Terrorizing with flowers is not just about throwing some petals around and expecting people to be scared. It's an art form.
First, you need to understand the psychology of your victim.
Do they fear snakes? Use some nice, realistic-looking cacti.
Do they fear heights? Dangle some flowers from a precarious cliffside location.
Do they fear the unknown? Create an entire field of identical, faceless, and slightly menacing-looking sunflowers.
And for the love of all that is good and holy, do not use daisies. Nobody fears daisies.