You can replant seedlings deeper in many cases, but not all plants will survive it. A leggy tomato can be sunk up to its lowest leaves and it grows new roots along the buried stem. A leggy sunflower buried the same way just rots in the soil. The plant you have decides the rule.
The reason comes down to one trait. Some plants grow roots along any stem that touches damp soil. These are called adventitious roots, and they form from the stem itself rather than from the base. So when you bury leggy seedlings of these types, the buried section sprouts a whole new set of roots. The plant ends up with far more root mass than it had before. Tomatoes do this best of all. Their cousins like tomatillos and peppers handle a deeper set too, though peppers root less freely than tomatoes do.
Most other plants cannot pull this off. A buried stem on a sunflower, a bean, a squash, or a cucumber stays a plain stem. It sits in wet soil with no way to make roots. Within a few days it softens and rots at the buried point. The rot then creeps up the stem and the whole seedling collapses. Single-stem flowers and root crops like carrots and beets fall into this same group. Burying a carrot stem does not give you deeper roots. It gives you a rotted carrot and a wasted seed.
Why does the buried tomato stem live while the buried sunflower stem dies? Tomatoes carry hidden root cells, called primordia, all along the stem. Bury that stem in moist soil and those cells wake up and push out real roots. A sunflower stem has no such cells waiting. The buried section can only do what a stem does, which is move water and sugar, and it has no defense against constant damp. That one fact about the cells is what tells you whether you can replant seedlings deeper at all.
Here is the part to memorize, because seedling planting depth is the whole game with leggy starts. For plants that tolerate it, bury the stem up to the lowest set of leaves. Strip off any leaves that would end up below the soil line, since buried leaves rot and feed disease. When you are not sure whether your plant can root along its stem, bury no more than a quarter of the total stem length. That shallow margin protects you from rot while it still steadies a wobbly seedling in the pot.
Bury tolerant plants up to their lowest leaves. When unsure, bury no more than a quarter of the stem and strip any leaves below the soil line.
So before you transplant, sort your seedling into one of three groups. The first group is bury-deep, and tomatoes lead this list with tomatillos close behind. The second is bury-a-little, where peppers and a few woody herbs take a small extra set within that quarter-stem limit. The third is do-not-bury, which covers sunflowers, beans, squash, cucumbers, melons, and every root crop. Make this call before you dig, not while the bare roots dry out in your hand. Knowing the group ahead of time is what saves the plant.
One more point makes this worth getting right. A leggy stem on a tomato is not a flaw you are hiding under the soil. The buried part turns into working roots, so a leggy tomato can finish stronger than a short, stocky one once those new roots take hold. The deeper roots also reach cooler, moister soil, which helps the plant through a dry spell. That same logic backfires on the do-not-bury crowd. With them, the fix for legginess is brighter light and a small fan for airflow, not a deeper hole. Plant each seedling for what it actually is and you will lose far fewer of them.
Read the full article: How to Fix Leggy Seedlings: 6 Proven Steps
