Ok, warning! Be aware any soil discussion can go sideways quick! This is just about my experience.
I have a number of prebonsai Satsuki, a couple of bonsai in refinement and some in the ground to grow out more.
I’ve spent a good bit of time researching soil types for both. I overwinter bonsai and bonsai in a cold frame.
My recommendation would be if you are going to repot this time of year, which is later than optimal... is to repot in 70% kanuma and 30% chopped sphagnum.
I’d repot, but I’m in the Pacific Northwest.
Have the kanuma mostly smaller grained with some larger mixed in and on the bottom. I used this mix on a bonsai that suffered from root rot and it brought that baby back to life in 5 months
Jonas D at Bonsai Tonight sells the stuff. I think he’s still got some in stock.
https://store.bonsaitonight.com/collections/bonsai-soil/products/kanuma-free-shipping
An alternative mix would be pure peat moss... but it’s gotta be non milled peat, the kind that breaks with difficulty into chunks. Use small chunks, but not powder. It’s a bit tricky. The only brand in the US that I’ve found that has this is the Sunshine brand and the latest bag I got was about 70% useable. Some folks report getting less then 40% useable. You could slip pot into this. I’m not sure how this mix would overwinter, but other people do plant azaleas in pure peat including Walter Pall.
The biggest challenge to begin with on a total repot that it would be best if you washed off most of the peat if you use kanuma. I do it using a hose sprayer with medium spray. The roots are awful fine so go slow and be careful not to damage them. On a repot in CO I’d not cut too many roots this time. Go for a deeper training pot then you would use. Next time put it in the Bonsai pot... which btw will always be deeper for Satsuki and other azaleas then other bonsai. I actually planted high for mine. Maybe it all goes optimal and you can then repot in a bonsai pot in the spring or next..
For either soil you will have to spray the top of the soil over time to get the kanuma/sphagnum or peat thoroughly wetted at first-then you can carry on with a gentle flush.
I don’t wire the roots tightly, just barely snug.
Finally find a good place to bootstrap the Satsuki into the environment. Like I said it’s gonna be tricky this time of year and likely that will take 3-4 weeks. I have mine in afternoon shade most of the time.
Remember that azaleas like to be moist at all times, but not soaking and either media will do a good job if you water carefully.
Finally think now about how you are going to winter over these plants.
Oh yes, and this may break your heart, but it would be best not to let the Satsuki flower much next year. Give it a year to grow strong.
One resource I use is the Bay Area Satsuki Aikokai site
I have a number of books on Satsuki... I like Bonsa Techniques for Satsuki by Naka etc, but there is a great book by Callahan... it’s pricey!
Cheers
DSD sends
