Cannabis PPFD, PAR and DLI: A Practical Guide

Cannabis is one of the most light-hungry crops grown indoors, and its response to light intensity is unusually linear — more photons, within practical limits, almost always means more yield. The terminology is the same as for any other plant, but the target numbers are crop-specific and worth understanding in detail. This guide covers what each measurement means for cannabis, what peer-reviewed research tells us about optimal ranges, and where the research stops and grower experience takes over.

Written by the GrowScope teamPublished 2026-09-06

The three terms that matter for cannabis lighting

PAR (Photosynthetically Active Radiation) is the wavelength range from roughly 400 to 700 nanometres that drives photosynthesis — it is a range, not a measurement. PPFD (Photosynthetic Photon Flux Density) is the number of PAR photons hitting one square metre of canopy every second, expressed in µmol/m²/s, and it is the number you measure at leaf level. DLI (Daily Light Integral) is PPFD accumulated over the entire photoperiod, expressed in mol/m²/day, and it is the number that determines whether the plant got enough total light for the day. For cannabis in flower running a 12-hour photoperiod, a PPFD of 900 µmol/m²/s yields a DLI of roughly 39 mol/m²/day.

What peer-reviewed research tells us about cannabis and light

Two studies from Youbin Zheng's lab at the University of Guelph provide the strongest published data on cannabis light response. Rodriguez-Morrison et al. (2021) tested canopy PPFD levels from 120 to 1,800 µmol/m²/s during flowering and found that yield increased linearly across the entire tested range with no saturation point — at roughly 900 µmol/m²/s (DLI ~39 mol/m²/day on a 12-hour cycle), predicted yield reached approximately 303 grams per square metre. Llewellyn et al. (2022) tested three fixed intensities — 600, 800, and 1,000 µmol/m²/s at a DLI of 25.9 mol/m²/day — and found that each additional 100 µmol/m²/s produced roughly 4.6 grams per plant of extra yield. Both studies confirm that cannabis is exceptionally light-responsive, with yield gains continuing well past the intensities that saturate many other crops.

Seedling-stage targets — where manufacturer data fills the gap

The peer-reviewed studies above focused on the flowering stage, and there is no published research establishing precise PPFD or DLI targets for cannabis seedlings. The commonly recommended range of 100 to 300 µmol/m²/s, corresponding to a DLI of roughly 6 to 16 mol/m²/day depending on photoperiod, comes from lighting manufacturers such as Thrive Agritech and Fluence — not from peer-reviewed studies — and should be treated as a practical starting point rather than a research-backed optimum. Seedlings have minimal leaf area and fragile root systems; light that a mature plant would barely register can overwhelm them. Start at the low end of the range and increase gradually as the first true leaves expand.

Vegetative and flowering targets — combining research with practice

During vegetative growth, experienced growers commonly run 400 to 600 µmol/m²/s at an 18-hour photoperiod, giving a DLI of roughly 26 to 39 mol/m²/day. Once the plant transitions to flower and the photoperiod drops to 12 hours, PPFD must increase to maintain the daily light budget. The Zheng lab data shows that flowering plants can productively use intensities of 800 to 1,000 µmol/m²/s and beyond, provided temperature, CO₂, and nutrition keep pace. Without supplemental CO₂, pushing past roughly 1,000 µmol/m²/s tends to hit diminishing returns — not because the plant cannot use the light, but because other factors become limiting.

Measuring light and adjusting over the cycle

A quantum sensor at canopy height is the reference method, but a smartphone app using the camera sensor with a white-paper diffuser can stay within about twenty percent of a real sensor — enough for practical positioning decisions. Take readings at several points across the canopy, not just directly under the centre of the fixture, and record them. As the plant grows and the canopy rises, the distance to the light shrinks and PPFD increases — what was safe two weeks ago may be causing light stress today. Adjust fixture height or dimming in small steps and watch the plant's response over the following three to five days before making further changes.

How GrowScope helps

GrowScope's physically modelled 3D light map accounts for both direct Lambertian output and reflected light from tent walls, so you can see PPFD distribution across the entire canopy area rather than relying on a few handheld readings. The platform lets you log PPFD measurements alongside plant height and growth rate, making it straightforward to spot when a canopy is drifting too close to the light before stress symptoms appear.

Key takeaway

Peer-reviewed research shows cannabis yield increases linearly with light intensity well past 1,000 µmol/m²/s in flower, making PPFD and DLI among the highest-leverage variables to measure and optimise — and a quantum sensor or a well-used phone app is all you need to start.