I dunno if it is easy to root cuttings.. On shadow exposition, the trunk produces aerial roots but i'm not sure if it is really roots... Maybe it is anchor filaments ? Dunno...
if there is one tree, that i'm sure to keep alive "forever" (as long as i keep it), it is this tree, because it has no known parasites or diseases.. i don't need to treat this tree. Hydrangea loves water (leaves hang down in case of lack of water) so water very frequently, use draining substrate (akadama), use a good amount of fertilizer (i use osmocote and organic fertilizer in pellets).
you should open a separate thread to show pics of your tree and its evolution