You have hung the light, plugged in the timer, and set the tray underneath — and now the only question is how far down the light should go. Too high and the seedlings stretch; too low and the leaves bleach. The answer is closer than most people hang it.
The rule: 2 to 4 inches for bars and tubes
For fluorescent tubes and LED bars — the lights most seed starters use — hang the fixture 2 to 4 inches above the tops of the seedlings. That gap is close enough that the light is strong and even, and far enough that the leaves never scorch. That gap matches the guidance from University of Minnesota Extension: no more than 4 inches above the tops, and as close as 2 inches is ideal.
Measure from the tops of the plants. A tray of fresh sprouts sits low, and the light should sit low with it — then rise as the plants do. Hang the fixture from chains or adjustable cords so raising it takes seconds.
Why closer beats higher
Light weakens fast as the gap grows, which is why a few inches matter more than they look like they should. A bar hung a foot above the tray is giving the seedlings a fraction of the light it gives at 3 inches, and the seedlings answer by stretching toward it — long pale stems, leaves spaced far apart, plants that flop when they meet real wind. If your seedlings look like that, the light is too far away, and the fix is to lower it tonight. Our guide to why seedlings go leggy works through the other causes if lowering the light does not fix it.
The same logic runs the other way for stronger lights. A high-output panel is a different fixture from a bar, and it hangs higher — follow the maker’s hanging-height guidance for that light rather than the 2-to-4-inch rule, and watch the leaves for the first few days. Bleached or curling leaf tips mean the light is too close; stretching means it is too far. Our grow light roundup covers which fixture fits which setup.
How long to leave them on
Give seedlings 14 to 16 hours of light a day, on a timer, with a dark night in between. They need the dark hours to grow properly, so leaving the lights on around the clock does more harm than good. Set the timer once and forget it — consistency matters more than the exact hour count.
Raising the light as seedlings grow
Check the gap every few days, because seedlings grow faster than the setup suggests. When the tops get within 2 inches of the fixture, raise it and restore the gap. A seedling that touches a bar can scorch a leaf tip, and a seedling left well below its light stretches to close the gap. The habit to build is a glance at the gap whenever you water. Our seed-starting station guide covers the chains, trays, and heat that make the whole setup easy to adjust.
The height that works
Hang the bars 2 to 4 inches above the seedlings, run them 14 to 16 hours on a timer, and raise them as the plants grow — and the next tray will stand up straight on its own.
