I don't see why she couldn't book with you now, but maybe that's her policy.
I don't think you can ever book anything too early, in fact, the earlier the less stress on you! I am happy to book ahead of time with clients; in fact I have been booked over two years in advance in the past. I honour the prices quoted at that time so I don't see why other businesses can't do the same. However, every business is different!
If this florist is the florist you really have your heart set on, ask her when she may consider booking & keep in touch with her until then to make sure your date isn't stolen!