Developing shohin for a specific pot

Hi all - I have a nice little Sara Rayner pot I won at a raffle, and a Japanese maple I’d like to eventually move from nursery soil to the pot. Is there a specific procedure for reducing a tree to fit in a specific pot? Mostly I’m worried by the time I prune the root ball back far enough the trunk might have developed too large. How do I know if a pot is too small for a tree, or avoid the tree outgrowing the pot?

Your question is multifaceted. Basically the combination of repotting techniques and pruning techniques will determine the growth and shape of your tree. If it takes longer to reduce the rootball to fit a suitable pot then growth may require management with appropriate regimes of watering, fertilizing and pruning to control growth as desired.
The type of rootball developed is very important for life in a bonsai pot, not just the size of the rootball.
Perhaps you could post a picture and dimensions of both the pot and the tree. That would present a better opportunity to provide more specifics.

The pot is about 3" wide by 4" long, about 1.5" deep. The maple’s a Mikawa Yatsubusa in a cut-down, I want to say one-gallon pot. This spring I reduced about half the root mass and cut the nursery soil with pumice. My plan for next spring was to further reduce the long roots into a smaller pot and switch it to fine-grain bonsai soil to hopefully get more feeder roots. The nebari right now would easily fit in the pot but the stuff under the soil is very long and spread out.

My approach in this situation would be to stage the transition over a couple of repots. Subject to what i saw when repotting. It is very difficult to give exact directions without actually seeing the root system. The degree of division and health condition of the root ball could vary greatly!

My first humble question is: is it ready to go into a bonsai pot?
And then, is this beautiful pot appropriate for a maple?

A very polite way of saying “this particular maple may not be worth the effort,” I appreciate it! I’m not so much interested in this specific tree, I was planning to use it to practice developing a nice bonsai root system from nursery material. You make a good point though, is this a useful exercise do you think, or would I be better off using that time and effort to develop good roots on a seedling or cutting rather than a larger nursery plant that has long winding roots in a big pot? Maybe a small crab or chojubai cutting would be a better subject?

As for the pot, I like it a lot but I admit nothing I have right now is really “worth” putting into it, but I feel like a pot ought to have a plant (you can see there’s just some stones and succulents in it right now) that can always come out later to make room for a better one.

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I sometimes put trees into nice pots ahead of schedule when I really like the pot and want to see it used. My usual approach is to focus on developing the roots and trunk and then move the tree into a bonsai pot when the lower part of the trunk is in good shape.

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If your primary goal is to use the tree to practice developing a great root system then the small pot is not the way to go. Better to develop the roots, trunk and primary branches in a larger container.
I like to move tree’s to nicer pots when they have entered the advanced ramification stage. I normally consider secondary, tertiary branching onward to be ramification.

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Thanks Frank! Yes that’s the plan - I just don’t know the specifics of how to go about developing better roots from a nursery tree, and if doing that over several years and repots, how to keep the trunk small enough. Ramified branches are going to thicken it considerably, but maybe that’s ok as long as the nebari is under control?

Controlling the apex will control the thickness.
Develop cuttings to do root grafts with in the future to improve the nebari.