I do slightly raised beds because of terrible soil in some places in my yard and the peonies do manage to expand out into that hard mess. The craziest I've seen is directly behind the house, a peony root actually extended around 3-4' to grow under a stone path, ended up pulling one of the large stones out of place to follow the root to the end....only to chop it off. There was no way that I was digging a hole large enough to replant it.
