[Jackypam]

If you sowed them like that and haven't thinned them I think you need to do that first - they won't enjoy being overcrowded to that extent and no amount of feeding will compensate.

About 1" apart should be sufficient to keep them growing until planted out.

Last year I tipped them out, separated them and replanted them. Was going to do the same again this year?  Thinning seems a bit wasteful.

[Goosegirl]

That what I do and they seemed fine after and appreciated a bit more space too.

[JayG]

If you sowed them like that and haven't thinned them I think you need to do that first - they won't enjoy being overcrowded to that extent and no amount of feeding will compensate.

About 1" apart should be sufficient to keep them growing until planted out.

Last year I tipped them out, separated them and replanted them. Was going to do the same again this year?  Thinning seems a bit wasteful.

Sorry, I meant replanting them, even though I didn't actually say so  , although if you sow them an inch apart to start off with you will only have to tip them out and transplant them once.

Both leek and onion seed are quite short lived, but I don't get many gaps sowing leek seeds at that spacing even with year old seed (some year old onion seed has been a quite different story unfortunately!  )