Skip to content
Sprout & Spectrum

Grow Lights for Tomato Seedlings

Tomatoes are the crop that punishes a weak light fastest, and the one that most rewards getting the number right.

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

Young tomato seedlings growing in small pots indoors

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
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.
One tray of tomatoes2.5 umol/J
2
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.
Growing on to transplant2.3 umol/J
3
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.
A whole shelf of themNot published

The reasoning

Tomato seedlings stretch faster and further than almost anything else you will start indoors. Give a pepper marginal light and it sits and sulks; give a tomato marginal light and within a week it is six inches tall, pencil-thin and leaning. That sensitivity makes tomatoes the best diagnostic on the shelf — and the crop where hitting the number actually pays.

Target for tomato seedlings before transplant
12–16 mol/m²/day

Seedlings generally want about 12 mol/m²/day (UNH Extension). Iowa State University Extension's high-light band — 12 to 16 mol/m²/day — covers seedlings intended for fruiting crops. Mature fruiting tomatoes want 20 to 30 (Virginia Cooperative Extension), but that is a greenhouse problem, not a windowsill one.

What that means at the tray

Reaching 14 mol/m²/day on a tomato tray
PPFD at the canopyHours per dayDLI delivered
200 µmol/m²/s16 h11.5 mol/m²/day
250 µmol/m²/s16 h14.4 mol/m²/day
300 µmol/m²/s14 h15.1 mol/m²/day
350 µmol/m²/s12 h15.1 mol/m²/day
400 µmol/m²/s10 h14.4 mol/m²/day

The practical sweet spot for tomatoes is 250 to 300 µmol/m²/s for 14 to 16 hours. That is comfortably inside what a single 100 W-class panel delivers over a couple of trays, and it is a stretch for a lone clip light over the same area.

Why tomatoes specifically stretch

Two things happen at once under low light. The plant elongates its internodes reaching for a brighter source — that is the visible stretch. And it does so at the expense of stem thickness, because the carbon it does capture is going into length rather than girth. The result is a tall seedling with a stem that cannot support it, which then needs staking, transplants badly, and is more vulnerable to wind and handling.

Tomatoes will root along a buried stem, so a stretched seedling can be planted deep and partly rescued at transplant. That is a genuine recovery route and worth knowing. It is not a reason to under-light them for eight weeks.

The setup that works

  1. Light on before they emerge. The stretch starts the hour the shoot breaks the surface. See whether seeds need light to germinate for the sequencing.
  2. 4 to 6 inches for strips, 12 to 18 for panels, raised weekly. Tomatoes grow fast enough that a fortnightly check is not enough.
  3. 14 to 16 hours. Longer does not help enough to be worth the electricity, and tomatoes do want a dark period.
  4. Pot up before they get leggy, not after. Tomatoes can be buried deeper each time they are potted on, which converts some of the stem into root.
  5. Brush the tops daily, or run a small fan. Mechanical movement thickens stems. It costs nothing and it visibly works.

Sizing a fixture for a tomato tray

One panel over two trays of tomatoes

Two 1020 trays side by side is about 3.2 sq ft, or 0.297 m². The Spider Farmer SF1000 publishes 249.21 µmol/s.

249.21 ÷ 0.297 × 0.8 uniformity = 671 µmol/m²/s. That is far more than tomatoes need at seedling stage — so raise it, or dim it, or spread it over four trays instead of two.

Over four trays (0.595 m²): 249.21 ÷ 0.595 × 0.8 = 335 µmol/m²/s. At 14 hours, DLI = 16.9 mol/m²/day. That is the right side of the target with margin at the edges.

Run your own numbers in the coverage calculator, and see PPFD targets by plant type for how tomatoes compare with everything else you might have on the same shelf.

Every pick in full

A square white LED grow panel hanging above leafy seedlings indoors
01

Spider Farmer SF1000

LED panel

One tray of tomatoes

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
02

Mars Hydro TS 1000

LED panel

Growing on to transplant

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.

Linked LED strip lights mounted under a wire shelf above trays of seedlings
03

Barrina T5 4ft Grow Light Strips

LED bar

A whole shelf of them

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.

Questions people actually ask

+How much light do tomato seedlings need?
Aim for 12 to 16 mol per square meter per day, which is roughly 250 to 300 µmol/m²/s at the canopy for 14 to 16 hours. Mature fruiting tomatoes want 20 to 30 mol/m²/day per Virginia Cooperative Extension, but that is a greenhouse target rather than a seedling one.
+Why are my tomato seedlings so tall and thin?
Too little light, nearly always. The plant elongates reaching for a brighter source and puts its energy into stem length rather than thickness. Lower the fixture, extend the photoperiod to 14 to 16 hours, and add a fan or brush the tops daily — mechanical movement thickens stems.
+Can I fix leggy tomato seedlings?
Partly, and tomatoes are unusually forgiving here. They root along a buried stem, so a stretched seedling can be potted on deeper — or planted deep at transplant — converting much of the excess stem into root. Fix the light at the same time or it simply happens again.
+How many hours of light do tomato seedlings need?
Fourteen to sixteen hours a day. Beyond about eighteen the returns fall away sharply, and tomatoes do benefit from a dark period. Use a timer rather than switching manually; consistency matters more than the exact figure inside that window.

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.