The three numbers that matter, the two that are marketing, and why the research says you need less light than you have been told.
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Search for a grow light and within about ninety seconds you will be looking at a $400 fixture with a cooling fan, a dimmer, a spectrum chart, and a claim about “1000W equivalent.” Somewhere behind it, a review will mention canopy penetration. You came here to grow eight tomato seedlings on a shelf in your basement, and now you are reading about photon efficacy.
Here is the thing the industry is not especially motivated to tell you: the published research on seedling light requirements points to substantially less light than most grow-light marketing implies. A modest fixture, hung at the right height and run for the right number of hours, will raise better seedlings than an expensive one used badly.
This guide is about spending the minimum that actually works — which, having raised seedlings commercially under lights at production density, is a subject we have opinions about.
Two numbers on the box that mean nothing
Lumens. A lumen measures brightness as perceived by the human eye, which is most sensitive to green and yellow wavelengths. Plant photosynthesis works across the whole 400–700 nm band — photosynthetically active radiation, or PAR — so a lumen figure tells you how bright a light looks rather than how useful it is to a plant. (An older teaching held that plants barely use green light. That has not aged well: green penetrates deeper into leaves and canopies than red or blue and contributes meaningfully to photosynthesis.) Two fixtures with identical lumen ratings can perform very differently on a seed tray, especially if they differ in spectrum or optical design.
“Equivalent watts.” When a 45-watt fixture is sold as “500W equivalent,” that number describes nothing measurable. It is a comparison against an unstated older technology, chosen by the marketing department. Look for the actual power draw at the wall — usually printed in small type — and treat the big number as decoration.
Neither figure tells you whether your seedlings will be stocky or stretched.
The three numbers that actually matter
PPFD — how bright it is where the plant is
PPFD (photosynthetic photon flux density) measures the density of photosynthetically active photons reaching a surface each second — what arrives, not what the leaf ultimately absorbs. It is measured in µmol/m²/s. Critically, it is a measurement at a specific distance — the same fixture might deliver 400 at 15 cm and 150 at 45 cm. A PPFD number quoted without a distance is meaningless.
DLI — the total your plants receive in a day
Daily light integral is the number that actually drives growth. Plants do not respond to peak brightness; they respond to accumulated photons over the day. DLI is measured in mol/m²/day, and it is simply intensity multiplied by time:
DLI = PPFD × hours × 3,600 ÷ 1,000,000
This is the single most useful equation in indoor seed starting, because within the normal range used for seedlings, intensity and duration can be traded against one another to reach a similar DLI. A dimmer light run longer can deliver much the same daily total as a brighter one run briefly. (Only within that range — plants show saturation behaviour, so you cannot compensate for very dim light by simply running it around the clock.) That is why an inexpensive fixture is a legitimate answer rather than a compromise.
Coverage area — the one people forget
A fixture concentrating light into a 30 × 30 cm footprint is useless over a 120 cm tray, no matter how impressive its centre reading. Light falls off sharply toward the edges of any fixture, and the seedlings at the ends of your tray are the ones that go leggy while the middle looks perfect. Match the fixture’s footprint to your tray, not to its headline number.
What seedlings actually need
This is where the research and the marketing part company.
Work on tomato seedlings points to a target around 250–300 PPFD and a DLI of roughly 10–13 mol/m²/day, with research reporting useful ranges of roughly 200–350 PPFD depending on cultivar and temperature. Broader guidance for seedlings runs about 6–10 DLI early and 10–15 as they develop. ⚠️ Treat all of these as approximate targets — species and cultivars genuinely differ, and your growing temperature shifts them (more on that below).
| Stage | Target DLI (mol/m²/day) | Roughly what that means |
|---|---|---|
| Just-germinated seedlings | ~6–10 | Modest light. Overdoing it here achieves nothing useful. |
| Developing seedlings, true leaves | ~10–15 | The main working range for tomatoes, peppers and brassicas. |
| Flowering crops indoors (for scale) | Much higher | This is what the expensive fixtures are built for. It is not your job. |
Now put that through the formula. To hit a DLI of about 11.5 you need only 200 PPFD for 16 hours. To hit roughly 17 — comfortably above what seedlings require — you need 300 PPFD for 16 hours.
Compare that with a fixture marketed for flowering crops, which may be built to deliver well over 1,000 PPFD. For raising tomato seedlings you would run it at a fraction of its capacity, or hang it far enough away to waste most of its output on your shelf. You would be paying for headroom you will never use.
For scale in the other direction: midday summer sun can reach around 2,000 PPFD, and a bright spring window might read anywhere from about 100 to 500 depending on orientation and weather — but only for a few hours, at a low angle, through glass. That is exactly why DLI matters more than any single reading. A window that peaks impressively can still deliver a poor daily total.
Distance is your intensity dial
Light intensity decreases rapidly as the fixture moves away — though not by a tidy formula, because the exact relationship depends on the fixture’s design. The inverse-square law people reach for applies to point sources; LED bars and panels are extended sources and behave somewhat differently. Either way, distance is the cheapest control you have, and most people never use it deliberately.
