Don’t believe everything you read on the internet.
– Abraham Lincoln
0 stars, would not recommend
Zone 8b/9a, PNW, near Mirai
I read the same about Mugos on the internet and tried an end of August repot on one a few years ago (before Mirai Live).
~40% root mass reduction from nursery can, into 60:40 pumice:nursery soil, screened
The tree survived, but BARELY pushed any new needles in the spring. The next year’s growth was a little better, but really took 3 years to get back to full vigor.
Finally got to style it last year (4 years) and repotted again into a new container in February this year. Candles are still extending right now, but I’m not seeing much loss of vigor due to repot. ~65% root mass reduction (lots of matted roots) into 1:1:1 akadama:pumice:lava. Much smaller pot, high confidence in the remaining roots
Early spring repots have more room for drastic work. On a first repot from a nursery can, you never know what you’re going to run into. Better to have more flexibility and a higher chance of success.
You can repot in late summer, just like you can grow a tree indoors. Having a tree survive is not the same as having it thrive, though.