I'm not an expert but personally I would leave them outside. Apples are very hardy and based on everything I have learned, they need cold weather to mature properly. They've been doing this for tens of thousands of years.

Thank you dmtaylor. Now both apples will survive.

The one which sprouted in July should be fine. But if I read that correctly, the 2nd sprouted in December (which is kind of strange...). I think that 2nd one would die outside. Plants need time to harden off for the winter. If the seed just germinated in the past week, I don't see how it could harden off in time for a zone 5b winter. I'd say keep that one inside this winter in a south facing window and leave it out next winter. Or better yet, plant it in the fall.

where are you in z5??

in my z5 MI .. i would tuck then in on the north side of the house.. when the ground freezes .. i would tip over the pot ... and as i shoveled snow of the sidewalk .. i would bury it in snow .. if any ... the snow actually insulates then from frigid winter winds.. and temp spikes ..

they are a deciduous tree.. they were supposed to lose their leaves months ago .. i suspect you may have been feeding them .. trees dont need a lot of fert in a pot ...

the sprouting seedling is way out of phase or kilter.. who knows if that one will live..

dormant trees do NOT need a lot of water .. and i tip them over.. so a frozen pot.. will not accumulate rain or melting snow.. in frozen media ... roots need air as much as other things.. and freezing them into an ice cube is not helpful ...

and NEVER leave a black pot in winter sun.. it can warm and heat too much ... making the roots come out of dormancy while the rest of the tree is dormant ..

this is all about my z5 adrian MI ... and i am sure all colder zone ...

ken