Techniques
Pricking out
Carefully lifting tiny seedlings from a seed tray and moving them into individual pots or modules once they have their first true leaves.
Pricking out is one of the first proper jobs after your seeds come up. When you sow into a tray or pan, the seedlings often grow close together in a little crowd. Pricking out simply means lifting each one gently and giving it its own space to grow on, usually in a module tray or a small individual pot.
When to do it
The right moment is once the seedlings have their first true leaves. The very first pair of leaves to appear are the seed leaves, or cotyledons, which look rounded and plain. The true leaves come next and look like miniature versions of the adult plant's foliage. Their arrival tells you the seedling has a working set of roots and can cope with being moved. Pricking out earlier than this is risky, because the roots are barely formed; leaving it much later lets the seedlings get tall, pale and tangled as they compete for light.
How to lift them safely
Always hold a seedling by a leaf, never by the stem. This is the golden rule of pricking out. A seedling can grow a new leaf if one tears, but it cannot replace a crushed stem. The stem carries water and food up from the roots, so even a light pinch can bruise it and kill the plant. A leaf, by contrast, is expendable. Loosen the roots from below with a dibber, an old pencil or a plant label, then support the seedling by one seed leaf and ease it free with the rootball intact.
Settling them in
Make a hole in fresh, damp compost, lower the seedling in so the seed leaves sit just above the surface, and firm gently. Water lightly to settle the compost around the roots and keep the new transplants out of harsh direct sun for a day or two while they recover. Handled this way, most seedlings barely notice the move.
Why bother at all
Crowded seedlings compete for light, water and nutrients, and the result is weak, drawn-out growth and a higher risk of damping off, the fungal rot that flattens densely sown trays. Giving each plant its own pot lets it build a sturdy root system and a stocky top. As it fills that pot, you simply move it on again into a larger one, a step known as potting on.
Pricking out feels fiddly the first time, but it is quick once you have the knack, and it is the difference between a tray of leggy weaklings and a batch of strong young plants ready to harden off and plant out after the last UK frosts.
In a UK garden
On a UK windowsill or in a propagator, seeds sown thickly in a tray quickly become crowded, so pricking out into modules in early spring gives each seedling room before it goes out after the frosts.
Example
When tray-sown tomatoes show their first true leaves around March, ease each one out with a dibber or pencil, hold it by a leaf, and lower it into its own 7cm pot of compost.