# 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](https://genetixbay.com/knowledge/glossary/ppfd.md) 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](https://genetixbay.com/knowledge/glossary/dli.md) 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](https://genetixbay.com/knowledge/glossary/thc.md)?** 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](https://genetixbay.com/knowledge/glossary/terpene.md) 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](https://genetixbay.com/knowledge/glossary/taco-leaf.md) 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 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–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](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) 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].

## Quick answers

- [Can I grow cannabis with household LED or CFL bulbs?](https://genetixbay.com/knowledge/answers/can-i-grow-with-household-bulbs.md): You can keep seedlings and clones alive under household LED or CFL bulbs, but you can't grow good flower with them.
- [How can I increase my yield indoors?](https://genetixbay.com/knowledge/answers/how-to-increase-yield-indoors.md): Increase indoor yield with more usable light first: dry flower yield rose linearly from 116 to 519 g/m² as canopy PPFD went from 120 to 1,800 µmol/m²/s.
- [How can I measure PPFD without a PAR meter?](https://genetixbay.com/knowledge/answers/measure-ppfd-without-a-par-meter.md): Without a PAR meter, estimate PPFD from the fixture's published PPFD map at your hanging height, cross-check with watts × µmol/J ÷ canopy area, and use a phone or lux meter only for comparisons.
- [How do I get bigger, denser buds?](https://genetixbay.com/knowledge/answers/how-to-get-bigger-denser-buds.md): Bigger, denser buds come mainly from more light at every bud site: 800–1,200 µmol/m²/s at canopy in flower at ambient CO₂, on an even canopy.
- [How much sun does outdoor cannabis need?](https://genetixbay.com/knowledge/answers/how-much-sun-outdoors.md): Outdoor cannabis needs at least 6–8 hours of direct sun a day, and more is better: full summer sun reaches 1,500–2,000 µmol/m²/s at midday and a daily light integral of roughly 40–60 mol/m²/day on clear days.
- [How much yield should I expect per watt or per square foot?](https://genetixbay.com/knowledge/answers/grams-per-watt.md): Expect roughly 100–500 g of dry flower per square metre (about 10–50 g, or 0.3–1.6 oz, per square foot) indoors, rising with light; well-run research rooms at 750–900 µmol/m²/s measured 500–750 g/m².
- [What should I look for when buying a grow light?](https://genetixbay.com/knowledge/answers/what-to-look-for-in-a-grow-light.md): When buying a grow light, look first for efficacy above about 2.5 µmol/J, a measured PPFD map over your tent's footprint showing 600–900 µmol/m²/s average with edges at least 70–80 % of the centre, and a real wattage at 
- [What wattage LED do I need for my tent?](https://genetixbay.com/knowledge/answers/what-wattage-light-for-my-tent.md): For flower, plan on roughly 25–40 W of efficient LED (2.5–3.0 µmol/J) per square foot, about 270–430 W/m², which gives an average of about 600–900 µmol/m²/s.
- [Why are my buds airy or larfy?](https://genetixbay.com/knowledge/answers/why-are-my-buds-airy.md): Airy, larfy buds are usually buds that got too little light: lower and inner sites often receive a fraction of the canopy-top PPFD, well below the 800–1,200 µmol/m²/s flower target.
- [Why don't lumens matter for grow lights?](https://genetixbay.com/knowledge/answers/lumens-vs-ppfd.md): Lumens don't matter for grow lights because lumens, lux and foot-candles measure light as the human eye sees it, weighted heavily to green-yellow, while plants use photons from 400–700 nm about equally.

## 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