I have a Japanese maple bonsai with a trunk diameter about 1" thick and a rootball 11" in diameter by 2" deep. It’s probably been growing for a while (5-10 years?). When I got it last fall the soil was in very poor condition. I’m repotting now before the buds start breaking.
I took it out of its pot and it’s pretty root bound, so I am cutting back the long roots to about where the soil starts. The question I have is, how much soil can I take off and how far to cut back the roots?
How was the health of the tree last fall? If it was healthy you can cut back quite a bit. If not, you should cut back less.
I generally cut off all of the circling roots and root wash out all of the soil on Japanese Maples like the one you describe, although I do it every 3 years or so. I then cut back the stronger roots so the weaker ones can develop better.
I use the low cost plastic saucers designed for under pots to develop these maples and they are in the size range you list. I melt several 1" (2.5 cm) holes in the bottom for very good drainage so I can fertilize strongly and push growth. I have tried pond baskets, but for maples I don’t get as much growth during our dry summers and the nebari does not develop as fast.
That’s interesting. 
I have two that I was going to put into a pond basket. Maybe I’ll just put them in grow bags. What kind of soil/substrate do you have yours in?
Thanks for your feedback, Marty. Before I got it, the tree had been a bit neglected. I decided to trim the roots 1" smaller than the pot, teased out some of the muck and then rinsed it off. The roots were pretty densely packed in the remaining soil, and never having repotted a maple before, decided to be cautious rather than taking off all the soil. I will check the tree again in a year if it doesn’t do well. Your comments have encouraged me to be a bit more aggressive with the next two that I have to repot.
I have normally used a pumice-lava-akadama mix when developing maples in the saucers. Normally without the 1/16" and often light on the akadama so it was more like 2-2-1. I am including more akadama in my mixes in the last year or two, although I still have not moved to solid akadama even for somewhat more refined trees (4 pumice & 1 akadama is typical for me to give a bit more structure). I also sift out the 1/16" - 1/8" (1 - 3 mm) and normally combine 1 part of that with 2 parts of the 1/8" - 1/4" (3 - 6 mm). I feel that these decisions are based upon my watering which is more garden center style than detailed bonsai bench style.
For plants in grow bags in the ground I am using a 50/50 mix of >1/8" (3 mm) diatomaceous earth (floor dry or similar) and < 1/3" (8 mm) garden soil that was sieved to remove gravel. Still to early to see how those work, but a couple of trees that were lifted from a grow bed of similar composition had roots that were fairly easy to clean up and get into a grow box to start developing into bonsai. I am hoping the grow bags result in a more compact root ball that is equally as easy to clean - root wash for maples, just knock off most of the garden soil for pines and such.