Too far away is the classic beginner error, and it produces exactly the stretched, pale seedlings people then blame on the light being underpowered. Hanging a modest fixture closer often solves the problem entirely, for free. For typical LED shop-light-style fixtures, somewhere in the range of 15–30 cm above the seedlings is a sensible starting point — then watch the plants and adjust.
Your seedlings will tell you. Stretching, long gaps between leaves, and leaning means too little light — lower the fixture. Pale, bleached or oddly bronzed leaves, curling margins and very tight growth means too much — raise it. Compact, dark green, short internodes means you have it right.
Because seedlings grow, this is not a set-and-forget adjustment. Rig your fixture on an adjustable hanging kit on Amazon.ca — a couple of dollars of ratchet hangers or even chain — so raising it weekly takes ten seconds. A light you cannot easily move is a light you will not move.
How long to run them
For most seedlings, 14 to 16 hours a day is the practical range. It delivers a solid DLI at modest intensity and matches how the equation actually behaves.
⚠️ Do not run lights 24 hours. It is tempting — more light, faster growth, surely? — but many plants, tomatoes very much included, carry out important metabolic and developmental processes during the dark period. Continuous lighting disrupts those processes and can cause physiological disorders even when water and nutrients are perfectly adequate. Tomatoes are the textbook example of continuous-light injury. You gain little and risk real damage.
Consistency matters more than precision here, which makes an outlet timer on Amazon.ca genuinely worth its few dollars. Set it once and stop thinking about it — remembering to switch lights on and off by hand for eight weeks is a system that fails in about week two.
The other half of the equation: temperature
Light does not work in isolation, and this is the piece most seed-starting guides leave out entirely.
Warmth drives a seedling’s growth rate. Light supplies the energy to fund it. When those two fall out of step — plenty of warmth, not enough light — the plant grows faster than it can build sturdy tissue, and you get exactly the pale, stretched seedlings you were trying to avoid. A warm seedling needs more light to stay compact than a cool one does.
Which gives you a second lever, and a free one. If your seedlings are stretching and you cannot add light, lower the temperature. Cooler growing conditions reduce stretching even under moderate light. This is exactly why commercial propagators drop the temperature slightly once germination is complete — and why you take a tray off its heat mat as soon as seedlings emerge. Bottom heat is a germination tool, not a growing tool.
Practically: if your grow shelf is in a warm room and your light is modest, the cheapest fix available to you may be moving the whole operation somewhere cooler.
One more nearly-free improvement while you are at it: reflective surroundings. White shelving, a white wall, or a sheet of Mylar or even white poster board around the sides of your setup bounces stray light back onto the tray and noticeably improves uniformity at the edges — which is where your weakest seedlings always are.
Spectrum: mostly solved, mostly marketing
Spectrum charts sell a lot of fixtures. For seed starting, the honest summary is that this question is largely settled and rarely your limiting factor.
Modern white full-spectrum LEDs work well for seedlings and have a large practical advantage: you can actually see your plants. Under white light you spot yellowing, damping-off, aphids and uneven watering at a glance.
The purple “blurple” fixtures — narrow red and blue diodes only — are older technology built on the reasoning that plants use red and blue most. They do grow plants. But assessing plant health under magenta light is genuinely difficult, and the efficiency advantage that once justified them has largely eroded.
Add-ons marketed as UV and far-red boosts are not fiction — both genuinely influence plant morphology, and far-red in particular affects stem elongation. They simply offer little practical benefit for raising vegetable seedlings destined for outdoor planting in eight weeks, and they are a common way to pay more for a fixture without improving your trays.
What is a waste of money
| Skip it | Why |
|---|---|
| High-output flowering fixtures | Built for many times a seedling’s requirement. You pay for capacity you will never use. |
| “1000W equivalent” claims | Not a measurement. Find the real wall draw. |
| Lumen-optimised shop lighting | Optimised for human eyes. Fine if it happens to be bright and close, but the number is not telling you what you think. |
| UV and IR add-ons | Aimed at flowering and finishing in other crops. Irrelevant for seedlings. |
| Elaborate dimmers and controllers | Distance and a timer do the same job for a fraction of the cost. |
| A fixture too small for your tray | The most common real mistake. Edge seedlings stretch while the centre looks fine. |
What to actually buy
For most home growers: a linked LED shop light. Modern LED shop lights on Amazon.ca are the value champion of seed starting. They are cheap, they cover a standard tray properly, several can be linked in a row, and hung 15–30 cm above your seedlings a decent one delivers intensity comfortably inside the range the research supports. ⚠️ Not every shop light qualifies — the cheapest units can produce surprisingly low output. Choose a modern LED fixture with real output rather than simply the lowest price on the page. Two of them over a shelf will out-perform one expensive fixture hung too high, for less money.
