to choose, to select, greens
芼 centers on selecting or picking out, especially in the context of edible plants. The meaning 'greens' is a specific application: chosen edible leaves or vegetables.
芼 combines 艹 (grass/plant) with 毛, which likely contributes the sound. The exact historical development is uncertain, but the structure suggests a plant-related word with a phonetic component.
The top 艹 is grass or plants, and the bottom 毛 looks like a hand picking something. Imagine selecting the best greens from a field by hand.
For ボウ, picture a bow made of selected greens: bow -> ボウ, and you choose the finest leaves to tie into a bow.