I've decided now after playing with a few different methods, that I like the greatstuff and drylock with rope for creating vines / root systems on this 29 gal scale. I once made some large mangrove roots by carving styrofoam and that worked nicely; but this has been much easier for making that curved look of roots growing across a rock structure.
I am a bit afraid of having too much of a harry potter look in my foam "tree roots". whaddya think? any advice??