How much light does cannabis actually need?

The short answer

In the most direct experiment published, flower yield went up linearly with light intensity across the entire tested range — from 120 all the way to 1,800 µmol·m²·s⁻¹ at the canopy — and never levelled off. Rodriguez-Morrison, Llewellyn & Zheng (2021) grew flowering cannabis under that whole spread of intensities and the yield line just kept climbing. Twice the photons, roughly twice the flower, all the way up to intensities brighter than most hobby lights can produce.

So the practical answer to “how much light does it need” is: cannabis will productively use about as much as you can afford to give it, provided the rest of the environment keeps up.

The number, for people who’ve never measured it

The unit that matters is PPFD — photosynthetic photons landing on a square metre of canopy each second (µmol·m²·s⁻¹). Multiply it by your hours of light and you get the daily total, DLI. A lux meter or a phone app won’t give you this honestly: lux weights light by human-eye sensitivity, and the conversion to photosynthetic photons changes with every lamp spectrum. If you’ve never measured PPFD at your canopy, you genuinely do not know where you sit on that yield line — and light is the one input in the tent that most growers have never measured. A borrowed quantum meter, or your lamp maker’s PPFD chart at your actual hang height, is where to start.

What more light does not do

The same body of work is just as clear about the other half: cannabinoid concentration barely moves with intensity. Potency was roughly flat across the tested range, only falling away at very low intensities. More light means more flower of about the same strength — not stronger flower. If someone sells you a lamp upgrade as a potency upgrade, the published data is not on their side. (Interestingly, the 2021 study did find bud density and terpenes improving with intensity — so the quality story isn’t nothing; it’s just not THC.)

The honest caveats

Three things keep the “no ceiling found” result from being a blank cheque. First, the leaf is not the canopy: the same study found that one leaf on its own stops making use of any more light well below 1,800 µmol. The whole-plant line keeps climbing because a bright canopy drives photons deeper, to leaves that were starved — which also means the result assumes a full, well-managed canopy. Second, the experiment kept everything else — water, nutrition, temperature, airflow — up to the demand that extra light creates. More light means the plant drinks more and the tent runs hotter; cranking the intensity in a tent whose climate can’t keep up buys stress, not grams. Third, linear grams are not linear dollars: electricity scales with the photons, so where the economics stop making sense is your call, not the plant’s.

Where my device fits, stated narrowly

Croplock ships with a canopy light sensor: it logs PPFD where your plants actually sit and adds it up into your daily light integral, so the yield lever above stops being a guess. Its light channel holds the schedule you declared — and on any fault it fails dark, never light, because an unscheduled interruption of the dark period in flower is the one mistake a grow can’t take back. What it deliberately does not do is grade your lamp’s absolute output against laboratory numbers. The canopy sensor is a spectral sensor rather than a laboratory PAR meter: its maker makes no PPFD claim at all, and the scale comes from our own five-point fit against a reference meter under one fixture. That is honest for schedules, trends and day-to-day comparison, and it is worth a few percent rather than a certificate — so the readout tells you which scale it is on instead of pretending otherwise.

Sources

• Rodriguez-Morrison, V., Llewellyn, D. & Zheng, Y. (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. DOI 10.3389/fpls.2021.646020.
• Zheng, Y. & Llewellyn, D. “Lighting and CO₂.” Ch. 6 in Handbook of Cannabis Production in Controlled Environments (Y. Zheng, ed.), CRC Press, 2022. DOI 10.1201/9781003150442.

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