Technically, a flower is a plant - as long as it's not a poisonous plant, there's no reason it cannot be edible. It's just most flowers are not very tasty.
Growing up, we had meadows full of purple flowering clover (I think that's what they were). We found if you pulled out the little flower pedals and sucked on the stems, they tasted like honey. We tried chewing the pedals, and those were very bitter. After that, we just stuck to sucking on the pedal stems.