How Much Light Do Seedlings Actually Need?
One number, one formula, and the two ways to hit it — turn the intensity up, or leave the light on longer.
By Scooter M. — An enthusiast who's genuinely into this. I read the manuals, compile the published specs, and do the math. No lab coat.
Published · How we pick
Photographs on this site are licensed stock and are illustrative of the fixture type. We do not own the units we write about.
How this page is funded: the buy links here are Amazon Associates links, and we earn a commission on qualifying purchases at no extra cost to you. We have not been paid to feature anything on this page, we accept no free product, and commission plays no part in the ranking. Full disclosure.
We have not tested these units. Everything below comes from the manufacturer’s own published specifications, or from arithmetic we have shown our working for. Where a figure is not published, we say so instead of estimating one. Read the method.
Quick picks
Efficacy is photons out per joule in. Where a manufacturer publishes it we quote theirs; where they publish photon output and wattage but not efficacy, we divide and label it as ours; where they publish neither, the cell says so rather than guessing. Tap a row for the full write-up.
| # | Product | Best for | Efficacy | Price |
|---|---|---|---|---|
| 1 |  Spider Farmer SF1000 Publishes PPF to two decimal places and an efficacy figure that actually divides out of it. That internal consistency is rarer in this category than it should be. | Lands on 12 mol/m2/day | 2.5 umol/J | |
| 2 |  Barrina T5 4ft Grow Light Strips The default seed-starting bar for good reasons — 4 ft long, linkable in series, cheap per foot — and the reason we built a coverage calculator, because Barrina publishes no photon data whatsoever. | A whole shelf of trays | Not published | |
| 3 |  AC Infinity IONBOARD S22 The highest published efficacy of any fixture here at 2.7 umol/J, on Samsung LM301H diodes, with dimming in real 20% steps rather than a vague knob. | Dimmable to the target | 2.7 umol/J |
The reasoning
Seedlings want roughly 12 moles of light per square meter per day. That figure comes from University of New Hampshire Extension, and it is the most useful single number in seed starting, because unlike “bright light” it can be checked.
University of New Hampshire Extension. Virginia Cooperative Extension puts propagation — seedlings and cuttings — at 5 to 10 mol/m²/day, so 12 is a healthy transplant-quality target rather than a bare minimum.
Two figures, one relationship
PPFD is intensity: photosynthetically active photons landing on a square meter every second, in µmol/m²/s. DLI is the day’s total: how many moles of those photons arrive over the whole photoperiod. One converts to the other with a single line of arithmetic.
Virginia Cooperative Extension gives the same relation with a worked example: 200 µmol/m²/s for 16 hours produces 11.5 mol/m²/day, which they note “supports seedlings through bedding plants”.
Hitting 12 mol: the trade between intensity and hours
Because the two multiply, there is more than one right answer. A dim light left on longer and a bright light left on for less time deliver the same daily total.
| PPFD at the canopy | Hours per day | Resulting DLI |
|---|---|---|
| 150 µmol/m²/s | 22 h | 11.9 mol/m²/day |
| 175 µmol/m²/s | 19 h | 12.0 mol/m²/day |
| 200 µmol/m²/s | 17 h | 12.2 mol/m²/day |
| 250 µmol/m²/s | 13 h 20 m | 12.0 mol/m²/day |
| 300 µmol/m²/s | 11 h | 11.9 mol/m²/day |
| 400 µmol/m²/s | 8 h 20 m | 12.0 mol/m²/day |
The right-hand rows are cheaper to run, because you are paying for fewer hours. UNH Extension makes exactly this point about fixture class: a high-output horticultural LED needed 10 hours a day to match what a fluorescent setup achieved in 22 hours.
There is a limit at the other end. Most seedlings want a dark period, and pushing past about 18 hours buys progressively less. The practical window is 14 to 16 hours at 200 to 300 µmol/m²/s, which is what most fixtures on a shelf will deliver anyway.
Where 12 mol sits against everything else
Context matters: seedlings need substantially more light than the houseplants sharing the shelf, and substantially less than a fruiting tomato.
| Category | DLI (mol/m²/day) | Source |
|---|---|---|
| Low-light foliage houseplants | 3–6 | Iowa State University Extension |
| Propagation — seedlings and cuttings | 5–10 | Virginia Cooperative Extension |
| Foliage and flowering houseplants, seedlings | 6–10 | Iowa State University Extension |
| Transplant-quality seedlings | about 12 | University of New Hampshire Extension |
| Flowering houseplants, succulents, leafy herbs | 12–16 | Iowa State University Extension |
| Leafy greens and herbs | 9–20 | Virginia Cooperative Extension |
| Fruiting crops — tomato, cucumber | 20–30 | Virginia Cooperative Extension |
What this means for buying a fixture
Turn the target into a purchase decision in one step: multiply your area in square meters by the PPFD you want, and you have the photon output the fixture has to deliver to that area.
Two 1020 trays side by side is about 3.2 sq ft, or 0.297 m². Target 250 µmol/m²/s.
0.297 m² × 250 µmol/m²/s = 74 µmol/s landing on the trays.
At a realistic 0.8 uniformity, the fixture needs to produce about 74 ÷ 0.8 = 93 µmol/s. That is a modest requirement — well inside a single 100 W panel, and roughly what a couple of linked bars should manage.
