Do not worry about cutting the roots on your trees.

Just lift them, and plant them in proper substrate come early spring, just as the buds are about to open.
Sharing a few pictures may help also assess what you are dealing with. If it is just young plants (say, less than 5cm thick) you can just go out today and lift them, and plant them up.

Do not try and get the plant in a bonsai container, but use one of the slightly bigger pots for it instead.

And for your inspiration:

Thanks, they are indeed very small trees, around 5 cm. There’s a few Acers and a couple of prunus. I’ll wait for spring I think, since this is mainly to learn. Thanks for adding the video, I’ve watched about all of your videos so far I think, I’ll watch this one again just before taking them out.

leatherback wrote:

Do not worry about cutting the roots on your trees.

I will disagree with Leatherback, a very rare event.

If all the feeder roots are at the end of long tap roots and you cut the tap roots off, also removing the feeder roots, the tree will probably die.

I would agree don't worry about losing some of the feeders, but don't cut them all off.

I had this beautiful elm about 7 feet/2 meters tall down by the creek. I cut it off about a foot/30 cm high leaving nice thickness and some decent curve, came back the next year, cut the tap root, including all the feeder roots, and it died. The voice of experience here.

Ivan Mann wrote:

I had this beautiful elm about 7 feet/2 meters tall down by the creek. I cut it off about a foot/30 cm high leaving nice thickness and some decent curve, came back the next year, cut the tap root, including all the feeder roots, and it died. The voice of experience here.

Interesting. Elms are amongst the toughest trees I know, and I have had branches of 2 inches / 5cm diameter root by just dropping them in substrate. Are you sure there is not something else that might have gone wrong? Could it e.g., be that the cut-back tree spent its resources growing foliage, but in a more shaded position and therefor ran out of reserves to regrow roots?

The OP is however asking about very small trees right?

leatherback wrote:

Ivan Mann wrote:

I had this beautiful elm about 7 feet/2 meters tall down by the creek. I cut it off about a foot/30 cm high leaving nice thickness and some decent curve, came back the next year, cut the tap root, including all the feeder roots, and it died. The voice of experience here.

Interesting. Elms are amongst the toughest trees I know, and I have had branches of 2 inches / 5cm diameter root by just dropping them in substrate. Are you sure there is not something else that might have gone wrong? Could it e.g., be that the cut-back tree spent its resources growing foliage, but in a more shaded position and therefor ran out of reserves to regrow roots?

The OP is however asking about very small trees right?

My point was don't cut the feeder roots.

This was ten years ago, so I may have mixed some stories up. It may have been a hawthorn or hackberry, but it was definitely in high shade and definitely went to high sun. The creek beside it kept temperatures down, and then it was hit by warmer temperatures.

I blamed it on cutting the taproot and have kept a lot of feeders since then.

Well, here’s a picture of each of the trees in a growing crate. Couldn’t really use a pot, as the tap roots were quite to the side in stead of growing down. But about three weeks since I dug them up, so at the moment it looks like they survived. Pruned them hard last year, the fresh wounds are from cleaning up the cuts. The one with wire was also wired before the winter.
The prunus is about 6cm thick at the base, the (field?) maples about 3.5cm. A long way to go, but the start is there.
I know not to do anything with them this year, but any tips about what next are always welcome.

I would say that next step depends on the development during the upcoming years. Too many posibilities, better to come back in a few years and show what you have.

Very cool project, and interesting trees.