You should almost always try to fix them first rather than restart leggy seedlings. A tall, thin seedling that still stands up straight has healthy roots and good leaves to build on. Give it brighter, closer light and it will most likely firm up and keep growing. The plants you should throw away are the ones that have already flopped over and gone soft or mushy at the base.
The difference comes down to one quick look. Is the stem thin but standing on its own? Then fix it. Has it collapsed, snapped, or turned slimy where it meets the soil? Then it is past saving. That single check sorts most of your tray into two clear piles without much guessing.
Here is why the fix versus resow math leans toward fixing. A merely tall seedling lost no working parts. The roots still pull water, the leaves still feed the plant, and the only real problem is a stretched stem you can support or bury deeper later. None of that is broken, so you keep weeks of growth you already paid for. Think of legginess as a cosmetic flaw rather than a death sentence. The plant grew in the wrong shape, but the engine inside it still runs fine. Fix the light and the next round of growth comes in short and sturdy, sitting right on top of the stretched part.
Standing on its own means fix the light. Collapsed, snapped, or mushy at the base means resow. You rarely need a third option.
Restarting throws all that progress away. A new sowing puts you back at day one, and most seeds need 6 to 8 weeks to reach transplant size. That is a long gap to give up when the seedling in front of you only needs better light. You also have to wait days just for the new seeds to sprout before the clock even starts. Resowing makes sense when the plant is truly gone, not when it merely grew tall. So before you restart leggy seedlings, separate the standing ones from the collapsed ones and treat each pile on its own.
So weigh how much of the season you have left before you reach for a fresh seed packet. Early in spring, you have plenty of room to resow a tray that failed, and starting over costs you little. Late in the season the math flips. Those lost 6 to 8 weeks can push a tomato or pepper right past its window to ripen before frost. At that point, fixing the seedling you already have is the smarter move. A tall plant that fruits beats a perfect plant that never gets the time to set fruit at all. This is the main reason you fix rather than restart leggy seedlings once the calendar gets tight.
Keep your standard short while you sort the tray. You are not trying to save every weak plant, just the ones that stand and have green, healthy leaves. A seedling can look alarmingly tall and still be fine. Bright light from above and a cooler spot will slow new stretch, and burying the long stem at transplant hides most of the damage. Tomatoes handle this best because they grow roots all along a buried stem. Many other plants do not, so go easy on burial when you are unsure which type you have.
When you do decide to throw away seedlings, do it without guilt and reuse the cell right away. A plant that rots at the soil line will not come back, and leaving it in the tray risks spreading that rot to healthy neighbors. Clear it, refill the cell, and resow that one spot so the rest of your tray stays clean and the season keeps moving.
Read the full article: How to Fix Leggy Seedlings: 6 Proven Steps
