Keep them cool and let them go fully dormant. But don't let them freeze! Japanese maple roots are very sensitive to cold temperatures so store somewhere above freezing but around 40-45F or less. The soil should just be damp to the touch. Don't let it dry excessively but never too wet either.

If you plan on planting in the ground in spring, no pressing need to repot at all. Just plant out directly at the appropriate time. Otherwise, you can pot up any time while still dormant.

Considering the size of the seedlings, I'd be inclined to pot them up and grow on for a year before shoving them out on their own to face the world :-)

I'll give you a slightly different opinion. Keep them up and running, cool but not dormant, for this year only. If they decide to go dormant, great. But for their first year as mini-seedlings, it's not strictly necessary and let them choose. Keep 'em on a windowsill (in a cool area) and keep them lightly damp, but don't deprive them intentionally of all light or force them to do anything.

They're entirely tolerant to Zone 8, and can survive the cooler areas of Zone 9, so they're not terribly cold-dependent, and yes, some plants really do need to undergo a cold period. This really is one of them. However, for one single year, it's not going to cost them their entire lifespan or anything. They won't complain too much for being by a nice, drafty window in a cool room with sunlight. Far less than a cold garage with no light to reset their clocks come spring (honestly, I can't see how that could possibly be better).

Definitely don't let them fall below freezing, the roots don't like that. If you want to enhance light, that's OK, but there's no real need to bother. I did with one I coddled over the winter before planting, but it's not at all necessary, these are not delicate plants that require too much care.

I don't see your location, but adjust to outside sun over the course of a week or two (follow any good "hardening off" guide) next spring after frost has passed, and plant in your garden. The slower, the better, really--I usually spend 10 days or so hardening off annuals.

where are you.. big city name

i dont know why a bare deciduous tree would need sun .. once dormant ... you dont want the pot itself heating up ...

imo .. they are not houseplants ... and the usual problem in winter with pots.. is accumulated water ... trees do not like to sit around in sopping soil ... but not bone dry either ...

so put them on the north side against a building and protected from rain/snow melt..etc ... and you should be all set ... in my MI ... we just tip pots on teh side.. and if they get buried in snow.. all the better ... its insulation ... against severe winter winds and sun on the pot itselfr

treat them like the trees they are.. not houseplants

ken

BTW: never had much luck in growing much of anything in a clay pot.. ever ... unless you have experience with such pots... skip them .. unless you do pot in pot .. in other words.. the clay is just decorative ... and im not talking about a ceramic pot ... thats a different answer ...

ken, a Japanese maple seedling in a container that small is unlikely to survive the winter outdoors in anything lower than a zone 8. The roots are very sensitive to cold weather and a drop into the low teens overnight - certainly possible in zone 8 - could prove fatal. Now if the OP were to bury the pots in the ground/heel them in and also mulch well, that could get them by. Not sure I'd risk even that in a colder zone. At that size, they would be much better off being kept above freezing as best possible.

And if by clay pots you mean terra cotta, they are much more than just decorative! They are extremely useful and in many cases the added porosity is a bonus.

One interesting quote regarding Japanese maples, "The roots can withstand temperatures as low as 14 degrees Fahrenheit."

Not that I'd be overly inclined to try with a sapling, but OP in MO could heel in pots in a very protected southern location in the house, mulch 6" deep up to within 3" of the "trunk," surround the stick with deer fence, and pile hay or more mulch around that for wind protection. I'd give it a 90% chance of survival, and simply undo it in March.

I do that yearly with my crepe myrtle, which is way out of its native zone, and it always makes it, including part of the canopy, even though we always dive under the point that will kill them.