I’m hoping to get your help regarding repotting Japanese maples. I purchased two garden center maples (mikawa yatsubusa and Sharp’s pygmy). I am wondering how much root mass one can take when repotting acer palmatum as bonsai. I am aiming to repot these in the spring, just as new buds emerge and am trying to order some pots ahead of time. I think I tend to use pot sizes that are too large. Right now, these trees are in 7 and 5 gallon pots. Any suggestions would help!
Great, thank you both so much for the helpful responses here!
I may be a lot more severe than others here, but all of my maples get the same treatment.
Find the best base/nebari. It may be lower in the pot than you realize. There may even be a few roots above the best base that need removed.
Wash/Clean the roots so that you know what you're looking at.
If possible cut away everything growing straight down to just a radial arrangement of roots. So long as everything is healthy there should be plenty of fine roots in that radial root mass.
You'll need to use your own judgement at this point on what is enough root mass. But I can tell you it's likely less than you think. Maples are tough!
I like to plant my maples in shallow containers. But there's no reason you can't do it incrementally if the root mass you need isn't available yet or you're not comfortable with it. From the pictures I would say that oval pots about the width of the containers they are in (possibly a bit wider on the right tree) with a 2"-3" depth would likely be plenty large for these. And like I said, go a bit deeper if you aren't comfortable with that.