Cannabis PPFD and DLI: The Commercial Lighting Guide
Quick Answer: PPFD (photosynthetic photon flux density) is the intensity of usable light hitting your canopy at a given moment, measured in micromoles per square meter per second. DLI (daily light integral) is the total amount of that light the plant receives over a full day, measured in moles per square meter per day. PPFD tells you how bright; DLI tells you how much. Commercial cannabis runs roughly 200–400 PPFD for clones and early veg, 400–600 in late veg, and 800–1,000+ in flower with CO2 supplementation — landing flower DLI in the 35–50+ range. The mistake that costs lb/light is treating light as a fixed setting instead of a steering input that has to be matched to irrigation, VPD, and growth stage.
PPFD vs. DLI: Intensity vs. Total Dose
These two numbers get used interchangeably, and that confusion costs yield.
PPFD is a snapshot. It is the photon intensity — specifically photons in the 400–700 nm photosynthetically active range — landing on one square meter of canopy in one second. When you put a quantum sensor at canopy height and read 850, that is PPFD. It tells you how intense the light is right now, at that spot.
DLI is the accumulation. It is every photon the canopy receives over the entire photoperiod, summed into moles per square meter per day. DLI is what actually correlates with biomass, because the plant's photosynthetic output over a day is a function of total light banked, not peak intensity.
The relationship is simple math: DLI is roughly PPFD multiplied by photoperiod seconds, divided by one million. An 850 PPFD canopy under 12 hours of light banks about 36.7 mol/m²/day. The same 850 PPFD under an 11-hour photoperiod banks about 33.7. Same intensity, different dose — and the plant responds to the dose.
This is why "what PPFD should I run" is an incomplete question. Two facilities at identical PPFD can be delivering meaningfully different DLI because their photoperiods differ. And a facility that drops to an 11/13 or 10/14 ripening photoperiod is cutting DLI even if the fixtures never change. Track both. PPFD is the dial you set; DLI is the result you are actually managing.
Use the reference chart below for stage targets, and use the interactive DLI calculator to enter your canopy PPFD and photoperiod and see the actual daily dose your plants are banking — plus where it sits against the stage range.
PPFD and DLI Target Ranges by Growth Stage
The numbers below assume a healthy plant and adequate CO2 in flower. Without CO2 supplementation, the top of the flower range is wasted light — the plant cannot use it and you are paying for photons that become heat.
| Growth Stage | Target PPFD (µmol/m²/s) | Typical Photoperiod | Resulting DLI (mol/m²/day) |
|---|---|---|---|
| Clones / seedlings | 100–300 | 18/6 | 6–19 |
| Early veg | 300–500 | 18/6 | 19–32 |
| Late veg | 400–600 | 18/6 | 26–39 |
| Early flower / stretch | 600–800 | 12/12 | 26–35 |
| Mid-flower (bulk) | 800–1,000+ | 12/12 | 35–43+ |
| Late flower (ripening) | 700–950 | 11/13 or 12/12 | 28–41 |
A few things experienced growers watch here. The jump into flower is not just a photoperiod flip — canopy PPFD usually steps up as the plant can handle more, and pushing too hard too fast during stretch invites bleaching at the colas before the canopy has filled in. In mid-flower, the ceiling is real: past roughly 1,000–1,200 PPFD, even CO2-supplemented plants hit diminishing returns, and you start trading photosynthesis for light stress, foxtailing, and bleach. Late flower often pulls intensity back slightly while the plant finishes — more light does not finish a plant faster, and it can degrade the top of the canopy.
How Light Interacts With Irrigation and VPD
Light is not an isolated system. It is the input that sets the plant's whole metabolic rate — and everything else has to be matched to it.
Light and irrigation. Higher DLI means more photosynthesis means more transpiration means faster substrate draw-down. A canopy banking 40 mol/m²/day pulls water harder than the same canopy at 28. If you raise DLI — by stepping up PPFD or holding photoperiod while the canopy fills in — but leave the irrigation program fixed, your dry-back gets steeper than intended. The plant is doing more work and you have not fed it more. Conversely, drop intensity for a ripening photoperiod and a fixed irrigation program will start over-watering, flattening the dry-back you wanted.
Light and VPD. Light drives leaf temperature, and leaf temperature drives the gap between air VPD and the leaf VPD the plant actually responds to. Crank PPFD and leaf surfaces run warmer, effectively raising VPD at the leaf even if the room sensor does not move. The two highest-energy inputs in the room — light and VPD — compound each other. High PPFD with high VPD can push transpiration past the point where stomata stay open, and the plant defends itself by closing down. Then your expensive light is heating a canopy that has stopped photosynthesizing efficiently.
The takeaway: light is a steering input. When DLI changes, the correct irrigation volume, the correct dry-back target, and the correct VPD band all move with it. A facility that sets lighting once and steers irrigation independently is fighting itself.
Common Lighting Mistakes in Commercial Facilities
Reporting fixture wattage instead of canopy PPFD. What the plant gets is canopy-height PPFD, not what the fixture draws. Hanging height, fixture age, reflectivity, and canopy position all sit between the spec sheet and the leaf. Measure at the canopy.
