Hubis posted:

'Sup maple-avalache buddy!

- So from my (modest but focused) reading and (almost non-existent) experience, big-leaf / native maples are probably going to look best as larger bonsai due to their... err... big leaves. Not huge, but probably in the 18-36" total height range. Smaller sizes are going to struggle with having the leaves and trunk/branches look balanced.
- Meanwhile, trees are (assuming good conditions) definitely going to grow best in terms of adding size in native soil.
- The best time to transplant native deciduous trees is probably the early spring or late fall. If you do it too close to the summer heat it will get dried out before it has a chance to build a good root system.
- On the other hand, saplings are incredibly energetic and resilient, and so you can be a lot more aggressive with them than larger trees.

So, based on all that, my suggestion (again, super limited expertise) would be, in order from most to least preferred:

1) If they are in an out of the way location, let them grow in-place for a year or two, pretty much unmanaged
2) If there somewhere not in the way but where you don't want a rogue seedling, let them grow through the summer and collect them in October/November
3) If you'd rather remove them immediately, dig them up gently and put them into a reasonably sized pot. Let them grow for a year or two, then try to move them to a training pot.
4) If you have a ton of seedlings and not a lot of space for pots, just pack them all into a few big pots and go through next spring to separate them based on how they grow.
5) If you feel adventurous, you could try to carefully bare-root one or two now and put them straight in bonsai soil so you're growing straight from the start

If you're like me you've probably got more sprouts than you know what to do with, so you could probably do some combination of all 5 and just see how they turn out and still have plenty to spare that will end up being weeded into the trash.

e: I've been doing a bunch of reading / youtube watching, and one of the sources I gravitated to was Peter Chan (just because he does such an amazing job of explaining sophisticated concepts while also making things seem not too terribly intimidating). One of his videos that really made me feel like I could do Bonsai myself and inspired me to take the plunge was this one:
https://www.youtube.com/watch?v=cvh0r0Yf59I

This shows how they grow them out in the field, the size / shapes they're looking for when they decide to pot and shape them, and the kinds of decisions he is making as he goes from raw material to bonsai in training. He goes through a ton of trees in like two 30-minute videos, and it was really illuminating for me in terms of mindset. It doesn't necessarily apply directly to what you're trying to do, but will maybe give you a good framework.

This is another good one, showing how to work specifically small tree starts:
https://www.youtube.com/watch?v=S7IHeDU-JjY