Seeds & growth
True leaves
The second set of leaves a seedling produces, which resemble the adult plant and signal it is ready to prick out.
When a seed germinates, the first leaves to appear are not really leaves at all in the everyday sense. They are the cotyledons, or seed leaves — a food store packed inside the seed that pushes up to get the seedling started. On most vegetables they look plain and rounded, and they often all look much the same whether you sowed a tomato, a cabbage or a courgette. Their job is simply to power the seedling until it can feed itself.
The true leaves come next. These are the seedling's first proper, photosynthesising leaves, and they look like a miniature version of the grown plant. A tomato's true leaves are jagged and tomato-shaped; a brassica's are slightly scalloped; a lettuce's are softly ruffled. Once you can tell what you sowed just by looking at the leaves, you are looking at true leaves.
Why true leaves matter
The arrival of the first true leaves is the single most useful milestone in raising seedlings, because it is the cue to act. Up to this point the seedling is living off its seed reserves and is too fragile to disturb. Once one or two true leaves have opened, the plant has a working set of leaves and the beginnings of a proper root system — strong enough to be moved.
That is the moment to prick out: lift each seedling from its crowded seed tray and move it into its own cell or small pot, where it has room to grow on. Seedlings sown thickly will be competing for light and turning leggy, so pricking out at the true-leaf stage spaces them out and sets them up for sturdy growth. Always handle a seedling by a leaf, never the delicate stem — a damaged leaf can be replaced, a crushed stem cannot.
For seedlings already in their own modules, the first true leaves are instead your signal that things are on track after germination, and that potting on into a larger container won't be far behind once the roots fill the cell.
A quick UK note
On a bright windowsill or in a heated propagator, spring-sown seeds such as tomatoes, peppers and courgettes usually show true leaves within one to two weeks of the seed leaves opening. In a cold, dim spell it can take longer, so go by the leaves, not the calendar. Wait for the true leaves to be properly open and a sensible size before you prick out — rushing it while only the seed leaves are showing risks setting the seedling back. Patience here pays off in stronger, healthier plants come planting-out time.
In a UK garden
On a UK windowsill in spring, most veg seedlings show their first true leaves a week or two after the seed leaves appear, depending on warmth.
Example
A tomato seedling first opens two smooth, oval seed leaves, then unfurls a jagged, tomato-shaped true leaf — your cue to prick it out into its own pot.