A person who lives to garden,has studied (formally or informally) plants,trees,etc. and can tell you more about them than you probably want to know.
Can be found in mostly sub-urban and rural areas.
Area of expertise is varied, but is usually related to their own "zone".
Gardenologist: Wow! That's an amazing senna!
Friend: Uh... yeah, nice.... (huh?)
Gardenologist: Hey! What's that weird looking plant by your foot?
Friend: I dunno, you're the gardenologist!
Gardenologist: Oh,how sweet, there's a name for people like me!( proceeds to pluck a leaf of unknown plant to identify later)