# PPFD and DLI by stage, with the evidence

> In the best indoor trial, cannabis flower yield rose linearly with canopy PPFD from 120 to 1,800 µmol/m²/s at ambient CO₂ (116 to 519 g/m²) with no change in cannabinoid potency. Practical targets: 100–300 µmol/m²/s for seedlings and clones, 300–700 (up to ~900) in veg, and 800–1,200 in flower, or more with CO₂, as long as the plant shows no stress and the electricity is worth it.

- Canonical: https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.html
- Category: Environment · Level: Intermediate
- Evidence: Supported · Status: Draft: awaiting expert review (some figures are being checked)
- Stage: Seedling, Veg, Flower · Systems: Any system
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Questions and answers

**What PPFD should I run in flower?** A practical PPFD for cannabis in flower at ambient CO₂ is 800–1,200 µmol/m²/s at canopy height, rising to 1,000–1,500 µmol/m²/s with 900–1,200 ppm CO₂. Research shows yield kept rising linearly up to 1,800 µmol/m²/s, so the ceiling depends on plant stress, root-zone health and whether the extra electricity pays. Measure with a quantum sensor grid, not the manufacturer's centre-point figure.

**What DLI does cannabis need?** Cannabis DLI needs by stage are roughly 3–10 mol/m²/day for unrooted cuttings, 10–19 for [seedlings](https://genetixbay.com/knowledge/glossary/seedling.md), 19–43 in early veg, 32–58 in late veg and 35–56 in flower at ambient CO₂, or 43–65 with CO₂. [Autoflowers](https://genetixbay.com/knowledge/glossary/autoflower.md) on 18–20 hour days run about 39–72. Most of these figures are grower practice, while the flower response is supported by controlled trials.

**Does more light increase THC?** No, more light does not increase THC percentage in cannabis. In two controlled indoor trials, raising canopy light from 120 to 1,800 µmol/m²/s multiplied dry flower yield about 4.5 times, from 116 to 519 g/m², while [cannabinoid potency](https://genetixbay.com/knowledge/glossary/potency-testing.md) per gram stayed flat and terpenes rose only modestly. Extra light buys more grams, not stronger buds.

**Is 1500 PPFD too much without CO2?** 1,500 µmol/m²/s PPFD is not automatically too much for cannabis without added CO₂: one trial saw yield keep climbing to 1,800 at ambient CO₂. Still, practical targets at ambient CO₂ are 800–1,200, and pushing past about 1,200 without CO₂ and healthy roots often causes stress. Raise light 10–15 % every 3–5 days and back off at bleaching, tacoing or leaves that stay praying upward.

**How do I convert PPFD to DLI?** To convert PPFD to DLI, multiply PPFD in µmol/m²/s by the hours of light and by 0.0036, giving mol/m²/day. For example, 600 µmol/m²/s for 18 hours equals 38.9 mol/m²/day, 900 for 12 hours is also 38.9, and 1,200 for 12 hours is 51.8. Use your measured canopy average, not a fixture's centre-point figure, and count dim ramps at their real PPFD.

**What PPFD for seedlings and clones?** Cannabis seedlings do well at about 150–300 µmol/m²/s PPFD on 18-hour days, and unrooted clones at 50–150 µmol/m²/s on 18–24 hours. General horticulture research supports a maximum of roughly 100–150 µmol/m²/s for cuttings at sticking, and young-plant DLI of 10–14 mol/m²/day gives better roots in other crops; the cannabis figures are grower practice.

**Short answer.** Light is the single biggest yield lever indoors. In a 2021 trial with [LEDs](https://genetixbay.com/knowledge/glossary/led.md), a 12 h flowering [photoperiod](https://genetixbay.com/knowledge/glossary/photoperiod.md) 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](https://genetixbay.com/knowledge/glossary/bud-site.md) 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](https://genetixbay.com/knowledge/glossary/internode.md), 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](https://genetixbay.com/knowledge/glossary/cotyledon.md)–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](https://genetixbay.com/knowledge/glossary/24-0.md) 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](https://genetixbay.com/knowledge/glossary/13-11.md) 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](https://genetixbay.com/knowledge/environment/photoperiod-light-leaks.md)). 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

1. Use a quantum ([PAR](https://genetixbay.com/knowledge/glossary/par.md)) sensor, not a lux meter. Lux-to-PPFD conversion depends on spectrum and is only rough.
2. Take a 9- or 16-point grid at canopy top height; record average and min/max. Aim for min ≥ 70–80 % of average.
3. Re-measure when plants grow; the canopy moves toward the light.
4. 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](https://genetixbay.com/knowledge/environment/heat-light-stress.md)).

## 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](https://genetixbay.com/knowledge/glossary/cannabinoid.md) 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].

## Sources

1. 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. https://doi.org/10.3389/fpls.2021.646020
2. Llewellyn D, Golem S, Foley E, Dinka S, Jones AMP, Zheng Y, 2022. Indoor grown cannabis yield increased proportionally with light intensity, but ultraviolet radiation did not affect yield or cannabinoid content. Frontiers in Plant Science 13:974018. https://doi.org/10.3389/fpls.2022.974018
3. Moher M, Llewellyn D, Jones M, Zheng Y, 2021. High light intensities can be used to grow healthy and robust cannabis plants during the vegetative stage of indoor production. Preprints 2021040417 (not peer reviewed). https://doi.org/10.20944/preprints202104.0417.v1
4. Chandra S, Lata H, Khan IA, ElSohly MA, 2008. Photosynthetic response of Cannabis sativa L. to variations in PPFD, temperature and CO2. Physiology and Molecular Biology of Plants 14(4):299–306. https://doi.org/10.1007/s12298-008-0027-x
5. Runkle E. The benefits of lighted plugs (DLI for young plants). Michigan State University Extension. https://www.canr.msu.edu/resources/the_benefits_of_lighted_plugs
6. Michigan State University. Lighting of cuttings (rooting-phase PPFD). https://www.canr.msu.edu/uploads/resources/pdfs/lighting-of-cuttings.pdf
7. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7