Run, don't walk, to see the English Garden in the formal Conservatory Garden collection, in the northwestern part of Central Park.
I passed through on my way to the North Woods but was transfixed here, lost track of time, spent my Canon's battery on pictures (using the Frenchman's telephoto - I was actually on a bird hunt!). I'll be writing story about the plantings for Gardenista, with lots more pictures.
[Here is that story!]
In the meantime, just go.