I would be totally flattered if you knocked on my door to ask about the plants/landscaping. Of course that probably won't be the case for another few years...if ever! I say go for it and just ask.
So my follow up question is: how do I control my enthusiasm and tendency toward the crazy eyes? ;) I don't know if I'd open the door for me.
I think it's more appropriate to walk through their gardens, including the backyard on your own and take photos. Then wave at them if they see you and high tail it out of there.
Will you bail me out if it goes badly? :)
Just don't get caught...