Grew ours from seed in February and just planted out on the allotment today.

plant leeks as you would onions. After danger of frost.

When they are about the thickness of a pencil is the usual advice.

The reason: They should be planted in a narrow hole at least 3 inches deep which you make with a dibber - the wrong end of a rake will do for this if you're careful with it) and watered in from a can without the rose on it. This washes down enough soil to cover the roots, and saves a lot of earthing up later. So small thin seedlings are easily overwhelmed if planted out too young.

You can shorten long roots and break some off the top of the leaves too. I had this advice from an old gardener many years ago and have always done it since.