For a tidier setup or a specific footprint: a purpose-built LED grow light. A dedicated full-spectrum LED grow light on Amazon.ca gives you a defined coverage area, published PPFD figures at stated distances, and usually a hanging kit in the box. Buy on coverage and PPFD-at-distance, not on wattage claims — and be wary of any listing that quotes intensity without saying at what height.
Two things worth adding, both cheap. A timer, as above. And if you want to stop guessing entirely, a PAR meter on Amazon.ca measures your actual PPFD at seedling height, which turns the whole subject from opinion into arithmetic. Research labs use Apogee quantum sensors as the reference standard and price them accordingly; inexpensive meters are fine for the comparative question most growers actually have, which is “is this spot bright enough?” It is a luxury for one tray and a genuine tool if you grow seriously.
What matters more than the fixture: hanging it at the right height, running it 14–16 hours on a timer, and covering your whole tray evenly. Get those three right with a cheap light and you will beat an expensive light used carelessly, every time.
The commercial-grower shortcut
One habit worth stealing from production growing: think in trays, not in plants.
Commercial propagation lights a defined area to a defined DLI and fills that area completely. It is dramatically more efficient than lighting scattered pots, and it is why growers get uniform seedlings while home setups produce a few good plants and several stretched ones.
Practically: fill your tray. Group pots tightly under the fixture rather than spreading them across a shelf. Keep the lit footprint packed, and move plants inward as you remove or pot up. Every centimetre of lit bench with nothing under it is light you paid for and threw away — and every plant sitting at the dim edge is one you will be disappointed by in May.
Common questions
Can I use a regular LED shop light for seedlings?
Yes — and for most home seed starting it is the sensible choice. Seedlings need far less intensity than flowering crops, and a shop light hung close delivers plenty. The catch is that shop lights are rated in lumens rather than PPFD, so you cannot verify output from the box; you judge by how your seedlings look and adjust the height accordingly.
How close should grow lights be to seedlings?
For typical LED shop-light-style fixtures, roughly 15–30 cm, adjusted as plants grow. High-output fixtures need considerably more distance. The plants are the real instrument: stretching means lower it, bleaching or bronzing means raise it.
How many hours should grow lights be on for seedlings?
14–16 hours a day, on a timer. Leave a genuine dark period — do not run them around the clock, particularly for tomatoes, which can develop injury under continuous light.
Why are my seedlings still leggy under a grow light?
Almost always the fixture is too far away, or the plant is at the dim edge of the footprint. Lower the light, pack the tray tighter, and check whether the leggy ones are all at the ends. Two other culprits compound it: excess warmth, which pushes growth faster than the light can support, and over-fertilising, since surplus nitrogen encourages soft elongated tissue. Gentle air movement from a small fan encourages stronger stem development, so that helps too.
Is a full-spectrum light necessary, or will purple work?
Purple blurple fixtures grow perfectly good seedlings. White full-spectrum is the better buy today mainly because you can see your plants properly — which matters more than most spectrum debates, since spotting a problem early is worth more than a marginal efficiency gain.
Are T5 fluorescent lights still worth buying?
T5 fluorescents raised excellent seedlings for decades and still work fine if you already own them. For a new purchase, LED has overtaken them on efficiency, lifespan, heat output and running cost, with no fragile tubes to replace. Keep a working T5 setup; do not buy one now.
Is 6500K a good colour temperature for seedlings?
Yes — 6500K is a cool daylight-white that works well for the seedling stage and has the practical benefit of showing plant colour accurately. Anywhere in roughly the 4000–6500K range is fine. Colour temperature is far less important than intensity at plant height, which is where people should be spending their attention.
Can seedlings get too much light?
Yes, though it is far rarer than too little in a Canadian basement in March. The signs are bleached or bronzed leaves, curling margins, and unusually squat growth. The fix costs nothing — raise the fixture. Genuine light damage in home seed starting almost always comes from hanging a high-output fixture far too close.
Is a 100W LED enough for seedlings?
Almost certainly, and probably more than enough — but wattage is the wrong question. What matters is the PPFD it delivers at your seedling height across your whole tray. A 100W fixture concentrated over a small footprint and a 100W fixture spread over a metre-wide bench give completely different results. Ask about coverage and intensity at distance, not watts.
Do I need a PAR meter?
Not for one tray. If you are running several shelves, or you want to stop guessing whether your light is adequate, it is the tool that answers the question definitively. Some smartphone apps approximate it, though accuracy varies considerably.
Get the timing right too
A good light setup only helps if you are starting seeds at the right moment. Start too early and you will be nursing overgrown plants under lights for weeks — see our guide to when to start tomatoes indoors in Canada, which counts back from your transplant date rather than your frost date.
Both depend on knowing your real season. Our free Zone Calendar is matched to your postal code and built around the two dates that govern a northern growing year. And for variety selection, sourcing and the rest of the season, start with the full guide to cold climate seed starting.
