The short version: Pot on a seedling when roots show at the drainage holes, or it has two sets of true leaves and planting out is still a fortnight away. Go up one pot size, hold a leaf not the stem, set it at the depth it grew (tomatoes can go deeper), water in and give it two days in bright shade.
When a seedling needs a bigger pot
- Roots at the drainage holes, or a root ball that holds its shape when tipped out.
- Two sets of true leaves (the leaves after the seed leaves) in a cell or punnet.
- Growth slowing and the mix drying out within a day of watering: the roots have filled the pot.
- Planting out is two weeks or more away because of frost, heat or an unfinished bed.
Skip the step where you can. If the seedling is hardened off and the bed is ready, plant it straight into the garden.
How to move it
- Water the seedling an hour before, so the mix holds together around the roots.
- Potting mix can carry Legionella bacteria: keep it damp, wear gloves and a P2 mask, and wash your hands after. Fill the new pot, one size up (a 5 cm cell to a 10 cm pot), to a centimetre below the rim and make a hole with a finger.
- Tip the seedling out: fingers over the mix, pot upside down, a tap on the base. Lever a punnet seedling out with a label, keeping as much root as will come.
- Hold it by a leaf, not the stem. A seedling can grow another leaf; a crushed stem is the end of it.
- Tease out any roots circling the base and set it at the depth it was growing. Tomatoes are the exception and can go in deeper, up to the seed leaves, because they root from the buried stem.
- Firm the mix gently, label the pot, water until it runs from the base, and give it two days in bright shade before full light.
When it goes wrong
Some wilting on the first afternoon is normal and should lift overnight. A seedling that stays down needs a look, not a wait: check the mix is damp through but not sodden, keep it shaded, and throw it out if the stem is pinched and brown at the mix line. A premium potting mix carries fertiliser for the first weeks; a regular mix may not, so read the bag and start a weak liquid feed after a week if it has none. Yellowing lower leaves later mean it wants a feed or, better, the garden.
One size up, a leaf to hold and a couple of quiet days in the shade are all a seedling asks.
Related reading
Start the next punnet from the vegetable seeds collection.