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Lights for beginners: how much light, PPFD and DLI made simple

PPFD is how much usable light lands on the canopy each second (µmol/m²/s); DLI is the day's total (mol/m²/day = PPFD x hours x 0.0036). Aim for about 150–300 µmol/m²/s for seedlings (DLI 10–20), 300–600 in veg (DLI 20–40) and 600–900 in flower at 12/12 (DLI 26–39). Set height and dimming by measuring PPFD at the canopy and watching the top leaves, not by a fixed distance.

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Short answer. PPFD is how much usable light lands on the canopy each second, in µmol/m²/s. DLI is the day's total, in mol/m²/day: DLI = PPFD x hours x 0.0036. Aim for about 150–300 µmol/m²/s for seedlings (DLI 10–20), 300–600 in veg (DLI 20–40) and 600–900 in flower at 12/12 (DLI 26–39). Set the height and dimmer by measuring at the canopy and watching the top leaves, not by a fixed distance.

What do you need to set up your light?

Setting up a grow light needs only a few generic tools:

  • [ ] A dimmable full-spectrum LED with a published PPFD map (choosing your setup)
  • [ ] Adjustable rope or ratchet hangers rated for the fixture's weight
  • [ ] A timer (mechanical or digital) or a controller
  • [ ] A quantum (PAR) meter, or at least a phone light app used only for comparisons
  • [ ] An IR thermometer to check leaf temperature
  • [ ] A tape measure and a notebook or journal
  • [ ] Eye protection: sunglasses rated for bright light, or simply don't look straight into the diodes

What are PPFD and DLI, in plain words?

PPFD is the light "speed" at the canopy and DLI is the "distance" the plant travels in a day. PPFD counts photons from 400–700 nm (PAR) landing on one square metre per second. DLI adds them up over the whole photoperiod. A plant cares about both: PPFD sets how hard leaves photosynthesise at any moment, and DLI sets how much sugar the day builds.

The conversion is fixed: DLI = PPFD x hours x 3,600 ÷ 1,000,000, or simply PPFD x hours x 0.0036.

PPFD (µmol/m²/s) Hours DLI (mol/m²/day) Where it fits
200 18 13 Seedling
300 18 19 Seedling to early veg
450 18 29 Veg
400 24 35 Veg on 24/0
600 12 26 Early flower, gentle
800 12 35 Flower
900 13 42 Flower on 13/11
1,000 12 43 Strong flower, experienced growers

Watts are not light. Lux and foot-candles are weighted for human eyes, not plants (lux, foot-candle). Only PPFD and DLI describe what the plant gets.

How much light does each stage need?

Each stage needs more light than the last, roughly doubling from seedling to veg and again into flower. These targets are for a beginner at ambient CO₂ (~420 ppm):

Stage PPFD (µmol/m²/s) Photoperiod DLI (mol/m²/day) Typical LED setting
Seedling, clones 150–300 18/6 10–20 25–40 % power, 45–60 cm (18–24 in)
Veg 300–600 18/6 to 24/0 20–40 50–75 %, 30–45 cm (12–18 in)
Flower 600–900 12/12 or 13/11 26–42 75–100 %, 30–45 cm (12–18 in)
With CO₂ and experience up to ~1,200 12/12 up to ~52 See CO₂ enrichment

The research ceiling is higher than this. In the best indoor trial, flower yield rose linearly from 120 to about 1,800 µmol/m²/s at ambient CO₂, with no change in cannabinoid concentration [1][2]. That doesn't mean a beginner should run 1,500: the plant must also have the water, nutrients, temperature and CO₂ to use it, and each extra µmol costs electricity [5]. Young plants tolerated high light well in a vegetative study [7], but seedlings and fresh transplants still do better gently. DLI for young plants follows extension guidance for other crops [4].

Photoperiod notes (house positions). 24/0 in veg is endorsed: cannabis needs no sleep in veg. Flower doesn't have to be exactly 12/12: 13/11 is endorsed, and some cultivars flower with 14–15 h, usually as about 12 h of full intensity plus sunrise and sunset ramps. Test your cultivar. The dark period must be truly dark (light leaks).

How do you set your light, step by step?

Set the light by measurement and by the plant, in this order:

