Grow lights for seedlings: how to choose and set one up

Grow lights for seedlings at a glance

Contents

• Grow lights for seedlings at a glance
• Is a sunny windowsill enough for seedlings?
• PPFD, DLI, PPE and Kelvin explained
• Full spectrum or red and blue: which should you buy?
• Fixture types compared: bars, panels, tubes and clamp lamps
• How much light does your growing area need?
• Distance and run time by crop group
• What do grow lights cost to run?
• Timers, dimmers and hanging the fixture
• How to set up a seedling light rig
• Signs of too little and too much light
• Zone table: when you need lights in USDA zones 3–6
• Common mistakes
• Frequently asked questions

Is a sunny windowsill enough for seedlings?

Not for seedlings you intend to keep. Plants do not care how bright a room looks; they care how many photons between 400 and 700 nanometres land on the leaves over 24 hours. That total is the daily light integral, in moles per square metre per day, and a vegetable start wants 10–15 of them. Outdoors near the spring equinox you can measure over 30 mol – one study logged 35.1 mol in Saskatoon, Saskatchewan. Inside a window the picture changes completely.

Three things eat the window light

Day length. In mid-February a zone 4 location gets about ten hours of daylight, and the sun stays low, spreading the same energy over a larger area.

The glass. Double and triple glazing with low-emissivity coatings transmits noticeably less light than old single panes. Blinds and dusty glass take more.

The angle. A plant on a sill gets light from one side, not overhead. Leaves facing the room sit in shade, and the seedling stretches toward the glass instead of building stem.

The net is often 1–3 mol per day, a fifth to a tenth of what the plant needs. Seeds still germinate, because germination runs on warmth. The damage happens the week after they come up.

What low light does to a seedling

Purdue University lists what low DLI produces: uneven and delayed germination, longer production time, poor root growth, stretched weak hypocotyls, and plants damaged during handling and transplanting. At the right DLI you get the opposite – short thick hypocotyls, well-developed roots, more stress tolerance.

Stretch cannot be undone. A tomato that reached 10 cm (4 in) before its first true leaf will always be weaker than one under a light from day two. The sowing side of the job is in the guide to starting seeds indoors.

PPFD, DLI, PPE and Kelvin explained

Four numbers are enough to buy well. Everything else on the box is derived from them or is marketing.

PPFD: how bright it is where the plant sits

PPFD is measured in micromoles per square metre per second and describes intensity at one point at one distance. The figure that recurs across extension sources for seed starting is about 200 µmol/m²/s over the growing area. The catch is that it always applies to a point: a manufacturer quoting "600 µmol/m²/s" without naming a distance has said nothing.

DLI: how much accumulates over a full day

DLI is PPFD multiplied by time, and the arithmetic is simple enough to do in your head:

DLI = PPFD × hours × 0.0036

So 200 µmol/m²/s for 16 hours gives 11.5 mol per day, right in the middle of the target. If a fixture only manages 120 µmol/m²/s at the height you can hang it, you reach 6.9 mol even at maximum run time, and it is too weak.

University of New Hampshire Extension makes the same point from the other end: an LED bar that hits the target in eight hours at 20 cm (8 in) above the crop needs sixteen hours at 50 cm (20 in). A four-bulb T8 shop light needs 22 hours, a genuinely poor fixture 34 – a polite way of saying it does not work.

PPE: how much light you get per watt

Photon efficacy, in micromoles per joule, separates a good fixture from a bad one: how many photons it makes per watt it draws. Two 40 W fixtures can differ threefold in usable light.

The DesignLights Consortium, which certifies horticultural fixtures in North America, raised its minimum qualifying efficacy to 2.5 µmol/J in April 2025. Use that as your benchmark at home. Good consumer fixtures sit at 2.0–2.9 µmol/J, fluorescent tubes around 1.0, the cheapest purple panels below that. If PPE is missing from the spec sheet, the number is usually embarrassing.

Kelvin: colour temperature, which matters less than you think

Kelvin describes whether white light reads cool and blue or warm and yellow: 6500 K is daylight white, 2700 K incandescent warm. For raising transplants the whole 3000–6500 K range works, and the growth difference is small next to the difference in quantity of light. Kelvin says nothing about how much light a fixture makes. In a room you also use, around 4000 K is the most comfortable compromise.

And the numbers you can ignore

Lumens and lux are weighted to the human eye, which peaks in green – exactly the wavelength plants reflect most. A lux meter under-reads red and blue and can give identical readings for two fixtures that grow very different plants. Watts tell you what the fixture costs to run, not what it produces. And "equivalent to 1000 W" on a budget panel means nothing.

Full spectrum or red and blue: which should you buy?

