Onions from seed - when to plant out.

[SimpleSimon]

My onions from seed are doing well but starting to wonder when people put them outside. My sets are in but I was told to wait until mid may for onions from seed. Not sure if this is true? These are gonna get pretty root bound unless I pot them up again. Which is a lot of pots and compost!

[ameno]

Mid-May sounds far too late. I'd plant those now, to be honest. They look a good size.
Onions are day-length sensitive, and before the longest day they will put most of their effort into growing leaves, and little into bulb, then after the longest day they will put very little into leaves and everything in swelling the bulb.
You want as much leaf as possible on the plant before the longest day, so as to fuel the growth of the bulb thereafter. But if you leave them in pots until mid-May it will slow and restrict their growth, so you'll end up with less leaf, and thus smaller bulbs.

[Snoop Puss]

I put my onion seedlings in any time from about now onwards and we might still get a touch of frost and certainly very cold nights for quite a bit yet even if no actual frost. They don't seem bothered.

Yours look like beauties. I agree with ameno. Not sure whereabouts you're located, but I'd be seriously considering hardening them off a bit if they've been grown indoors and then bunging them in the ground rather than potting them up again.