I wasn't aware plants didn't work in caves. Perhaps it was only a single version has that. Otherwise, the plant layer has the same restrictions it always did: Plants will only spawn on the appropriate blocks. If you create a cave layer without specifying floor materials, plants will spawn on the patches of dirt auto-generated by the default underground material, but not on any of the stone blocks.


The main problem I can spot so far is that worldpainter is not yet up to the point where it considers 'moss' a block that plants can spawn on.