The coverage calculator runs this in reverse from a fixture you are considering, and our seed-starting roundup sizes several specific fixtures against this target.
Every pick in full
Spider Farmer SF1000
LED panelLands on 12 mol/m2/day
Publishes PPF to two decimal places and an efficacy figure that actually divides out of it. That internal consistency is rarer in this category than it should be.
What Spider Farmer publishes
- Power draw
- 100 W +/-5% at 100-277 V AC (source)
- PPF (photon output)
- 249.21 umol/s (source)
- Efficacy
- 2.5 umol/J (source)
- Stated coverage
- 2 x 2 ft core, 3 x 3 ft maximum (source)
- Spectrum
- 650-665 nm, 730-740 nm, 2800-3000 K, 4800-5000 K (source)
- Diodes
- Bridgelux (source)
- Dimming
- Yes, dimming knob (source)
- Dimensions
- 32.5 x 29.0 x 5.9 cm (source)
- Rated life
- 55,000 hours (source)
Our arithmetic
- Does the published efficacy check out against the published PPF?: Yes — 2.49 umol/J computed against 2.5 umol/J published
249.21 umol/s / 100 W = 2.4921 umol/J. The published 2.5 umol/J is that number rounded. Not every brand's two figures agree like this, which is exactly why we divide them. - Average PPFD across its 2 x 2 ft core footprint: about 536 umol/m2/s
2 ft x 2 ft = 4 sq ft = 0.372 m2. 249.21 / 0.372 = 670 umol/m2/s at perfect uniformity; at a 0.8 uniformity factor, about 536 umol/m2/s.
Who should skip it
Skip it if raw output is what you are buying — the TS 1000 delivers 38% more photons per second, at the cost of 50 W more draw.
Barrina T5 4ft Grow Light Strips
LED barA whole shelf of trays
The default seed-starting bar for good reasons — 4 ft long, linkable in series, cheap per foot — and the reason we built a coverage calculator, because Barrina publishes no photon data whatsoever.
What Barrina publishes
- Power draw
- 20 W per fixture; an 8-pack draws 160 W (source)
- Spectrum
- 5000 K (source)
- Dimming
- No; on/off switch per fixture, linkable up to 8 in series (source)
- Dimensions
- 4 ft strip, 120-degree beam angle, 120 V AC (source)
What Barrina does not publish
- PPF in umol/s — for any strip in the range
- PPFD at any stated distance
- Photon efficacy in umol/J
- LED count per strip, and rated lifespan
Who should skip it
Skip it if you need to know what you are getting before it arrives. There is no published PPF, no PPFD and no efficacy figure, so nothing about its output can be verified in advance.
AC Infinity IONBOARD S22
LED panelDimmable to the target
The highest published efficacy of any fixture here at 2.7 umol/J, on Samsung LM301H diodes, with dimming in real 20% steps rather than a vague knob.
What AC Infinity publishes
- Power draw
- 115 W (source)
- Efficacy
- 2.7 umol/J (source)
- PPFD
- 1098 PPFD (no measurement distance published) (source)
- Stated coverage
- 2 x 2 ft (source)
- Spectrum
- Samsung LM301H, plus 660 nm, 730 nm, 3000 K, 6500 K (source)
- Diodes
- 272 diodes (source)
- Dimming
- Yes, 0-100% in 20% increments (source)
- Dimensions
- 30.0 x 30.0 cm board, 1.1 cm thick (source)
- Rated life
- 50,000 hours (source)
Our arithmetic
- Photon output implied by the published wattage and efficacy: about 311 umol/s
115 W x 2.7 umol/J = 310.5 umol/s. AC Infinity does not print PPF directly, so this is our arithmetic from two figures it does print.
What AC Infinity does not publish
- PPF in umol/s — only efficacy is given, so photon output has to be derived
- The measurement distance for the 1098 PPFD figure, without which the number cannot be compared to anything
Who should skip it
Skip it if you want to place it by the numbers. AC Infinity prints 1098 PPFD with no measurement distance attached, which makes that figure impossible to use or compare.
Questions people actually ask
+How much PPFD do seedlings need?
+How many hours of light do seedlings need per day?
+Can seedlings get too much light?
+Do seedlings need darkness?
Sources
- University of New Hampshire Extension — Growing Seedlings Under Lights — consulted September 2, 2026
- Virginia Cooperative Extension — Calculating and Using Daily Light Integral (DLI), SPES-720 — consulted September 2, 2026
- Iowa State University Extension — Important Considerations for Providing Supplemental Light to Indoor Plants — consulted September 2, 2026
- University of Minnesota Extension — Lighting for indoor plants and starting seeds — consulted September 2, 2026
Read next
Coverage & Sizing
PPFD Targets by Plant Type
Published light bands mapped to common indoor plants, with the DLI each implies.
Spectrum & PPFD
PAR, PPFD and DLI, Explained
The three photon units, what each measures, and the conversions between them, shown.
Seed Starting
How Far Should Grow Lights Be From Seedlings?
Distance by fixture type, in a table, with the inverse-square arithmetic that explains why an inch matters so much.
Seed Starting
The Best Grow Lights for Seed Starting
Sized against a 12 mol/m2/day seedling target using published photon output, not marketing claims.
Where this sits
This page belongs to Grow Lights for Seed Starting. Every figure on it is either quoted from a named published source or derived in view — the full method is on our methodology page, and how the site is funded is on the affiliate disclosure.