Chill hours really only have relevance to fruit and nut trees that require a specified number of hours at cold temperatures to develop flower buds and therefore fruit set.

Dormancy is a different concept altogether and all temperate region deciduous shrubs and trees require it. You can overlook it for a season, especially with small containerized seedlings, but I wouldn't risk it for a second winter.

So it's already dormant is what I'm reading? Which in MO, it certainly should be in December. That's sufficient. It's going to stay dormant for a while yet anyway even if you bring it indoors until the genetic "keys" go off and it "realizes" that temperatures and light levels are high enough to bring it out (quotes because plants don't have direct keys or volition).

I might put them on windowsills for a while to let them adjust to the increase in temperature, which is pretty sharp, first. Then put them under lights later, around Christmas. However, if they begin to sprout leaves, just put them under lights.

You can raise deciduous trees indoors all year without a full dormancy (the topic's covered online on many sites, I won't quote them here but a simple Google search will see you clear on that), and it's something I do regularly with several mini-trees in a hachi no ki (potted tree, a larger, free-er forerunner to bonsai) style. They do just fine.

Try "can you grow deciduous trees indoors" and you'll find a ton of resources. Some trees will drop into a light dormancy to one level or another naturally. Some don't. Some don't even lose leaves until the new ones are ready to grow. I don't do birches so you'd have to experiment to find out, but the tree will choose what's right for them and do just fine.

If you prohibit a plant from entering a natural state of dormancy consistently over several seasons you are altering its natural life cycle and you cannot do that without some sort of repercussions. The usual response is to significantly shorten its lifespan. And this too is clearly documented on many horticultural websites.

"Tree species that have well-developed dormancy needs may be tricked to some degree, but not completely. For instance, if a Japanese Maple (Acer palmatum) is given an "eternal summer" through exposure to additional daylight, it grows continuously for as long as two years. Eventually, however, a temperate-climate plant automatically goes dormant, no matter what environmental conditions it experiences. Deciduous plants lose their leaves; evergreens curtail all new growth. Going through an "eternal summer" and the resultant automatic dormancy is stressful to the plant and usually fatal. The fatality rate increases to 100% if the plant does not receive the necessary period of cold temperatures required to break the dormancy."

One does have to choose some trees carefully, but in this case, he's allowed dormancy, he can't leave the pots exposed to a Missouri winter, and something's gotta be done.

Of course trees raised indoors and in pots have shorter lifespans away from their native environments, that's obvious. But as I look at the hachi no ki my grandfather started and that I inherited, I can certainly see that good care will allow for decades of very, very good life.

So I'm going to call foul on that "100%" claim from the start, as will the crepe myrtle I kept indoors for five years until it was strong enough to go outside. (Turns head) There are two deciduous hibiscus under lights now as it snows outside that are simply, by my estimate, too small to survive the winter with any reliability. They'll be kept up and running through this year at least. They'll be just fine. Been there, done that, many times.

However, the conversation is moot, so I'd suggest simply tabling it, as I'm going to when the other person is, quite bluntly, completely wrong and unable to change their minds and update their knowledge with some research and some flexibility as to the unbelievable vastness of species on the planet. Some, even many of which might actually not conform to one's limited expectations, eh?

The OP's trees are already dormant and likely to remain so for weeks yet. In MO, they went dormant no earlier than late October. They won't break dormancy until early January at the absolute inside. That's more than sufficient for the southern range of the tree, It might be February or March given the described size of the small sapling.

The pots can't stay outside over the winter as the roots won't survive in a pot. Something has to be done. Therefore, one way or the other, it's lights or at least a windowsill.

Therefore, there aren't a lot of choices here.

Rather astounding at the flagrant dismissal some can give to documented evidence - not just babble back anecdotal reports - at the risks involved in indefinitely delaying dormancy of temperate zone plants! We are not talking about all the number of many species on the planet, only those that are native to temperate regions of the globe where a necessary period of dormancy is part of their genetic biological make up.

And I could also suggest that rather than tossing about cheap (and unsubstantiated) statements such as "the other person is, quite bluntly, completely wrong and unable to change their minds and update their knowledge with some research" that the poster do their own research, as they appear to be grossly uninformed. Even any reputable bonsai site will state the same. Completely wrong?? So then I guess is the vast majority of the rest of the population!! Just a little arrogance happening here.

Bonsai Learning Center: "the vast majority actually need the cold in order to put them into “sleep mode” for the winter. If they don’t get it, they are like marathon runners who never rest. They begin to slow down and gradually lose vigor. Eventually over time, they simply run out of energy and die. This is what happens to temperate bonsai which are brought inside the house during the winter months because their owners fear that exposure to cold weather will kill them"

Bonsai4me: "Some bonsai beginners feel that their trees may perish if subject to the harsh conditions of the winter months and bring their trees indoors to 'protect' them. This continuation of heat and light through the winter prevents dormancy in temperate trees. The resulting continual growth throughout the year goes against the trees' internal clock which is requiring a dormant period, the clock can be tricked to an extent; the tree will continue to grow inside. It may even grow continuously for as long as two years after which, whatever the season or conditions, deciduous species will drop all leaves and evergreens will stop all growth. This out of season dormancy usually results in very sickly trees and even death."