The purple fixture has a real origin: photosynthesis uses red around 660 nm and blue around 450 nm most efficiently, and early horticultural LEDs built in only those two colours. Then white LEDs got more efficient, and research showed plants use the whole visible spectrum, green included, with green penetrating deeper into the canopy.

What the difference actually is

Erik Runkle's work at Michigan State University sets out how white LEDs differ: a cool-white peaks around 432 nm with about 3 percent far-red, a warm-white around 637 nm with about 11 percent. Pure red-plus-blue fixtures emit at most 1 percent. Far-red drives stem extension, unhelpful when you want compact transplants. In trials of white against blue plus red, growth differences were small once light quantity matched.

Practical reasons to choose white

• You can see the plants. Under purple light there is no telling pale yellow from deep green, or spotting spider mite damage.
• The room stays livable. Ten weeks of purple light in a living room is a bigger sacrifice than it sounds.
• Quality correlates. The cheapest fixtures are almost always purple, so choosing white screens out a whole price tier.

Buy white full spectrum and put the money into higher PPE and wider coverage rather than spectrum detail. In seed starting the limiting factor is the quantity of light, not its colour.

Fixture types compared: bars, panels, tubes and clamp lamps

The table gives typical ranges for fixtures sold to home growers. The PPE column is a guide per technology – the only figure that counts for your fixture is on its own spec sheet. Coverage means the area lit fairly evenly, not the area the light reaches.

LED bars are usually the right answer

Two to four linear bars over a shelf give what a point source never does: even distribution across the whole area. The gap between 200 µmol/m²/s under the centre of a panel and 60 in the tray corner is the gap between twelve good seedlings and four good plus eight stretched. A panel, by contrast, throws a cone: the higher you hang it the more even but weaker the light, and a shelf rarely has the headroom it wants. A free bench with ceiling above is the panel's home ground, and a 15 W clamp lamp covers roughly one pot diameter – fine for basil on the counter, useless over a 72-cell flat.

Why the budget purple panel rarely pays

At low PPE you buy more watts for the same light, and pay for them every hour for ten weeks. A fixture at 2.6 µmol/J gives as much light from 40 W as one at 1.3 gives from 80 W. Over a 1,360-hour season that is 54 kWh, enough to pay back part of the better fixture's premium in year one.

How much light does your growing area need?

Step 1: measure the growing area

A standard 1020 tray is about 52 × 26 cm (10 × 20 in), roughly 0.13 m². Four trays on a shelf come to about 0.55 m² (6 sq ft), a typical bench 0.8–1.2 m² (9–13 sq ft).

Step 2: convert to wattage

The working rule is at least 100 W per square metre for fluorescent and about 30 W for modern LED, both landing near 200 µmol/m²/s. With LED that means:

• One 1020 tray (0.13 m²): 8–12 W of LED
• Two trays (0.26 m²): 12–20 W
• A shelf with four trays (0.55 m²): 20–35 W
• A full bench (1 m² / 11 sq ft): 40–60 W
• Three shelves (2 m² / 22 sq ft): 80–120 W total

The ranges are wide because of PPE: at 2.8 µmol/J the lower figure is enough, at 1.8 the upper.

Step 3: check against the fixture's PPF

PPF, without the D, is total photon output in µmol/s. Divided by the growing area it gives roughly the PPFD to expect, less spill at the edges:

PPFD ≈ PPF ÷ area in m² × 0.7

A fixture rated at 90 µmol/s over 0.26 m² gives about 240 µmol/m²/s, which is plenty. Use 0.85 inside a reflective enclosure. University of Minnesota Extension notes PPF on common home grow lights runs 10–50 µmol/s – at the bottom of that range, do not ask one fixture to cover more than a single tray.

Step 4: decide how many seedlings you will actually have

The expensive mistake is buying a light for the garden you picture, then sowing twice as much. Anything that does not fit under the fixture stretches. Quantities and dates are in the seed sowing calendar.

Distance and run time by crop group

The table gives DLI targets, PPFD targets, run time and hanging height per crop group. Distances assume low-heat LED bars or panels and are measured from the fixture to the tallest leaf; fluorescent and HPS need more clearance.

The grouping follows Purdue University, which splits transplant production into two tiers: lettuce, cucumber, squash and cabbage at 10–15 mol per day, eggplant, pepper and tomato at 15–20. That is why peppers stretch first in a mixed rig – they want the most light on the shelf and get sown earliest. See the guides to when to sow tomato seeds and how to grow chillies.

Why height matters so much

Light from a point source obeys the inverse square law: double the distance, quarter the intensity. Move a panel from 20 to 40 cm (8 to 16 in) and you lose three quarters. Long bars fall off more gently, but the principle holds. It is why a ceiling fixture never works, and why a mediocre light close in beats an expensive one far off.

