Hi, Rose...
I swear I answered this previously...perhaps it didn't go through.
I think all the previous answers are on the right track.
Also--
• I have successfully created basins a bit at a time over the course of years, to avoid massive root disturbance
• Trees chronically end up too deep. We have an
awesome new mulch basin drawing (one of several in the new edition) that shows a variety of measures to combat this...comments welcome.
Yours,
Art