My husband had a good idea for limited thawing; get a fairly long heating cable - the sort you would use to keep a water pipe from freezing - lay it on the ground in a circle about 8" away from the clump, insulate the upper surface of the cable circle so the heat goes down and not everywhere, and leave it there for - - probably a day + (plugged in of course
) Depending on where you are (not shown) you may only have a couple of inches of frozen soil to get a spade through. With this method you won't thaw the root ball out very much, if at all.