Are you actually dating her or just "talking" to her for a month? Are you official bf/gf? I don't believe in giving flowers unless you're actually official and you know that she's feeling you the way you're feeling her (I assume you're feeling her or you wouldn't want to bring her flowers) I think giving someone flowers before you're in an official relationship if putting pressure on the gal to like you back and they are always usually appreciated but they sub-concsiously put a bit of anxiety in our heads.
However; If you are definately going to give them to her then I'd do it in person. IMO finding them on my car would be kinda stalkerish.. It would be weird that you were there and didn't come in to see me and it would be weirder that you didn't give them to me in person if you DID come in to see me. That's my personal take on it anyway.
“The willingness to accept responsibility for one’s own life is the source from which self-respect springs.” ~Joan Didion