If the tree has lots of fine roots you can get rid of any tap root as required to get it into a pot. You could either put it into a nursery pot or a bonsai training pot, whichever it fits into. I would hang onto your shovel until the tree has gone dormant for the year. If you are going to dig it in the fall you should provide good winter protection this winter. Mine went into a cold frame, and still does, for winter. It still freezes in there, but generally doesn't get below 25 degrees or so.
I dug my forsythia in October after it had lost it's leaves for the year and put it into a large nursery pot. It stayed in that for two years and then went into a bonsai pot. As I recall I had to leave some of the taproot in tact to retain a good quantity of small roots. The taproot was then reduced dramatically at the first repotting since the tree had generated plenty of fine roots.
This is a picture from June 1996, the summer after I dug it.
- forsythia_6-96.jpg (214.4 KiB) Viewed 722 times
