Potting on tomato seedlings: when to do it and how

When the first true leaves are properly open and the pot is starting to feel cramped, it is time. Roots appearing from the drainage holes means you have already left it a little long — the seedlings will not be ruined, but they will be glad to move.

Move into something a size up — 9 or 10cm pots, ordinary multipurpose compost — but bury the stem deeper than it sat before. Tomatoes root all along any buried stem, so a deeper planting makes a stronger plant. A leggy seedling that has been stretching for light can be buried right up to its lowest leaves and will usually recover well. It is one of the more forgiving things about growing tomatoes.

Some varieties will need moving on twice before they go outside — from smaller pots into litre pots, then into the ground or final containers once frost is no longer a risk. Roots at the drainage holes means it is time to move them on.

Plants from a windowsill have never dealt with wind, direct sun, or cold nights. Moving them straight outside will set them back, sometimes badly. The fix is gradual — an hour or two outside in a sheltered spot to begin with, building up over a couple of weeks until they can manage a full day and night out. Bring them in if frost is forecast. After two weeks of this they are ready.