Grow lights for seedlings, explained: PPFD, hours and distance
By FarmerTested · a network of working growers based in Waxhaw, NC
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If your seedlings have ever come up tall, pale and leaning hard toward the window, they were short on light. It's the most common seed-starting problem and the easiest to fix. This guide explains the numbers on grow light boxes in plain English, so you can buy the right light and hang it at the right height without overspending.
One thing up front: I haven't tested specific grow light models side by side, and I won't pretend I have. What follows is how plant lighting works and how to read the specs. When I test lights myself, I'll post the measurements here.
Why a window isn't enough
A south window in winter looks bright to us, but our eyes are a poor light meter. Short days, a low sun angle and glass all cut the light that actually reaches a seedling. Seedlings respond by stretching: they grow long stems fast, trying to reach brighter light. A stretched stem is weak, and the plant often flops over or never quite recovers. A modest light hung close fixes that.
The terms on the box
Watts: how much power it uses, not how much light it makes
Watts tell you what the light costs to run. They don't tell you how much useful light reaches the plant, because some fixtures are much more efficient than others. Use watts for the electric bill, not for comparing brightness.
Lumens: light measured for human eyes
Lumens weight light by how bright it looks to people, who see green best. Plants use a broader range. Lumens aren't useless (a brighter white LED shop light generally grows better seedlings than a dim one), but they're not the plant-grower's number.
PAR and PPFD: light measured for plants
PAR (photosynthetically active radiation) is the band of light plants use, roughly 400–700 nanometers. PPFD (photosynthetic photon flux density) is how much of that light lands on a spot each second, measured in micromoles per square meter per second (µmol/m²/s). PPFD is the number that matters at seedling height, and it drops quickly as you raise the light.
Seedlings don't need much compared with fruiting plants. Commonly cited ranges for vegetable seedlings run roughly 100–300 µmol/m²/s, with leafy greens happy toward the lower-middle of that range. Treat that as a starting range, not a rule, and watch the plants.
DLI: the daily total
DLI (daily light integral) is the total light a plant gets in a day. It combines intensity and hours, and you can calculate it yourself:
DLI (mol/m²/day) = PPFD × hours of light × 0.0036
For example, 200 µmol/m²/s for 16 hours is 200 × 16 × 0.0036 = 11.5 mol/m²/day. The same light for 12 hours gives 8.6. This is why running a modest light for longer can do the same job as a brighter light for fewer hours.
How many hours?
Most growers run seedling lights 14–16 hours a day on a timer. Plants also need a dark period, so leaving lights on 24 hours isn't a shortcut. Some crops respond to day length, onions especially, but for ordinary seedlings, 14–16 hours on and the rest off is the practical standard. A $10 timer does this better than any of us remember to.
How far away?
It depends on the fixture, so start with the maker's numbers:
- Look for a PPFD map. Good grow light makers publish a chart of PPFD at different heights and spots under the light. Pick the height that puts your tray in the seedling range.
- Low-power LED shop lights and fluorescent tubes are dim, so they usually hang just a few inches above the leaves. Raise them as the seedlings grow.
- High-power LED panels are much stronger and usually hang well above the canopy, often a foot or more. Too close can bleach seedlings.
- Edges get less light than the center. Rotate trays, or put slower crops in the middle.
A handheld PAR meter takes the guesswork out, but it's an extra cost most home growers don't need. Phone light-meter apps can show relative differences, but don't rely on their PPFD numbers. (see PAR meters on Amazon)
Read the plants
- Too little light: long, thin stems, wide gaps between leaves, pale color, leaning. Lower the light or add hours.
- Too much light: bleached or yellow-white patches on top leaves, leaf edges curling up, crispy tips. Raise the light.
- About right: short, sturdy stems, leaves close together, good deep color.
The plants are a better meter than anything you'll buy. Check them every day for the first week after you change anything.
Spectrum: don't overthink it
Pink and purple "blurple" lights were popular for a while. For seedlings, a plain full-spectrum white LED works well and lets you see what your plants actually look like, which matters for spotting problems. Daylight shop lights in the 4000–6500K range are a common, inexpensive choice. (see full-spectrum LED shop lights on Amazon)
What it costs to run
This is the part brands rarely put on the box. Electricity = watts × hours ÷ 1000 × your rate:
- A 40 W light for 16 hours a day uses 0.64 kWh a day, or 19.2 kWh a month. At $0.15/kWh, that's about $2.90 a month.
- Two 40 W lights over a 60-day seedling season use 76.8 kWh, about $11.50.
Use your own rate from your power bill. Many Carolina customers pay somewhere around the $0.15 we use as a default, but check yours.
That's why the wattage matters more for an indoor hydroponic tower, which runs its lights all year, than for a seedling shelf that runs two months. We track it for every system on our comparison table and in the true-cost calculator.
A simple setup that works
- One or two full-spectrum LED shop lights, hung on chains or hooks so you can raise them. (see full-spectrum LED shop lights on Amazon)
- An outlet timer set for 16 hours on and 8 off. (see plug-in outlet timers on Amazon)
- Lights a few inches above the seedlings to start (or at the maker's recommended height). Adjust based on what the plants tell you.
- A small fan on low, for sturdier stems.
Follow along
This winter I'll be starting seedlings under lights and comparing them with the grow lights built into the hydroponic systems I test. Light is where most of a tower's electric bill comes from, and it's a big part of whether these systems are worth their price. I'll share what I find, good and bad, in the newsletter.
Next: the full budget setup and its true cost per plant.