  1. Read the fixture's PPFD map. Note the PPFD at the centre and edges at 30, 45 and 60 cm (12, 18, 24 in). Ignore "equivalent watts".
  2. Hang the light level over the centre of the canopy, at 45–60 cm (18–24 in) for seedlings.
  3. Set the timer. 18/6 for seedlings and veg (or 24/0); 12/12 or 13/11 for photoperiod flower; 18/6 or 20/4 for autoflowers throughout.
  4. Dim to the stage target. Start seedlings at 25–40 % power. Measure PPFD at the top leaves at the centre, the four corners and halfway out.
  5. Work out your DLI. Multiply the average PPFD by hours by 0.0036 and compare with the table.
  6. Raise intensity in steps. Increase power by about 10 % every 2–3 days as the plant grows, or lower the light by 5 cm (2 in) at a time. Never jump from seedling to flower settings in one go.
  7. Check leaf temperature. With an IR thermometer, the top leaves should read about 24–28 °C (75–82 °F) under LED; above about 30 °C (86 °F), dim or cool first [3].
  8. Watch the top leaves for 48 h after every change. Leaves praying slightly upward and a deep green are fine; bleached, yellow-white or upward-cupped tops mean back off 10–20 % (heat and light stress).
  9. Keep the gap as plants grow. Re-measure weekly, and daily in the 2–3 week stretch after the flip, when plants can grow 2–5 cm (1–2 in) a day.
  10. Log every change: height, power %, PPFD and DLI.

Does the inverse square law apply to grow lights?

Only roughly. The inverse square law says light from a small point source falls to a quarter when you double the distance. A large LED board or multi-bar fixture is not a point source: at 30–60 cm (12–24 in) its intensity falls more gently, and reflective tent walls return some of the light. So moving a big LED from 30 to 60 cm might cut centre PPFD by about a third to a half, not by three-quarters. The rule is still useful as a warning: small changes in height matter most when the light is close. Measure rather than calculate.

Can you trust a phone light meter?

A phone light meter is good for comparisons, not for absolute numbers. Phone sensors measure lux weighted for human vision, and the lux-to-PPFD conversion depends on the spectrum: roughly 1 µmol/m²/s per 54 lux in sunlight, but different for white LEDs and different again for red-heavy fixtures. Readings can be off by 20–40 % or more. Use a phone app to find hot and dim spots, and to check that today matches last week. Use a quantum meter (or the fixture's PPFD map) for the actual targets.

Does spectrum matter for a beginner?

Spectrum matters much less than the amount of light and the fixture's efficacy. A cannabis trial found that less blue light slightly increased yield, but that efficacy (µmol per joule) mattered more for cost [6]. UV has repeatedly failed to raise cannabinoids in controlled trials [2]. Any reputable white full-spectrum LED is fine; see light spectrum.

How will you know it's working?

You'll know the light is right when growth is compact and even, and the top leaves look healthy:

  • Seedlings: short, sturdy stems; the first node sits close to the soil; no leaning toward the light.
  • Veg: internodes of 2–5 cm (1–2 in), leaves held flat or slightly up, mid to deep green.
  • Flower: tight, even colas across the tent; no bleached tops; lower buds not much smaller than upper.
  • Readings: canopy PPFD within about 10–15 % of target at the centre and within 30 % at the edges; leaf temperature under 30 °C (86 °F).

What mistakes do beginners make with lights?

Beginners usually have too much light on seedlings and too little in flower:

  • Full power on seedlings. Dim to 25–40 % or raise the light.
  • Setting height by a generic chart instead of the fixture's map and the plant.
  • Buying by watts or "equivalent". Compare µmol/J and the PPFD map [5].
  • Ignoring the corners. Edge plants can get half the centre's light.
  • Forgetting the stretch. Tops can hit the light within two weeks of the flip.
  • Big jumps. Raise intensity 10 % at a time.
  • Light leaks in flower. Even dim light at night can delay flowering in sensitive cultivars and reveal herm-prone genetics.
  • Trusting phone-app numbers as if they were a quantum meter.

Evidence

  • Yield rising linearly with PPFD up to ~1,800 µmol/m²/s with no potency change: Supported by cannabis trials [1][2].
  • Photosynthesis response to light and temperature: Supported [3]. High light tolerated in veg: Supported, preprint [7].
  • DLI targets for young plants: Supported, extrapolated from other crops [4].
  • Efficacy over spectrum for cost: Supported [5][6].
  • Stage PPFD bands, LED settings, phone-meter error ranges: Grower practice.
  • 24/0 in veg and 13/11 in flower: house positions; see photoperiod and light leaks for the evidence.

Questions this article answers

What is the difference between PPFD and DLI?

PPFD (photosynthetic photon flux density) is the number of usable photons hitting a square metre of canopy each second, in µmol/m²/s. DLI (daily light integral) is the total for the whole day, in mol/m²/day. DLI = PPFD x hours of light x 0.0036. So 400 µmol/m²/s for 18 hours is a DLI of about 26, the same as about 600 for 12 hours.

How much light does a cannabis seedling need?

A cannabis seedling needs about 150–300 µmol/m²/s PPFD for 18 hours, a DLI of roughly 10–20 mol/m²/day. That is often an LED dimmed to 25–40 % at 45–60 cm (18–24 in). Too little light makes a long, leaning stem; too much makes small, pale, upward-cupped leaves. Raise the light gradually as true leaves appear.

How much light does cannabis need in flower?

