Phono-semantic compound (形声). The radical 艹 (grass/plant) provides the semantic element, while 化 (change, か ka) supplies the sound. A flower is a plant that transforms — budding, blooming, and falling.
The grass radical 艹 on top plus change 化 below... a plant that transforms into something beautiful... that's a 'flower'...! Sensei wants to press it in a book...