Light intensity, in horticulture, means the photon flux density at the plant: PPFD in µmol/m²/s. It is the single biggest lighting factor in cannabis yield. Spectrum and photoperiod matter, but within sensible ranges neither moves yield as much as giving the canopy more photons.
In practice
Llewellyn and colleagues (2022) found indoor cannabis yield rose in proportion to light intensity across the range they tested, and Rodriguez-Morrison and colleagues (2021) saw gains up to about 1,800 µmol/m²/s in flower. Established. The practical ceiling is set by everything else: heat removal, VPD, CO2, water and nutrient supply, and the cultivar.
Typical progression: seedlings 100–300, veg 400–700, flower 700–1,100 µmol/m²/s without CO2 enrichment. Raise intensity over a week or so rather than in one jump; leaves acclimate. Signs you have outrun the rest of the environment: upward-cupping (tacoing) top leaves, bleached tips, and slower rather than faster growth.
"Intensity" in casual talk often means fixture wattage or "how bright it looks". Neither is a measurement of what the plant receives; distance, reflectors and spectrum change everything.
Don't confuse with
- DLI: intensity accumulated over the day.
- Wattage: electrical input, not light output.
- Brightness (lux): eye-weighted, misleading under grow lights.
Photosynthetic photon flux density: the number of PAR photons landing on one square metre each second, in µmol/m²/s; the standard measure of light intensity at the canopy.The daily length of light versus dark; photoperiod cannabis flowers when nights grow long enough, rather than at a fixed age.Vapour pressure deficit: how much more water vapour the air could hold at its temperature, in kPa; it is the main environmental driver of transpiration.CO2 enrichment raises carbon dioxide in a sealed grow space from about 420 ppm to roughly 800–1,500 ppm, letting plants use higher light intensities productively.A seedling is a young cannabis plant from emergence of the cotyledons until it has several sets of true leaves, typically the first 2-3 weeks.Bag appeal is how good flower looks in the bag or jar, frost, colour, density and trim, which sells cannabis but does not measure its quality.Daily light integral: the total number of PAR photons a square metre of canopy receives in one day, in mol/m²/day; PPFD multiplied by hours of light.The light saturation point is the PPFD above which photosynthesis stops rising; for cannabis it is unusually high and moves upwards with CO2 and temperature.The inverse square law says light from a point source falls with the square of distance; grow fixtures are not point sources, so it only roughly predicts PPFD.
