Hey apologies if I sound like I'm greatly underestimating your maturity but it's probably better to act on the side of caution.
Flowers are lovely I really hope she likes them! I also have a best friend who I really want to be happy and would probably appreciate nice flowers, although I'd personally steer away from valentine's as I wouldn't want them to be interpreted romantically.
As for that, I trust you know if you want them to be interpreted as a romantic gesture and you should absolutely let her know they're from you and if they are or aren't.
If you want it to be mysterious if they're from you, absolutely expect her to either figure it out or assume they're from someone else secretly and show them gratitude, it's a rare person who would receive something anonymously and not try to presume the sender.
It is a really thoughtful gesture to get flowers for someone you care about today in the want for them to be happy, but it's not fun for anyone if it's misinterpreted and someone ends up less happy for it.