Whenever you buy leek seedlings I would recommend that you get them as big as possible. Leeks really need to be planted deeply, the deeper you plant them the more white stem you will get without having to earth up them up.
Many of the leek seedlings you can buy are too small which means you can’t plant them deep enough and they are easily consumed by snails. Try and get them at least as big as the ones below.
The only downside of buying seedlings this size is that they can end up root bound in the pot. While looks like a major issue, the roots on leeks are quite tough and roots like this actually means the pant will have a good root run as soon as they are planted. Just take care as you tease them out and you should be fine.
I plant my leek seeds using a dibber, which in this case is the handle of the pointing trowel I use in the garden. I stick the handle of the trowel in as deep as it will go, then gently pull it out. I then poke a leek seedling into the hole as deep as it will go without jamming it in. Don’t worry if the roots don’t appear to go in easily, they will be fine. As you can see from the third photo they go in quite deeply, with really just the top third of the plant poking out above the soil. This method only works with soil moist enough to hold it’s shape. If yours is too dry or too sandy you may need to dig a trench instead.
Once you have planted all your seedlings the final thing is to go along and give all of the leeks a good soaking. This will collapse the hope around the leek and cover the roots up giving the plant a firm foundation. Easy huh.