Top grass radical - three compressed strokes for the standalone 艸. As the indexing radical it places 蒲 in the botanical family alongside 草, 花, 苗, 茶. Cattail and bulrush are reedy marsh plants, so the grass cap is a literal semantic marker.
Bottom 浦 supplies the sound - pǔ drifting to pú with a tone shift. It also adds a faint semantic boost: 浦 means waterside or riverbank, and cattails grow at the water's edge. Same phonetic appears in 蒲's relatives 莆 and 葡 (grape vine).
