Grow lights for seed starting should hang directly above seedlings, remain 2–6 inches from the canopy for most LED setups, run 14–16 hours daily on a timer, and get raised as plants grow to prevent stretching.
The difference between stocky, dark-green seedlings you want in the garden and pale, leggy stems that flop over on day three is almost always light management. Seedlings stretch not because they are growing toward the sun—they are emergency-growing because they cannot find it. Get the light close enough, keep it on long enough, and give it a dark break each night, and you will own the indoor start.
How Close Should the Light Be?
Distance depends on the fixture type, and getting this wrong is the single most common mistake. LED bar lights can sit 1–6 inches above the canopy—the lower end for small seedlings, the higher once plants are several inches tall. Fluorescent tubes belong about 6–8 inches above; the light intensity drops off sharply at the ends of the bulbs, so position trays under the middle two-thirds and slide the tray a few inches in from each bulb tip to avoid weak light spots. Pendant-style bulbs or adjustable full-sun lamps for tomatoes and peppers need 12–24 inches of clearance to avoid scorching the crown while still delivering enough intensity for fast growth.
Check the seedlings daily: if they are leaning sideways or reaching up, the light is too far. If leaf edges show bleached or crispy spots, it is too close or the run time is too long.
Light Schedule and the Darkness Rule
14 to 16 hours of light per day is the sweet spot. Seedlings need a solid 8-hour dark period—leaving lights on 24 hours disrupts the metabolic cycle that builds strong stems and healthy root systems. An automatic outlet timer is worth the five dollars it costs; human memory is not reliable for turning lights off at the same time every night. Set the timer so the lights click on around sunrise and shut off mid-evening, and you will not have to think about it again until you raise the fixture.
Turn the lights on as soon as the first green hook breaks the soil surface. Germination itself needs warmth, not bright light—so if the room stays warm enough, wait for that first visible sprout. Then flip the switch and drop the light into position immediately.
Placement, Air, and Avoiding the Obvious Mistakes
The light should hang directly over the tray—seedlings more than a few inches sideways from the bulb will stretch sideways looking for it. Hanging the fixture on chains and S-hooks makes it easy to ratchet up as the plants climb. Expect to adjust every two to three days during the rapid-growth phase.
Supplemental heat from the lights will dry soil faster than a dark windowsill would. Check moisture every morning; the seed-starting mix should feel damp but never wet, and never crusty dry.
If you are setting up your first indoor seed station for the season, our guide to the best seed-starting grow lights covers the fixtures that actually hold up through a full germination cycle.
| Fixture Type | Optimal Distance Above Canopy | Notes |
|---|---|---|
| LED bar lights (low-profile) | 1–6 inches | Start at 2 inches for small seedlings; raise as stems grow. |
| Fluorescent tubes (T5 / T8) | 6–8 inches | Keep trays 3 inches from bulb ends; use full 4-foot bulbs. |
| Pendant/bulb style (full-sun plants) | 12–24 inches | Required for tomatoes, peppers, and other heavy-light crops. |
What the Common Mistakes Actually Look Like
Leggy seedlings—thin stems with wide spacing between leaves—mean the light is too far or the hours are too short. Leaves that cup downward or show pale spots mean the fixture is too close or the run time exceeds 16 hours. Using a standard household lamp bulb (not a grow light) will produce weak, spindly growth because the spectrum is wrong for photosynthesis. A fixture that is shorter than the tray leaves the edges of the tray in near-darkness; a 4-foot bulb serves a 10×20 tray far better than two 2-foot bulbs.
FAQs
Can I leave grow lights on 24 hours a day for faster growth?
No. Seedlings need at least eight hours of uninterrupted darkness per night to complete respiration and build strong cell structure. Running lights around the clock produces weaker plants and can stress leaves.
How do I know when to raise the light?
Raise the fixture when the tallest seedling is about one inch below the light panel or bulb. The goal is to maintain the same 2–6 inch gap as plants grow; check every two to three days during active growth.
Do seedlings need the same light distance for different crops?
Not exactly. Full-sun crops like tomatoes, peppers, and eggplant tolerate and benefit from higher-intensity light; pendant-style bulbs may need 12–24 inches for these. Most greens and flowers grow fine 2–6 inches under LED bar lights.
References & Sources
- Oklahoma State University Extension. “Grow Gardening: Starting Seeds Indoors.” Provides guidance on light distance, duration, and seedling stretching prevention.