All things bonsai: "It is quite common for beginners in bonsai to want to over
protect trees by bringing them into the house or into a heated conservatory. This
may result in the trees not actually entering the dormancy period. If this
happens the trees may carry out growing throughout the winter. This distortion
of the natural cycle will harm and can eventually kill the tree. The dormancy
period is essential to the health of trees from a temperate climate. Trees from
tropical parts of the world do not need this dormancy period as they do not
experience it in their natural environments. They can therefore be kept indoors
all year round."

Yes, now that we've thrown some personal insults, and done superfluous research that means nothing, I'm sure some posters feel better now and can we get back to the OP's question?
Choose the tree correctly rather than relying on Internet quotes. Alrighty, then, because I don't argue with those who simply toss insults and cherry-pick Google results. (Glances at hachi no ki) They do fine when the species is correctly chosen as I already stated. Or, some should learn to pay attention, because once again, I'm done with the likes of some posters. (flips hand in dismissive gesture) For obvious reasons.

Yes, OP, bring the tree in. It'll break dormancy when it feels like, then grow on.

And to be blunt, if you expend the energy caring for it as was done on superfluous research, it'll be gorgeous.

I'm guessing you'll plant them outside when they're big enough, yes? So next fall, you let them drop into dormancy, then bring the pots inside when it starts to get too cold. You can't let them freeze as the roots, in pots, can't deal with it. A cool cellar or garage for a while would be fine, then indoors on a windowsill.

You've had them dormant so the...er, statement...above doesn't apply. And you don't need to worry about it even if it were a problem, which for a couple years, it really wouldn't be much of one. This isn't a forever thing. As I noted, I did it with a crepe myrtle for years before it was strong enough to tolerate a Pennsylvania winter, but that's an oddity of a plant I wanted in a place that's not well-suited for it!

Even if it is, just keep that up and you'll be OK. Just keep root trimming and top-trimming to maintain the tree at a size and shape that you can transport easily. At that point, you're actually doing "hachi no ki" just not formally. :-) You can look up more formal designs for that if you like, they're easy and fun and not nearly the challenge bonsai is (the rules also don't apply and it's not anywhere near as hard, either!)

By fall, though, they might be large enough to plant outside. It depends on the care you give them.

what is your goal with this tree ... release it back to nature in spring ... or bonsai???

many trees .. only have one basic growth spurt in spring .. and thats it for the year ... others of course.. grow all season ...im not researching this one for you ...

but if its really only the basic spring spurt.. what do you accomplish.. other than doing it early???

is an additional month or two really going to get growth significant enough to take the risk of growing a tree as a houseplant??

of course.. if its not rare to you ... and you have spares .... experiment away ...

unless you just have lots of space under existing lights ... frankly ... i would just leave it outside ... im sure there are other things you can fool around with under the lights ... i was also thinking about how close lights need to be to the plants ... and if you have this 2 foot tree ... will you be able to utilize the lights on anything much lower ..????

ken

What, you didn't hear that a mild winter and remaining in foliage will apparently kill them instantly or something? Apparently, anyway, quite apart from "anecdotal" reports or some such. :-)

But yeah, if it stays that mild, no worries. For the OP, he lists Missouri as his state, the north end. Even for Jefferson City, MO (mid-State), the average January temp is listed at 19. I wouldn't risk it for a potted first-year tree (really, any potted tree). OP's locale is likely to be colder.

Just sink the pot into the ground for the winter.

Yes, that's the insurance Lacey, should work fine even if a really bad winter.

The problem with bringing it in now, or soon, to grow all winter is you have to deal with figuring out how to acclimate it to being back outside in the spring. After being used to a constant photoperiod, constant temperature and little to no direct sunlight, you have to figure out how to adapt that plant to being outside again. All sorts of things can go wrong trying to do that. Much easier to put it into storage of some sort and letting it regrow on its regular cycle next year. It's a river birch, it will grow really fast no need to push it.

Fortunately, that's not too bad. Just follow any good hardening off guide and take it slow. Roughly, give it an hour of sunlight the first day and second. Two hours the third and fourth and fifth. Three hours the sixth to eighth. Four hours days 9-12. After that, it can probably go out all day, but one can continue a slow adjustment if you'd like (or take it faster if the plant is adjusting well).

Leafing could be a little weird next year due to that with tired leaves, partial re-leafing, or other strange effects (I've seen this happen under experimen...I mean "anecdotal" conditions) But unless one is worried about proper flowering or the like, as with a poinsettia, photo period doesn't matter too terribly much. The tree will adjust itself properly and be just fine for Year 3.

I don't know about River Birch in particular. However, I can say that I have had excellent survival rates when I left all of the following trees (and many more shrubs and plants) outside in VT in gallon or larger pots with no protection but the natural snow cover: Morus alba 'Tatarica', Prunus subhirtella, Pinus longaeva, Ulmus americana, Platanus occidentalis, Pinus contorta var. latifolia. There were dozens of the Prunus, a few of the Pinus longaeva, one of the Platanus, half a dozen of the Ulmus, and a couple dozen of the Pinus contorta in the group I left out last year. The one and only Morus alba 'Tatarica' I had grown indoors for a whole year, and it was seven feet tall in a five gallon bag of indoor potting soil. It died back about a foot but filled in nicely and grew some this past summer. The only trees and the only shrub or small tree that just crapped out en mass were Gleditsia triacanthos and Amelanchier canadensis.