To avoid girdling roots, whenever I up-pot or transplant to the ground I usually partially bare root the plant and then prune and spread the roots out. Alternatively a quick and dirty approach is to cut vertically down the rootball in 3-4 locations around the rootball in order to sever the roots the circle around the rootball. Of course, it is a good idea not to mess with the roots at all in the heat of the summer.