Short answer. Light is the single biggest yield lever indoors. In a 2021 trial with LEDs, a 12 h flowering photoperiod and ambient CO₂, dry flower yield rose linearly from 116 g/m² at 120 µmol/m²/s to 519 g/m² at 1,800 µmol/m²/s (about 4.5×) and did not saturate; cannabinoid potency did not change with light, terpenes rose modestly [1]. A 2022 trial on a high-THC cultivar found the same proportional yield response [2]. Practical targets: seedlings/clones 100–300, veg 300–700 (up to ~900), flower 800–1,200 µmol/m²/s, higher with CO₂ if the plants and your power bill tolerate it.
What's happening
- PPFD (photosynthetic photon flux density, µmol/m²/s) is the instantaneous photon rate hitting the canopy, 400–700 nm.
- DLI (daily light integral, mol/m²/day) is the total for the day:
DLI = PPFD × hours × 0.0036. - Individual leaves saturate well below the canopy's limit; whole-plant yield keeps climbing because lower leaves and bud sites receive more light as the top gets more [1]. Leaf light-response curves are a poor predictor of yield [1].
- Higher light also shortens internodes, thickens leaves and makes denser, more compact plants in veg [3].
Numbers that matter
| Stage | PPFD (µmol/m²/s) | Hours | DLI (mol/m²/day) | Evidence |
|---|---|---|---|---|
| Unrooted cuttings | 50–150 | 18–24 | 3–10 | Supported for horticultural cuttings: max ~100–150 at sticking [6] |
| Seedlings (cotyledons–2 nodes) | 150–300 | 18 | 10–19 | Grower practice; young-plant DLI 10–14 gives better roots in other crops [5] |
| Early veg | 300–500 | 18–24 | 19–43 | Grower practice; 24/0 house-endorsed (no sleep period needed in veg) |
| Late veg | 500–900 | 18 | 32–58 | Supported (preprint): healthy, compact growth up to ~900 [3] |
| Flower, ambient CO₂ | 800–1,200 | 12–13 | 35–56 | Supported: yield linear to 1,800 [1][2]; 13/11 house-endorsed |
| Flower, 900–1,200 ppm CO₂ | 1,000–1,500 | 12 | 43–65 | Supported (gas exchange) [4]; grower practice for yield |
| Autoflowers | 600–1,000 | 18–20 | 39–72 | Grower practice |
Conversions: 600 µmol at 18 h = 38.9 mol; 900 µmol at 12 h = 38.9 mol; 1,200 µmol at 12 h = 51.8 mol; 400 µmol at 24 h = 34.6 mol.
Photoperiod in practice. 24/0 is endorsed in veg; 13/11 is endorsed in flower, and some cultivars flower at 14–15 h (see photoperiod). In practice a GenetixBay flowering day is about 12 h at high intensity, with dimmed sunrise and sunset ramps adding the rest of a 13–15 h photoperiod. The ramps are low-intensity light, so they add little DLI, but they count as day: the uninterrupted dark that follows is what the plant measures. Count the ramps at their real PPFD when you work out DLI.
The economics
The same 2021 trial estimated ~0.54 g/kWh at 900 µmol/m²/s; doubling light raised yield about 70 % but cut energy-use efficacy about 15 % [1]. Fixture efficacy (µmol/J) matters more than any other lighting specification for cost per gram; current good LEDs exceed 2.5–3 µmol/J [7].
How to measure
- Use a quantum (PAR) sensor, not a lux meter. Lux-to-PPFD conversion depends on spectrum and is only rough.
- Take a 9- or 16-point grid at canopy top height; record average and min/max. Aim for min ≥ 70–80 % of average.
- Re-measure when plants grow; the canopy moves toward the light.
- Phone apps are ±20 % or worse; fine for trends, not for setpoints.
How to raise light safely
Step up 10–15 % every 3–5 days in veg and early flower and watch the top leaves. Signs you've overshot: upward-praying leaves that stay that way at lights-on, bleached or yellow-white top bracts, tacoing, and pale new growth at the top only (heat and light stress).
Common mistakes
- Using the manufacturer's centre-point PPFD at 30 cm as "my PPFD".
- Hanging lights for the tallest cola and starving the rest.
- Pushing above ~1,200 µmol/m²/s without CO₂ and good root-zone health, then blaming the strain.
- Expecting more light to raise THC percentage. It raises grams; potency per gram stayed flat [1][2].
Evidence
Linear yield response to light in flower is Supported by two controlled indoor trials from one group [1][2]; it needs replication across more cultivars and with CO₂. No effect of light intensity on cannabinoid concentration is Supported [1][2]. Veg targets rely on a non-peer-reviewed preprint [3] and grower practice. Seedling/clone values are Grower practice plus general horticulture [5][6].
