How Much Light Do Seedlings Really Need?

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Seedlings are tiny, so it's easy to think a soft glow will do. It won't. Your seedlings need less light than a flowering plant, but probably more than you're giving them. Too little light, and they stretch. Too much, too fast, and the leaves bleach. Here is how plants really count light, the daily total to aim for, what a seedling study found, and how to set your light, with or without a meter.

The short answer ▶ 0:29

Measure light the way a plant does, in photons, not lux. Add it up over the whole day. For seedlings, aim for a daily total of at least 10 to 12 moles: that is just a way of counting light, explained below. Then turn it up gradually, and let the stem tell you if it's right.

Counting light ▶ 0:48

An ordinary light meter reads lux: brightness, as our eyes see it. Our eyes are most sensitive to yellow-green light. Plants use every colour from blue to red. So the same lux reading can mean quite different amounts of useful light, depending on the colour of the lamp.

Source · Thimijan & Heins, HortScience (1983)

So growers count what a leaf counts: photons, the tiny packets of light that power photosynthesis, landing on each square metre every second. That's PPFD, measured in micromoles per square metre per second (µmol/m²/s). The unit sounds scary, but a mole is just a counting word, like a dozen, only for an enormous number of photons. Bigger number, more light.

Source · McCree, Agric. Meteorol. (1972)

Full summer sun gives around 2,000. A seedling needs only a fraction of that.

But a leaf doesn't care about one moment. It cares about the whole day. Multiply the brightness by the hours the light is on, and you get the daily light integral (DLI): the day's total, counted in moles.

With the light on 18 hours a day, a PPFD of 100 makes about 6.5 moles. 150 makes nearly 10. 200 makes 13. 300, nearly 20.

That's why the hours matter as much as the brightness. 150 for 24 hours gives the same daily total as 200 for 18 hours. (For how hours work with autos, see the best light schedule for autoflowers.)

What a seedling study found ▶ 2:06

In a 2018 study, seedlings of three garden flowers grew for four weeks under LEDs at three levels: 105, 210 or 315 µmol/m²/s, 16 hours a day. That's about 6, 12 and 18 moles a day.

Source · Craver, Boldt & Lopez, HortScience (2018)

The brighter they grew, the better they were built. At the top level, stems were 9 to 21% shorter and 10 to 14% thicker, and roots and shoots kept getting heavier, right up to the highest light.

That fits the long-standing advice for raising seedlings: a daily total of at least 10 to 12 moles. At 18 hours a day, that's roughly 150 to 200 µmol/m²/s. These were flowers, not cannabis, and we found no published test like it on cannabis seedlings.

Source · Pramuk & Runkle, HortScience (2005)

But young cannabis has been tested. In a Canadian study, plants grown from cuttings got anything from about 135 to over 1,400 µmol/m²/s for three weeks. The more light they got, the shorter the gaps between their leaves, and their weight kept climbing until it levelled off near the top. So by cannabis standards, 200 is gentle.

Source · Moher et al., Ind. Crops Prod. (2022)

Too little light is the classic seedling problem. A dim seedling leans towards whatever light it can find, reaching for a window. And as we explain in why seedlings stretch, it pours its energy into a long, thin stem.

Too much is rarer, but it happens when a light goes from gentle to full power overnight. Leaves are built for the light they grew up in, and a sudden jump can bleach them. So turn it up over several days, not all at once.

Source · Taiz et al., Plant Physiology and Development (textbook)

How high to hang your light ▶ 3:42

How high should it hang? That depends entirely on the light. University gardening advice puts fluorescent tubes just 10 to 15 cm above seedlings, and small LED seed-starting lights about 20 to 30 cm. A tent LED panel is far stronger, so makers often suggest starting seedlings at 45 to 60 cm or more, often dimmed.

Source · Iowa State Univ. Extension; LED makers' guides

For a panel, the best guide is the maker's light map: the PPFD across the floor at a few hanging heights. Pick the height or dimmer setting that gives around 150 to 200 under your seedlings, and remember the corners get less than the middle. (Still choosing a space? See what size grow tent you need.)

No map and no meter? Then start high, or dimmed. Every two or three days, lower it a few centimetres, or turn it up a notch. Light spreads out as it travels, so small changes close to the light make a big difference. And don't trust the back of your hand: an LED can be too bright for a seedling without feeling hot.

Phone light-meter apps can get you close, but their accuracy depends on the phone and the colour of the light, and we found no independent study testing them. Treat them as a rough guide.

And a windowsill? In December, the daily total outdoors across the northern United States averages only about 5 to 10 moles. Behind glass, lit from one side, a seedling gets less still. That's why winter windowsill seedlings so often stretch.

Source · Korczynski et al., HortTechnology (2002)

Then let the seedling tell you. A short, sturdy stem with its leaves close together means the light is right. A long, thin stem means it wants more. Pale or bleached top leaves mean it's too close or too strong.

Plants count photons, not lux. Aim for a daily total of 10 to 12 moles or more, remember the hours count as much as the brightness, and turn it up slowly, then watch the stem.