Sorry, no pictures this time.
So after a full year inside a mesh colander, yesterday I took one of my Niwa Ume to repot it in a proper pot and...
The roots were EXACTLY the same as when potted in the colander. That is why the plant didn't grow at all during this year.
But the culprit was very visible, the inorganic soil that surrounded the original rootball was never touched by the new roots; those kept inside the original rootball, and lived by using the exhausted original organic soil.
My fault, of course.
Last year, afraid of messing around too much with the roots I left much more original soil than it was needed, and on top of that I didn't scratch the sides ou under the rootball to loosen up the tips of the roots, so instead of growing into the new substrate, they just kept using the soil they were used to.
Lesson learned.
Last week I have repotted another Ume in 100% inorganic soil, this tree being much bigger, chopped off all root tips and thick roots, and almost bare rooted it. The result?
It is thriving one week later, full of new shoots and leaves.
So if it's done at the right time and using the proper techniques, one can get away with really heavy root pruning at the repotting time.
As for the Niwa, one whole season was lost...

As part of the Great Azalea Project I was digging up the azaleas my son planted in the Alabama clay soil around here. One, probably the last one, he had dug a hole exactly the same size as the nursery container, and the root ball after fifteen years had grown inwards and not out. I put it in an azalea akadama based mix and it is growing like a weed with more flowers than it probably had in ten years total.

Pretty close to the same story.