The right grow light distance depends on your bulb type. Keep fluorescent shop tubes 2 to 4 inches above the tops of your seedlings. Hang LED panels 4 to 6 inches above them instead. Both gaps put strong light on young plants without scorching the leaves, and they give you a clear starting point for grow lights from seedlings on day one.
"Set the shop light a foot over the tray," a friend told me when I started my first batch indoors. I did. Two weeks later my tomato seedlings on the basement shelf were pale and leggy, leaning and falling over their own stems. I dropped the fixture down to three inches above the leaves. The next round of growth came in thick and firm, with short stems that held the plants upright.
Distance controls intensity, and that is the part most people miss. Light fades fast as it moves away from the source. A bulb hung a foot up acts like a dim window rather than a strong light. Your seedlings still get their hours, but the weak glow makes them stretch and reach for more. You can run the light 14 to 16 hours a day and still grow weak plants if the fixture sits too high.
The numbers come straight from university extension programs, and they hold up in real trays. Use this as your reference point when you set up a shelf.
Fluorescent tubes run cooler and softer, so you can bring them in close to 2 to 4 inches without burning anything. LED panels push more light, which is why the University of Minnesota suggests a wider 4 to 6 inch gap. Watch the plants for the answer too. Tight, stocky seedlings with green leaves mean your height is right. Pale stems that stretch upward mean the light sits too far away.
That gap is not a one-time setting, and this trips up new growers more than anything else. Seedlings grow toward the light every day. A fixture that sat three inches above the soil on Monday can sit half an inch over the leaf tips by the weekend. Get that close and the heat starts to dry out the tops. Check the gap every few days so it never sneaks up on you.
Raising grow lights as the plants gain height keeps the same distance over the canopy. The fix is simple once you set it up right. Hang the fixture from chains with S-hooks, or set it on an adjustable stand. Either way you can lift it an inch at a time as the plants climb. Measure from the bulb to the tallest leaf, not the soil, and reset that gap each time you look.
Watch for warning signs between checks. Leaves that curl down or fade at the edges sit too close to the light. Stems that thin out and lean tell you the light hangs too far. Both problems show up within a day or two, so a quick look each evening catches them early. A short, stocky seedling with deep green leaves means your height is right and you can leave it alone.
Keep this routine through the four to six weeks of seedling growth and you get short, strong transplants ready for the garden. The whole job takes a few seconds each time. Lift the light, measure the gap, and move on. Do that one small thing and your seedlings stand firm instead of flopping over before they ever reach a pot of soil.
Read the full article: How to Fix Leggy Seedlings: 6 Proven Steps
