What to do now
Mildest step first.
- Hang fluorescent tubes 2–4 inches above the leaves; start LEDs around 8–12 inches unless the maker says otherwise.
- Put the lights on a timer for 14–16 hours a day.
- Raise the lights every few days to keep the same gap as the seedlings grow.
- If seedlings stretch, lower the light; if top leaves look bleached or scorched, raise it.
Fluorescent lights
Light from a fixture weakens quickly with distance, so a few inches make a real difference. For fluorescent tubes, including ordinary T8 and T5 shop lights, University of Maine Extension says to keep them 2 to 4 inches above the tops of the seedlings, and Illinois and Utah State give the same range. That feels close, but fluorescent light fades quickly with distance, and that gap is what keeps seedlings stocky.
LED lights
LEDs are brighter for the power they use, and the right height varies more from one fixture to another. Iowa State suggests LED fixtures can often be set a little farther away, at about 8 to 12 inches from the plants. A powerful LED panel may need more distance than that, and a weak LED strip may need less. Start with the maker's recommendation for seedlings, then let the plants tell you whether to adjust.
Signs of too far or too close
Seedlings that are too far from the light stretch toward it: long, stems with wide gaps between leaves, often leaning. A windowsill alone usually produces the same result, since late-winter days are short and the light comes from one side. UNH Extension points out that once a stem has stretched, adding light won't shrink it back, so it's worth getting the height right early. Light that's too close or too strong shows up on the top leaves first: Iowa State describes leaves turning yellow or brown and burning, starting with the upper leaves, and K-State notes that bleached-looking leaves mean the light should move back. Raising a fixture a few inches lowers the intensity a lot.
Hours and adjusting
Distance and hours work together. Most extension guides suggest 14 to 16 hours of light a day, on a timer so you don't have to remember. Utah State suggests turning the lights off at night to give plants a rest. Hang fixtures from chains or adjustable cords, or raise the trays on books or blocks, so you can keep the gap the same as the plants grow. Turning the trays occasionally helps the plants at the ends of a fixture, where light is weaker, keep pace with the ones in the middle.
- TomatoThe warm-season anchor of the summer garden.
- PepperA tropical perennial grown as an annual — patient, slow, and particular about warmth.
- BasilThe summer companion — to tomatoes, to pasta, and to the gardener with a south-facing window.
- LettuceA cool-season leaf crop that thrives in spring and fall, sulks in summer heat.
- KaleThe cold-weather workhorse that improves when everything else quits.
- Why are my seedlings tall, pale, and floppy?Seedlings stretch toward inadequate light — the usual fix is moving the light closer, to about 2–6 inches above the leaves, and running it 14–16 hours a day.
- When should I start seeds indoors?Count backward from your last frost date using the seed packet's weeks-to-transplant number — tomatoes usually go in 6–8 weeks before last frost, and peppers a little earlier, about 8–10 weeks.
- When should I pot up seedlings?Soon after the first true leaves open, or when leaves from neighboring cells start to overlap and roots show at the drainage holes. Move tomatoes, peppers and eggplant into pots about 3 to 4 inches across. Squash, cucumbers and melons are better sown directly in their own pots and not moved until planting out.
- Why are my seedlings turning purple?Most often the roots are too cold to take up phosphorus, even when there's enough in the mix. Purple leaf undersides and stems are common on tomato seedlings under lights and on cabbage-family seedlings. Warm the roots first; feed with a dilute fertilizer if the seedlings have been in the same mix for weeks.