Rootbound or not, I try to remove as much of the soil in potted/purchased plants as I can as it often times is some sort of spongy wet type of stuff or it is mostly bark/mulch. The spongy stuff, during our cold wet winters, soaks up water, freezes and the plant rots. The really loose mulch type stuff drains too well and is too loose and the plant will still die over the winter. I have best luck shaking and working all that off, and then planting it in the garden soil.
