/J. When comparing grow lights, efficacy and PPFD distribution are the numbers that matter. A fixture with high PPF but poor optics delivers a lot of photons toward the tent walls rather than the canopy. The SANlight EVO Series uses secondary optic technology specifically to address this, concentrating photon delivery across the target footprint.
PPFD Targets by Growth Stage
Propagation and seedlings: 100 to 300 µmol/m2/s. Young plants have not developed the leaf structure and pigment density to handle high light intensity.
Vegetative growth: 400 to 600 µmol/m2/s. Most crops in active vegetative growth respond well in this range.
Flowering crops with CO2 supplementation: 600 to 900 µmol/m2/s. With CO2 raised above ambient levels, the light saturation point of many crops increases.
High-demand flowering crops without CO2: 600 to 800 µmol/m2/s. Cannabis, tomatoes, cucumbers, and similar crops in flower can utilise light effectively up to around 800 µmol/m2/s under standard atmospheric CO2 levels.
Leafy greens and herbs: 200 to 400 µmol/m2/s. Most lettuce, basil, spinach, and leafy greens are light-saturated at relatively low PPFD. Running flowering lights over leafy crops wastes electricity and can cause tip burn and bleaching.
DLI: The Number That Ties It All Together
DLI stands for Daily Light Integral. It measures the total number of photons a plant receives over a full day, calculated by multiplying average PPFD by the number of light hours. DLI targets for common crops: Leafy greens 12 to 17 mol/m2/day, vegetative cannabis and tomato 20 to 30 mol/m2/day, flowering cannabis 35 to 50 mol/m2/day, fruiting tomatoes and cucumbers 25 to 35 mol/m2/day. Understanding DLI helps you decide whether to increase light intensity or extend photoperiod when plants are underlit.
Matching Fixtures to Tent Size
The most common mistake in light selection is choosing a fixture by wattage rather than by target PPFD across the specific footprint. For an 80x80cm tent targeting 600 to 800 µmol/m2/s in flower, the SANlight EVO 3-80 is purpose-engineered for that exact footprint. For a 120x120cm tent in flower, the SANlight EVO 4-120 or EVO 5-120 delivers the coverage footprint that the space requires. Match the fixture to the footprint first, then consider wattage as a secondary factor.
PPFD and Australian Summer
Some growers reduce light intensity by raising fixtures or dimming during the hottest weeks rather than fighting the resulting heat with additional cooling. This is a valid approach. A plant receiving slightly lower PPFD in a stable 24 degree environment will generally outperform a plant receiving peak PPFD in a 32 degree environment. For the full range of grow lights available for Australian indoor growers, browse the Grow Lights collection.
What to Buy to Hit Your PPFD Target
The PPFD numbers this guide walks through translate directly into one buying decision, an undersized light simply can't hit flowering-stage PPFD across a full canopy no matter how you position it. The SANlight EVO 3-80 is built to deliver the PPFD levels this guide recommends for an 80x80cm footprint specifically, buying a smaller or cheaper fixture for the same space means under-lighting the canopy edges even if the centre reads fine. If you're growing a larger tent, don't assume one light scales linearly, check the EVO range size-for-size rather than assuming a single bigger fixture covers double the area at the same PPFD.
Frequently Asked Questions
What PPFD do I need for flowering cannabis or tomatoes?
For high-demand flowering crops without CO2 supplementation, target 600 to 800 µmol/m2/s at canopy level. With CO2 raised above ambient, the saturation point increases and plants can productively use up to 900 µmol/m2/s or more. Pushing beyond 800 without CO2 produces diminishing returns for most hobby growers.
How is PPFD different from watts or lumens?
Watts measure how much electricity a fixture consumes. Lumens measure light as perceived by the human eye, not by plants. PPFD measures the photons that actually drive photosynthesis, which is the only number that tells you how much useful light is reaching your plants. Two fixtures with identical wattage can produce very different PPFD readings at the canopy.
What PPFD do leafy greens and herbs need?
Most lettuce, basil, spinach, and similar crops perform well at 200 to 400 µmol/m2/s. Running high-intensity flowering lights over leafy green crops wastes electricity and can cause tip burn and bleaching from excess intensity. Purpose-built propagation fixtures are better suited to leafy green growing than full-power LED bars.
What is DLI and why does it matter?
DLI (Daily Light Integral) is the total number of photons a plant receives over a full day. It accounts for both the PPFD intensity and the number of light hours. Two different combinations of intensity and hours can deliver the same DLI. Understanding DLI helps you decide whether to increase light intensity or extend photoperiod when plants are underlit.
Should I hang my grow light higher or lower to increase PPFD?
Lowering the light increases PPFD intensity at the canopy but reduces coverage area and can create hotspots in the centre. Raising the light distributes photons more evenly across a larger area but lowers average intensity. Check the manufacturer's PPFD maps for your specific fixture at different hang heights rather than guessing based on wattage.