You can plant vidalias up north but they won't be the giants you get from Georgia. You can plant the little italian cippolinis in the deep deep south but they'll be really tasty 'green' onions that never really make a bulb. I'd like to grow the 'ailsa craig' onions but don't dare to waste the money on seed.
If you go to johnny's and look at the ailsa craig:
It will tell you what latitudes they're optimal for. *most* but not all seed catalogs will tell you if one is long or short day. Some like Johnnys will even tell you the specific latitudes they're suited for.
Up north, onions might not be hardy through the entire winter so they're started very early in spring and transplanted out as soon as the ground is workable. They bulb/make in mid/late summer IIRC. Down south onions are started in september and transplanted out in october/nov to overwinter and make in early summer.
Hope this helps and hope I'm right! I've found most of this out the hard way. Planted the entirely wrong variety in late spring and they never ever bulbed. Made beautiful green onions though, if expensive ones.
I got most of this info from various state run ag sites. ixquick is your friend.
Oh, to add to the onion post, we planted the granex onion variety that ‘vidalias’ are IIRC last year. Our were just as sweet and possibly sweeter than the vidalias we got from the store for comparison. It’s apparently a soil dependent thing. We’ll definitely be doing that again this year.
My goodness! Thanks for both of your very helpful and informative replies! I’d had no idea about the onions at all. The bleach solution sounds quite useful as well.