Keep the gap constant, not the fixture

Seedlings put on 2–5 cm (1–2 in) a week during the fast weeks, so a fixed fixture drifts out of its own spec within days. Raise it once a week.

About run time

Fourteen to sixteen hours is standard, and do not go past eighteen. Seedlings need darkness for respiration and sugar transport, and tomato is especially sensitive: under continuous light its leaves develop chlorosis. Six to eight hours of dark is part of the growth, not lost time.

What do grow lights cost to run?

Less than most people fear, and worrying about the bill is what makes gardeners switch off too early. A seed-starting season runs about twelve weeks, so at 16 hours a day, roughly 1,360 running hours:

kWh = watts × 1,360 ÷ 1,000

The two right-hand columns are worked examples. The US residential average sat at 18.34 cents per kWh in September 2026, with the Northeast and California well above it. For scale: two shelves of LED for a full season cost about what ten nursery-bought tomato plants do, and under the lights you get a hundred instead, in varieties nobody sells.

Three ways to cut the cost without hurting the plants

• Buy higher PPE. 2.6 µmol/J instead of 1.3 halves the electricity for the same light.
• Fill the area. Half-empty trays under a full fixture are pure waste.
• Build a reflective enclosure. White walls or a foil-lined back panel can lift edge PPFD by a fifth for free.

Timers, dimmers and hanging the fixture

The timer is not optional

Switching by hand works for three days. Then a weekend gets away from you, the seedlings get six hours instead of sixteen, and the stretch begins. Choose a digital timer over a mechanical dial – dial timers have a fifteen-minute minimum and lose time in an outage. Set 5 a.m. to 9 p.m. and leave it all season.

A dimmer if you can get one

Dimmable fixtures let you run at 40 percent the first week after emergence and ramp up, which cuts the risk of bleached cotyledons. Without a dimmer, hang the fixture higher at the start and lower it weekly.

Hanging that adjusts

Adjustable height is what makes the distance table usable. Rope ratchets are fastest and chain with S-hooks cheapest; on a ready-made rack it is often simplest to leave the fixture fixed and stand the trays on blocks you pull out as the seedlings rise.

Heat, moisture and safety

LEDs run cooler than fluorescent and HPS but still get warm, so leave air above the fixture. Never run an unrated fixture over a tray you water from above: check the IP rating, route cords so no drip follows the cable into the outlet, and plug into a GFCI.

How to set up a seedling light rig

1. Measure the area you want lit. Length times width – the number the purchase rests on.
2. Decide the hungriest crop. Peppers and tomatoes set the bar at 250–350 µmol/m²/s; lettuce and brassicas need 150–200.
3. Work out the wattage. About 30 W of LED per square metre, more if the PPE is low.
4. Check PPE and PPF on the spec sheet. PPE 2.0 µmol/J minimum, 2.5 preferred. No figures, no sale.
5. Match the fixture shape to the area. Long narrow shelf: two to four bars. Open bench with headroom: a panel.
6. Hang it so it moves. Ratchets, chain or blocks under the trays. Fixed mounting is a mistake you find in week three.
7. Set the starting height. 35–45 cm (14–18 in) until the cotyledons are up, then down toward 15–30 cm (6–12 in) by crop.
8. Plug in the timer at 14–16 hours. One fixed schedule, not one you fiddle with weekly.
9. Check the corners the first evening. If tray corners sit in gloom, move the plants in or add a bar.
10. Raise weekly, then harden off. Outdoor light is several times stronger, so give the plants a week of increasing part shade first.

Signs of too little and too much light

The plants tell you plainly. Shortage is far more common indoors, but overdosing happens when a strong panel sits too close to a fresh flat.

Signs of too little light

• Long gaps between leaves. The stem elongates between nodes – the clearest early sign.
• Leaning toward the source. Under a centred fixture seedlings should stand upright.
• Pale yellow-green colour. The RHS describes mid-green foliage as the mark of the right amount of light.
• Small thin leaves on a long stem. The plant is buying height instead of leaf area.
• No visible growth in a week despite correct temperature and moisture.
• Seedlings that fold at potting-on. The stem cannot carry its leaves.

Signs of too much light

• Bleached, almost white patches on the top leaves. Chlorophyll breaks down faster than it is rebuilt, and only nearest the fixture.
• Leaf edges curling upward. The blade cups into a taco shape.
• Dry, brittle brown margins. Common when the room is warm and dry too.
• Purple cast on cotyledons. Often alongside cold roots and poor phosphorus uptake under a strong light.
• Extremely short plants with crinkled leaves. Compact is good; stalled is not.