In flower a beginner should aim for about 600–900 µmol/m²/s at the canopy for 12 hours, a DLI of about 26–39 mol/m²/day. Cannabis yield rose roughly linearly with light up to about 1,800 µmol/m²/s in a controlled trial, but above about 900–1,000 without added CO₂ the risk of light stress and the electricity cost climb faster than a beginner's returns.

How far should my LED be from my plants?

There is no single right distance: hang the LED where the canopy reads your target PPFD, usually 30–45 cm (12–18 in) for a mid-power LED in veg and flower and 45–60 cm (18–24 in) or dimmed for seedlings. Check the fixture's PPFD map, measure at the top leaves, and back off 10–20 % if top leaves bleach, cup upward or feel hot.

Can I use a phone app to measure PPFD?

A phone app gives only a rough PPFD estimate. Phone cameras and lux sensors are built for human vision, and converting lux to PPFD depends on the spectrum, so readings can be off by 20–40 % or more. Use one to compare spots and track changes in your own tent, not as an absolute number. A quantum (PAR) meter is the proper tool.

Should I run my lights 24 hours a day in veg?

You can. GenetixBay endorses 24/0 in veg: cannabis does not need a dark period while vegetating. 18/6 is the common default and saves 25 % of the power. If you run 24/0, lower the PPFD so the DLI stays sensible: 400 µmol/m²/s for 24 hours is already about 35 mol/m²/day. Photoperiods need 12/12 or 13/11 with a truly dark night to flower.

Why do my top leaves look bleached under my LED?

Bleached, yellow-white top leaves or bracts under an LED are usually light stress: too much PPFD for that plant, often with high leaf temperature. Raise or dim the light by 10–20 %, check leaf temperature is under about 30 °C (86 °F), and make sure roots are taking up water. Lower leaves stay green because they get less light.

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. 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. doi.org/10.3389/fpls.2022.974018
  3. 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. doi.org/10.1007/s12298-008-0027-x
  4. Runkle E. The benefits of lighted plugs (DLI for young plants). Michigan State University Extension. canr.msu.edu/resources/the_benefits_of_lighted_plugs
  5. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. doi.org/10.1038/s41438-020-0283-7
  6. Westmoreland FM, Kusuma P, Bugbee B, 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE 16(3): e0248988. pmc.ncbi.nlm.nih.gov/articles/PMC7987162/
  7. 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). doi.org/10.20944/preprints202104.0417.v1

Related

EnvironmentSupportedIntermediate · Draft

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.

EnvironmentContestedIntermediate · Draft

Light spectrum: blue, red, far-red and UV, what's real

Photon quantity (PPFD/DLI) and fixture efficacy matter far more than spectrum. Less blue slightly increased cannabis yield (about 12 % from 4 % to 20 % blue). Far-red stretches plants; its yield effect is contested and seems to trade off against cannabinoid concentration. UV has repeatedly failed to raise cannabinoids in controlled trials and can cut yield at high doses.

EnvironmentSupportedBeginner · Draft

Heat and light stress: taco leaves, bleaching and foxtails

Heat and light stress show at the top of the plant, closest to the light: leaves curl up or taco, bracts bleach white or yellow, buds may foxtail. Measure leaf temperature and canopy PPFD before blaming nutrients. Typical fixes: dim or raise the light 10–20 %, bring leaf temperature under about 30 °C (86 °F), restore airflow and root-zone water uptake.

EnvironmentSupportedBeginner · Draft

Photoperiod and light leaks: 18/6, 12/12 and the dark period

Photoperiod cannabis stays vegetative under long days (18/6 up to 24/0; no sleep period is needed in veg) and flowers when the uninterrupted night is long enough: 12/12 or 13/11, and some cultivars flower at 14–15 h, usually as ~12 h at high intensity plus sunrise and sunset ramps. Light during the dark period, even very dim, can delay or weaken flowering in sensitive cultivars; leaks can also expose herm-prone genetics, triggering male flowers on susceptible plants. Some cultivars yield more at 13–14 h days, at a possible cost in THC concentration.

Grower practiceBeginner · Draft

Choosing your setup: tent, space and budget tiers

Choose the space first, then size everything to it. A 60 x 60 cm (2 x 2 ft) tent suits one or two plants and a 100–200 W LED; 80 x 80 to 90 x 90 cm (about 3 x 3 ft) suits two or three plants and 200–320 W; 120 x 120 cm (4 x 4 ft) suits two to four plants and 400–650 W. Size the exhaust fan to change the tent's air about once a minute, plus 25–50 % for the carbon filter and ducting.

SupportedBeginner · Draft

Your first grow, start to finish: the master guide

A first indoor grow takes about 3–5 months from seed to jar: 1 week to set up and test the room, 1–2 weeks of germination and seedling, 3–6 weeks of veg (photoperiods), a 10–14 day transition after the flip, 7–10 weeks of flower, 7–14 days of drying and at least 2–4 weeks of curing. Use one to four feminised plants, a tent, one good LED, a fan and carbon filter, 11–19 L pots, a pH pen and an EC meter, and keep a journal.

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