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Have you ever wondered: are my grow lights too far away? When it comes to young seedlings, getting the light just right—both in strength and distance—is super important. Too close, and you might burn them. Too far, and they get tall and spindly.
So, what’s the perfect distance? And does it change depending on the type of grow light you use?
Today, I want to share a super interesting experiment from a gardening YouTuber who tested exactly this. What I love is that he didn’t just rely on theory—he actually ran a real test. And when the first round didn’t give great results, he started all over again for round two.
He spent a full 60 days just trying to answer one question: how far should your grow light be from your seedlings?
I’ve put together his findings here, and I hope they’ll help you if you’re starting seeds at home.
The YouTuber set up six seed-starting stations, each with a different light height: 6, 12, 18, and 24 inches above the plants.
He also tested a few types of grow lights, including standard LED bulbs and his go-to full-spectrum lights.
As for the plants, he chose three common veggies: tomatoes, broccoli, and lettuce.
What’s especially thoughtful is that he didn’t just set the starting height and leave it. As the seedlings grew taller, he adjusted the lights to stay at the exact same distance from the plants the whole time. That way, the results wouldn’t be affected by how tall the seedlings got during the test.
Distance (inches) | номинальная стоимость（μмоль/м²/с） | Seedling Condition | Legged or not | Burned or not |
6 | 1600 | Leaves turning yellow, burned, growth stalled | No | Yes |
12 | ~900 | Thin stems, slight legginess | Yes | No |
18 | ~600 | Weak seedlings, clearly leggy | Yes | No |
24 | ~300 | Severely leggy seedlings, pale color | Yes | No |
After 30 days, the results weren’t what he had hoped for.
Seedlings under the 6-inch lights showed clear signs of stress — burnt and yellowing leaves, especially on the tomatoes and broccoli. That meant the light was definitely strong enough, but too intense for young seedlings to handle.
At the other end, the 24-inch group had the opposite problem: not enough light. The seedlings stretched way too much — classic leggy growth.
Even some of the setups at 18 inches — which you’d think would be the sweet spot — still showed signs of stretching.
Using a PAR meter, he measured the light intensity. At 6 inches, the PAR reading was 1600 μmol/m²/s — way above the recommended range of 200 to 800. But at 24 inches, the PAR level dropped too low to support healthy growth.
The weirdest part? Even when the PAR numbers looked “perfect” some seedlings still got leggy. So clearly, light height and intensity aren’t the whole story.
To figure out what went wrong in the first experiment, the blogger ran a second round — this time with tighter controls and smarter tweaks:
He kept the seed trays exactly the same and only changed the lighting setup.
He used just one type of high-quality grow light and set it to a fixed height of 12 inches. Then, he adjusted the light intensity directly — for example, dialing it in to exactly 800 μmol/m²/s.
One group had a small fan running — on for 30 minutes, off for 30 minutes — to simulate airflow and see if that could help reduce legginess.
And of course, he included a control group using the same setup from round one that had performed reasonably well.
After 25 days, the second round of the experiment showed much better results:
Seedlings grown under light intensities around 800 μmol/m²/s, with the lights placed 6 to 10 inches above, performed the best overall. They had sturdy stems, healthy leaf spread, and strong color.
The group with the fan didn’t show a huge difference compared to the no-fan group, but good airflow could still help prevent disease, so it’s worth watching in future tests.
Even when the PAR values were the same, the quality of the grow light and how evenly it spread the light made subtle but noticeable differences.
He also tried something small but clever — adding reflective material to the sides of the light setup. That bumped the PAR value from 770 up to 880, proving that reflectors really can boost light efficiency.
After nearly 60 days of testing, this gardener came away with a few key lessons:
Light distance isn’t the only factor—what really matters is how much light the top of the seedlings actually gets. Aim for 600–800 μmol/m²/s at the canopy level.
The often-recommended “24-inch distance” found in many manuals is on the safe side. It may work for mature plants or hardy crops, but it’s too far for seedlings, and can lead to stretching.
A light distance of 6 to 10 inches, with properly controlled intensity, gave the best results across the board.
While things like reflective panels and good airflow aren’t the main focus, they’re worth including. Reflectors boost light efficiency, and airflow can help reduce disease—both are easy upgrades with real benefits.
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