The Flower Card is far from dangerous. It's a friendly card who's just looking for a good time. It likes to party and dance, plus it likes to spread it's petals everywhere. It chose a bad time to do this in Episode 10, during the Athletics Meet at Reidenton Elementary.
The petals kept increasing more and more as time passed, causing people to become buried. Sakura managed to summon the Fly Card to escape, where they found Flower's above form on the roof of the school. After a quick dance(^_^), Sakura managed to seal it. When summoned, it spreads petals and creates flowers.
