Hi, you don't HAVE to cut the roots, but it generally is easier on the tree (and you, in the end) if you do it. You get as sharp a spade as you can, then plunge it straight down through whatever roots might be under it, as far out from the trunk as the branches/foliage are spread, but don't lift the tree til the next year. Many people do it, in a ring around the tree, but alternate the cuts with untouched gaps as it may be easier on the tree to do the job in stages over two years (those gaps are addressed in the following year). However, you said your tree's been in the ground 6 yrs, which makes me wonder just how large it is now... you could end up with a serious weight problem needing a lot of extra help from lifting devices, etc. How big is the tree now? And BTW, don't try to get it into a 'bonsai' pot right off the bat - at the size it's going to be when out of the ground, a training box of some kind for the first year at least is probably goiung to be needed, if only so you don't end up pruning off (once the tree's dug) 3/4 of the root mass all at once, yet having very few new feeder roots actually up near the trunk - having ringed the tree in the ground, it will need a couple of seasons at least to form a pad of new ones able to sustain itself.

I have three California live oaks i grew from acorns started about5-6 years ago.Never in more than a 6" pot.Wow,what tough plants they are.I have put them where i have forgotten about them..and they looked very good.Even letting them get way too long ,then cutting far back,-they took it in stride. One is in a Bonsai pot,the others just now are in larger pots to get some girth to the trunks. A long road to see them with the fat trunks of some I have seen at Bonsai nurserys for thousands of dollars.

Burleigh, that sounds like it could be a fantastic start, but yes, after 6 years how large is the tree? You mention that you have been cutting it back. Could you post some pictures so that we may see what you are working with?