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Sprout & Spectrum

How Far Should Grow Lights Be From Seedlings?

Four to six inches for a strip light, and up from there as the seedlings do. The reason is the inverse-square law, and it is unforgiving at close range.

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

A close-up of a young seedling emerging from potting soil

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.

#ProductBest forEfficacyPrice
1
Linked LED strip lights mounted under a wire shelf above trays of seedlings
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.
Runs cool enough to sit closeNot published
2
A square white LED grow panel hanging above leafy seedlings indoors
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.
Dim instead of raising2.5 umol/J
3
A flat white-light LED grow panel suspended above a tray of young plants on a wire rack
Mars Hydro TS 1000
The most photons of anything on this list, and one of only three fixtures here that publishes both PPF and efficacy so the claim can be checked against itself.
Height to spare2.3 umol/J

The reasoning

Keep LED strip lights four to six inches above the tops of seedlings, and raise them as the seedlings grow. That is University of Minnesota Extension’s figure, and it is the one to start from. Fluorescent shop lights sit closer still — Kansas State University Extension puts them roughly an inch above the leaf tops, because a fluorescent tube runs cool and is much dimmer at the canopy.

Starting distance from the top of the seedlings, by fixture type
Fixture typeStarting distanceAdjust when
T5 / T8 fluorescent tube1–3 inchesRaise weekly. Fluorescents run cool enough to sit very close; the risk is contact, not heat.
LED strip / bar (20 W class)4–6 inchesRaise as the tallest seedling gets within about 3 inches.
LED panel (100–150 W class)12–18 inches, dimmed if possibleThese are built for a mature canopy. Over bare seedlings, raise them and turn the dimmer down.
Grow bulb in a desk lamp6–12 inchesCheck for leaf curl or bleaching; bulbs concentrate output into a narrow cone.
Clip light (10 W class)4–8 inchesThese are weak. Closer is usually better, up to the point of touching.

Why an inch matters this much

Light from a small source falls off with the square of the distance. Double the gap and you get a quarter of the intensity — not half. UNH Extension demonstrates the practical consequence neatly: the same LED bar needs 8 hours a day when mounted 8 inches above the crop, but 16 hours a day at 20 inches to deliver the same daily light integral.

The same fixture, two heights

Take a fixture producing 400 µmol/m²/s at 6 inches. Move it to 12 inches — double the distance — and intensity falls by a factor of 2² = 4, to roughly 100 µmol/m²/s.

At 6 inches and 14 hours: DLI = 400 × 14 × 3600 ÷ 1,000,000 = 20.2 mol/m²/day. Comfortably above the 12 mol seedling target.

At 12 inches and the same 14 hours: DLI = 100 × 14 × 3600 ÷ 1,000,000 = 5.0 mol/m²/day. Less than half the target — and the seedlings will tell you within a week.

The inverse-square relationship is exact only for a point source, and a 4 ft bar is nothing like a point source, so treat the arithmetic above as a demonstration of the magnitude rather than a precise prediction. The general rule survives: at seedling distances, small changes in height are large changes in light.

Reading the seedlings instead of a meter

You do not need a PPFD meter to diagnose distance. The plants report it clearly, in both directions.

What the seedlings are telling you
What you seeWhat it meansWhat to do
Long pale stems, leaves far apart, leaning toward the lightToo little light — the classic stretch (etiolation)Lower the fixture, or add hours, or both. Fix it early; a stretched stem does not un-stretch.
Leaves curling down or cupping away from the lightToo much intensity, or too much radiant heat at close rangeRaise the fixture two inches at a time and wait 48 hours between changes.
Pale or bleached patches directly under the fixture centerPhotobleaching from a hot spotRaise it, or dim it if the fixture supports dimming.
Sturdy short stems, leaves close together, even greenAbout rightNothing. Raise it as they grow so the gap stays constant.

The one rule that matters more than the number

The distance is not a setting, it is a moving target. Seedlings can add an inch a week under good light. Whatever gap you start with, plan to raise the fixture roughly weekly so the gap stays constant — chains with S-hooks or a ratchet hanger make this a ten-second job rather than a project you keep postponing.

For the general case beyond seedlings, see how far a grow light should be from plants, which covers mature houseplants and the fixture classes at greater heights. If you want to know whether your setup hits the target at all, the coverage calculator works it out from your fixture’s published photon output.

Every pick in full

Linked LED strip lights mounted under a wire shelf above trays of seedlings
01

Barrina T5 4ft Grow Light Strips

LED bar

Runs cool enough to sit close

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.

A square white LED grow panel hanging above leafy seedlings indoors
02

Spider Farmer SF1000

LED panel

Dim instead of raising

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.

A flat white-light LED grow panel suspended above a tray of young plants on a wire rack
03

Mars Hydro TS 1000

LED panel

Height to spare

The most photons of anything on this list, and one of only three fixtures here that publishes both PPF and efficacy so the claim can be checked against itself.

What Mars Hydro publishes

Power draw
150 W +/-5% at 100-277 V AC (source)
PPF (photon output)
343 umol/s (source)
Efficacy
2.3 umol/J (source)
Stated coverage
2.5 x 2.5 ft vegetative, 2.3 x 2.3 ft flowering (source)
Spectrum
660-665 nm, 730-740 nm, 3000 K, 5000 K (source)
Diodes
Bridgelux (source)
Dimming
Yes, 0-100% dimmer knob (source)
Dimensions
406 x 326 x 50 mm (source)

Our arithmetic

  • Average PPFD across its stated 2.5 x 2.5 ft vegetative footprint: about 470 umol/m2/s
    2.5 ft x 2.5 ft = 6.25 sq ft = 0.581 m2. 343 umol/s divided by 0.581 m2 = 590 umol/m2/s at perfect uniformity; applying a 0.8 uniformity factor gives about 472 umol/m2/s average.
  • Electricity used at 14 hours a day for 30 days: 63.0 kWh a month
    150 W x 14 h x 30 d / 1000 = 63.0 kWh.

Who should skip it

Skip it if you are lighting a bookshelf. 343 umol/s concentrated into a 2.5 ft square is far more light than a monstera wants, and you pay 150 W to deliver it.

Questions people actually ask

+How far should LED grow lights be from seedlings?
Four to six inches above the tops of the seedlings for LED strip and bar fixtures, per University of Minnesota Extension, raised regularly as the seedlings grow. Larger 100 W-class LED panels belong 12 to 18 inches away and dimmed if they support it, because they are designed for a mature canopy rather than bare trays.
+Can a grow light be too close to seedlings?
Yes. The symptoms are leaves cupping or curling away from the fixture, and bleached pale patches directly beneath its brightest point. Raise it two inches at a time and give the plants 48 hours to respond before adjusting again.
+Why are my seedlings stretching even under a grow light?
Almost always the fixture is too far away, or on for too few hours, or both. Intensity falls with the square of distance, so a light at 12 inches delivers roughly a quarter of what it delivers at 6 inches. Lower it first, then extend the photoperiod to 14 to 16 hours.
+Do I need to move the light as seedlings grow?
Yes, roughly weekly. Seedlings under good light can add an inch a week, and the distance that was right at germination is wrong by the time they have their first true leaves. Hang the fixture on chains or a ratchet hanger so raising it takes seconds.

Sources

Read next

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.