What to do now
Mildest step first.
- Look for the first true leaves, the ones that look like the plant's real leaves, above the round seed leaves.
- Water an hour ahead, then lift each seedling out with a butter knife or popsicle stick, holding it by a leaf, not the stem.
- Set it in a 3- to 4-inch pot of moist potting mix at the same depth; tomatoes can go deeper.
- Keep it out of strong light for a day or two, then put it back under lights 14–16 hours a day.
When they're ready
Seeds sown together in a flat or small cell need more room soon after their first true leaves appear. Those are the leaves that form after the round seed leaves (cotyledons) and look like a small version of the plant's real foliage. University of Maryland Extension calls that the ideal time to move them and says leaving seedlings in the seed flat too long is one of the most common mistakes; Purdue warns that waiting produces crowded, spindly plants. Clemson allows up to two sets of true leaves. Utah State uses two other signs: leaves starting to overlap with neighboring plants, and roots visible when you slide a plant out, before they form a dense mat.
What size pot
For tomatoes, peppers and eggplant started in small cells, the next step is a pot about 3 to 4 inches across; Maryland suggests cells or pots of 9 to 16 square inches for these crops. Bigger is not better at this stage: a large pot of wet mix around a small root ball stays soggy and invites root rot. Fast-growing tomatoes may need a second move into a larger pot if planting-out is still weeks away. Broccoli and cabbage do well in somewhat smaller cells, around 4 to 9 square inches.
How to move them
Water the flat an hour before so the mix holds together. Lift each seedling with a butter knife, chopstick or popsicle stick, keeping as much mix around the roots as you can, and tease crowded seedlings apart gently. Hold them by a leaf, not the stem: a crushed stem can't repair itself, but a torn leaf is soon replaced. Set the seedling in its new pot at the depth it grew before and firm the mix lightly. Tomatoes are the exception; they root along the buried stem, so you can set them deeper, up to just below the seed leaves.
Aftercare
Newly moved seedlings may droop for a day. Purdue suggests keeping them out of strong light for a few days; under fluorescent lights they usually do fine. Then return them to 14 to 16 hours of light a day. Fertilizer depends on your mix: Maryland notes that most potting mixes with added fertilizer carry enough for about 6 weeks, so wait. If your mix has none, water with a half-strength soluble fertilizer every week or two. Too much fertilizer makes soft, growth.
Crops to leave alone
Squash, cucumbers, melons and pumpkins don't tolerate root disturbance well. Colorado State says the only workable way to transplant them is to start seedlings in pots and move them without damaging the roots. Maryland suggests sowing 2 or 3 seeds per container, growing them for only 2 to 4 weeks, and skipping entirely. to one by snipping the extras with scissors rather than pulling them. Corn and beans are handled the same way, or sown directly in the garden.
- TomatoThe warm-season anchor of the summer garden.
- PepperA tropical perennial grown as an annual — patient, slow, and particular about warmth.
- EggplantA heat-demanding nightshade that needs more warmth than the tomato next to it.
- BroccoliA cool-season brassica that heads up for a short window and then turns on you.
- CabbageA long-maturing head that splits if you water it wrong at the wrong time.
- 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.
- Should I bury my tomato plants deep when transplanting?Yes — tomatoes grow roots along any buried stem, so planting deep creates a larger root system and a more resilient plant. Grafted tomatoes are the exception.
- My transplants look wilted and sad after planting — is this normal?Some wilting and leaf drop in the first few days after transplanting is normal; if a plant is still wilting after a week and well-watered, the roots may have been damaged.
- How do I harden off seedlings before transplanting?Gradually expose indoor seedlings to outdoor conditions over 7–10 days, starting with an hour of shade and building up to full sun and overnight temperatures before planting.
- 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.
- How close should grow lights be to seedlings?Fluorescent tubes work best 2 to 4 inches above the tops of the seedlings. LED fixtures can usually hang farther away, often 8 to 12 inches, but follow the maker's guidance for yours. Run the lights 14 to 16 hours a day and raise them as the seedlings grow.