What to do

For too little light, lower the fixture first. If it is already as low as heat allows, extend to 16 hours, group the plants under the brightest zone, or add a fixture. Stretched seedlings can be potted deeper – tomato and brassicas root along the stem – but prevention is the only real fix.

For too much, raise the fixture 5–10 cm (2–4 in) or dim to 60–70 percent for a week, and check the plants are not simply dry, since light damage and water stress look almost identical. Lettuce complains first; see the guide to how to grow lettuce.

Zone table: when you need lights in USDA zones 3–6

Two things set the need: when you sow, and how fast the day lengthens where you live. Further north the sowing date slips later, but day length climbs faster in spring, so the switch-off date moves less than the sowing date. The table starts from the earliest crops, peppers and chillies.

Between "full lighting" and "lights off" you can usually drop to eight or ten hours morning and evening, letting daylight carry midday. Switching off assumes a south, east or west window – in a north window or behind shade trees, add two or three weeks.

In zone 3 and cold parts of zone 4 it is often smarter to sow two weeks later and run the light for a shorter stretch than to sow early and fight the dark for six weeks. Five weeks of properly lit growth beats nine of gloom. Early tomato timing is in the guide to how to grow tomatoes.

Common mistakes

• Buying on watts instead of PPE. Two 40 W fixtures can differ threefold in usable light.
• Hanging the fixture too high. Double the distance, quarter the light. A cheap bar at 20 cm (8 in) beats an expensive one at 60 cm (24 in).
• Measuring with a lux meter or phone app. Lux is weighted to the eye's green sensitivity and under-reads red and blue.
• Running around the clock. Seedlings need six to eight hours of dark, and tomato gets leaf chlorosis under continuous light.
• Forgetting to raise the light. A gap that was perfect in week one is wrong by week three.
• Lighting an area you do not fill. Half-empty trays cost full electricity.
• Ignoring the corners. Light is strongest in the middle. Rotate the trays twice a week.
• Going straight from the rig to full sun. Without a week of hardening off, the leaves scorch in an afternoon.

Frequently asked questions

Do seedlings really need a grow light?
Yes, if you sow before about mid-April in zones 3–6. A February window often delivers 1–3 mol of light per square metre per day, while a vegetable start wants 10–15. Seeds germinate anyway, since germination runs on warmth, but what comes up is long, pale and weak. From mid-April a bright south window carries most crops.

How many hours a day should a grow light be on?
Fourteen to sixteen hours is standard for seed starting, and do not exceed eighteen. Seedlings need a dark period for respiration and sugar transport, and tomato develops leaf chlorosis under continuous light. Always use a timer – switching by hand drifts, and a single forgotten day is enough to start the stretch.

How far should grow lights be from seedlings?
For LED bars and panels, 35–45 cm (14–18 in) from sowing to cotyledon, then 25–35 cm (10–14 in) for lettuce, brassicas, onions and herbs, 20–30 cm (8–12 in) for cucumber and squash, and 15–25 cm (6–10 in) for tomato, pepper and chilli. Measure from the tallest leaf and raise the fixture weekly.

How many watts per square foot do I need?
About 3 W of modern LED per square foot, roughly 30 W per square metre, which lands near 200 µmol/m²/s. Fluorescent needs 100–200 W per square metre for the same light. Watts are not a light unit, though: what you get depends on photon efficacy. At 2.8 µmol/J you need far fewer watts than at 1.5.

What do PPFD and DLI mean?
PPFD is intensity at one point, in micromoles per square metre per second. DLI is the total light over 24 hours, in moles per square metre. The formula is DLI = PPFD × hours × 0.0036. So 200 µmol/m²/s for 16 hours gives 11.5 mol, which sits in the middle of the target for most vegetable seedlings.

Is full spectrum better than red and blue light?
For home seed starting, yes, though mostly for practical reasons. Growth differences are small once the light quantity matches. White light lets you judge leaf colour and spot pests, keeps the room usable, and screens out the cheap purple panels that tend to have poor photon efficacy. Spend on PPE and coverage instead.

Will a regular LED bulb work as a grow light?
Partly. A bright white household LED emits the right wavelengths, but photon efficacy is low and the output scatters in all directions. It is fine for one pot of herbs on a sill. Over a 72-cell flat it is not – you cannot lift PPFD across the whole tray no matter how close you put it.

What do grow lights cost to run for a season?
A 20 W LED bar running 16 hours a day for twelve weeks uses about 27 kWh. A full 80 W shelf uses about 109 kWh. At the US average of roughly 18 cents per kWh that is about $5 and $20 respectively for the whole season. Per seedling raised it rounds to pennies.

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