Somewhere around two to four weeks in, your seedlings outgrow their first home. The little cell that was perfect for germination becomes a cramped box of tangled roots and depleted mix, and the seedling stalls if you leave it there. Moving it into a larger pot with fresh, richer mix - potting up - gives the roots room to run and the plant food to grow on, bridging the gap between the tiny starter cell and the garden. It is a gentle operation, but a fragile seedling can be killed by a careless hand, so it is worth doing with a little care and knowing the few rules that keep casualties near zero.
When to pot up: read the signal, not the calendar
The reliable cue is the leaves. A seedling emerges with seed leaves (cotyledons) - the first, simple pair, often rounded and plain, which are really extensions of the seed itself. Then come the true leaves, which look like miniature versions of the adult plant’s foliage. Once a seedling has its first one or two sets of true leaves, it has the root system and the vigour to handle being moved, and it is usually starting to need more room and more food than its starter cell can give. That is your window.
Two supporting signs confirm it. Roots appearing at the drainage holes mean the plant has filled its cell and is looking for somewhere to go. And a seedling that has stalled or begun to yellow in a small cell after making a few true leaves is telling you it has spent its packed lunch and exhausted the starter mix - potting up into richer mix is often exactly what un-sticks it. Do not pot up a seedling still on its seed leaves alone; it is too young and its roots too slight to move safely. Wait for true leaves.
Not everything needs potting up. Fast crops going to the garden soon, or crops started in generously sized cells, may go straight from their first cell to the ground without an intermediate pot. Potting up is for the crops that spend a long stretch indoors - peppers, tomatoes, eggplant and the like started early - which would otherwise become root-bound and starved weeks before the garden is ready. If a seedling will be transplanted outdoors within a week or two of filling its cell, you can often skip potting up and go straight out.
The golden rule: handle by the leaf, never the stem
This is the single most important sentence in the guide. When you handle a seedling, hold it gently by a leaf - never by the stem. The stem is the plant’s lifeline: crush it, kink it, or pinch it and you sever the pathway that moves water and food, and the seedling dies, often not immediately but over the next day or two as the damage tells. A leaf is expendable by comparison - a seedling can lose a leaf and grow a new one, but it has only one stem.
So the technique is: loosen the whole root ball from below (push up through the cell’s drainage hole, or squeeze a flexible cell, or lever gently with a dibber or the tip of a pencil), and lift the seedling by cradling its root ball and steadying it by a leaf, never by pinching the stem. Move slowly. A seedling out of its cell with its root ball intact and its stem untouched transplants without even noticing; one levered out by its stem is a casualty whether or not it looks fine in the moment.
The technique, step by step
Fill the new pots - a step up in size, not a giant leap; a small seedling in an enormous pot sits in a reservoir of wet, airless mix and can rot, the same too-big problem we meet with houseplants. A modest pot now, and another step later if needed, beats one huge jump. Use a richer mix than your starter blend, because the plant is now big enough to want feeding: a general potting compost, or your starter mix boosted with worm castings or a little balanced fertiliser, is right at this stage. Make a hole in the fresh mix with a dibber or finger, big enough for the root ball to drop in without cramming.
Loosen and lift the seedling as above, holding a leaf and supporting the roots. Set it into the hole and firm the mix gently around it - firm enough for good root-to-soil contact, not packed hard. Then water it in straight away with a gentle pour or by bottom watering, which settles the mix around the roots and closes air gaps. Keep the freshly potted seedlings out of the harshest, brightest light and any heat for a day or two while they re-establish - not in darkness, just easier conditions - then return them to full light under your lamps. A day of recovery, and they carry on growing, now with room and food.
Planting deeper: a trick for some crops, a trap for others
At potting up (and again at final transplant) you can bury some seedlings deeper than they were growing, and for the right crops this is a genuine advantage. Tomatoes are the star: their stems grow roots anywhere they touch moist soil, so burying a leggy tomato up to its lowest leaves converts that weak, stretched stem into a big, deep, vigorous root system - the single best rescue for a leggy tomato and a good practice even for a healthy one. Other members of the same family, and a few crops that root along the stem, can be treated similarly.
But this is not universal, and getting it wrong kills. Many plants must not be buried deeper - their stems will rot if soil is piled against them. Most seedlings should be replanted at the same depth they were growing, with the soil line unchanged. As a safe default, plant at the same depth unless you specifically know the crop tolerates deep planting (tomatoes being the famous exception you can rely on). When unsure, keep the original soil line and you will never go wrong; reserve deep planting for the crops you have confirmed like it.
When to start feeding, and how gently
Potting up into richer mix often supplies enough food for a while, but as seedlings grow on indoors they eventually need feeding, and the rule is little and weak. Start a dilute liquid fertiliser at quarter to half the recommended strength, no more than once a week, once the seedlings have their true leaves and especially if they are in low-nutrient mix or showing the pale, hungry look of depletion. Young seedlings have tender roots that a full-strength feed can burn, and overfeeding produces soft, sappy, weak growth that is more prone to pests and to flopping - the opposite of the tough transplant you want. Under-feed rather than over-feed at this stage. A pale seedling wants a gentle top-up, not a banquet.
Watch, too, that you are diagnosing correctly before feeding, per the troubleshooting guide: a yellow seedling in soggy mix is over-watered, not hungry, and fertiliser will not help it. Feed the ones that have genuinely outgrown their food supply, weakly, and let good light and room do the rest.
Where this leaves you
Potted up on the true-leaf signal, handled by the leaf, set into fresh richer mix at the right depth, watered in and fed lightly, your seedlings now have everything they need to grow into stocky, garden-ready plants over the following weeks. They are bigger, stronger and better fed than the fragile things that emerged from the starter cells. But they are still soft - raised in the windless, weatherless comfort of your home under gentle light. Between them and the garden lies the step that trips up more gardeners than any other, and kills more otherwise-perfect seedlings than all the earlier hazards combined: the transition to the outdoors. That bridge, hardening off, is the next guide - and it is the one you must not skip.