They can talk about all sorts of things, but I dont know of Harry Being interested in gardening. And IMO they will probably stick to non contentious subjects like how Archie is, sicne I can't imagine Charles is very happy that his son is living in LA, having walked out on his job and is now it appears doing a Diana by the use of a book which makes the RF look bad.
Seeing as we don't have any idea at all what Charles, Harry and Meghan talk about (except for Archie), this is quite preposterous imagining presented as truth. How do you know Charles never thought about walking out of the job himself, seeing how his little brothers coped and being happy for Harry that he did it? Only because the queen's and her parents' generation bedevilled the Duke of Windsor for chooseing his freedom over the Crown doesn't mean Charles doesn't understand Harry and wants to keep talking to them.