One PPFD reading per room. Center-of-light PPFD and edge-of-bench PPFD can differ by 200–400 µmol/m²/s. A single reading per room hides the exact non-uniformity that produces uneven canopy and uneven harvest weight across a zone.
Pushing flower PPFD without CO2. Above roughly 600–700 PPFD, ambient-CO2 plants are largely light-saturated. The extra intensity becomes heat and light stress, not biomass. You are paying for photons that hurt.
Ignoring DLI when changing photoperiod. Switching to an 11/13 ripening schedule cuts DLI by roughly 8% at the same PPFD. That is a real change in the plant's daily dose, and the irrigation program should acknowledge it.
Treating light as fixed. The most expensive mistake is conceptual: setting lighting at the start of a stage and never reading it as a variable that interacts with everything else. Light changes as the canopy fills in, as fixtures age, as photoperiod shifts. If it is not part of the steering picture, the steering picture is incomplete.
Why Light Belongs in the lb/Light Conversation
The metric is literally pounds per light. Lighting is not adjacent to the yield-efficiency conversation — it is the denominator. Every photon you deliver is an energy cost, and lb/light is the measure of how efficiently those photons become harvest weight.
That makes lighting decisions economic decisions. Running 1,100 PPFD when the canopy is light-saturated at 950 does not raise yield — it raises the energy denominator while the numerator stays flat, so lb/light falls. Letting DLI drift because photoperiod changed and nobody adjusted irrigation means the plant banks light it cannot fully convert. The waste does not show up as a dramatic failure. It shows up as a quietly worse cost-per-pound.
And as with VPD, the gap between facilities is rarely the target numbers — it is consistent execution across every zone. This is where the nightly read matters. Hyper Yield pulls live Aroya data for every zone each night and evaluates each zone's conditions against the facility's own SOP, including how the irrigation directive should respond to that zone's growth stage and light environment. When DLI steps up because a canopy filled in, the morning P1/P2 directive reflects it. The head grower sets the lighting strategy and reviews every directive; the system makes sure all 109 zones get their irrigation matched to their actual light load, the same way, every day — with every directive and override logged. Light stays a managed variable instead of a forgotten setting.
Frequently Asked Questions
What is a good PPFD for cannabis flower? With CO2 supplementation, commercial mid-flower targets generally run 800–1,000+ µmol/m²/s at the canopy. Without CO2, plants are largely light-saturated around 600–700, and pushing higher mostly produces heat and light stress rather than biomass. Always measure at canopy height, not by fixture spec.
What is the difference between PPFD and DLI? PPFD is intensity — how bright the light is at a given moment, in µmol/m²/s. DLI is total dose — how much light the plant receives over the full photoperiod, in mol/m²/day. PPFD is the dial you set; DLI is the accumulated result the plant actually responds to. DLI roughly equals PPFD × photoperiod seconds ÷ 1,000,000.
What DLI should cannabis get in flower? Mid-flower commercial DLI commonly lands in the 35–50 mol/m²/day range with adequate CO2 and a 12/12 photoperiod. The exact figure depends on PPFD and photoperiod together — which is why a facility moving to an 11/13 ripening schedule is reducing DLI even with no change to the fixtures.
Can cannabis get too much light? Yes. Past roughly 1,000–1,200 PPFD, even CO2-supplemented plants hit diminishing returns, and the extra intensity drives bleaching, foxtailing, and light stress instead of yield. Because lb/light has light in the denominator, over-lighting actively lowers yield efficiency — you raise the energy cost without raising harvest weight.
Does photoperiod change my DLI? Yes, directly. At the same PPFD, an 11-hour photoperiod delivers roughly 8% less DLI than a 12-hour one. Any ripening-phase photoperiod reduction is a real cut to the plant's daily light dose, and the irrigation program should be adjusted to match the lower transpiration demand.
How does lighting affect crop steering? Light sets the plant's metabolic rate, so it drives transpiration and substrate draw-down. Higher DLI means faster dry-back and harder nutrient pull; lower DLI means the opposite. Light also raises leaf temperature, which shifts leaf-level VPD. That makes light a steering input — when DLI changes, the correct irrigation volume, dry-back target, and VPD band all move with it.
Summary
PPFD is the intensity of usable light hitting your canopy; DLI is the total daily dose the plant banks — and DLI is what correlates with biomass. Commercial targets climb from roughly 200–400 PPFD in early growth to 800–1,000+ in flower with CO2, but the target numbers are not where facilities lose yield. They lose it by treating light as a fixed setting instead of a steering input that has to be matched to irrigation and VPD, by reporting fixture wattage instead of canopy PPFD, and by ignoring DLI when photoperiods change. Because lb/light has light in the denominator, every wasted photon is a worse cost-per-pound. Putting the nightly read on a system that evaluates every zone against your own SOP — and matches each zone's irrigation directive to its actual light load — is how light stays a managed variable across all 109 zones instead of a forgotten one.
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