It is recommended that you transplant leeks to their final spot once they have grown to the “size of a pencil”. I suppose that means the thickness of your little finger or thereabouts. Personally, I have always transplanted leeks when they have been smaller than this, and they have always struggled to grow on to full size.
This season, I’ve decided to transplant the young leeks into a larger seed tray and grow them on before the final transplant. The larger seed tray allows for more space to grow, it also replenishes the compost, and I will feed with a nitrogen rich feed every week until they have grown large enough to plant out.
Even very young leeks have long roots, and I like to trim them back before planting on. Just nip the roots to leave a neat length that fits in the hole you have made. I’ll leave the leeks outside now as it is warm enough for them to thrive.
I’ve uploaded a video to my YouTube Channel, you can see it below…