evergreen mulberry
榕 centers on a specific type of tree: the evergreen mulberry, a tree that stays green year-round.
榕 combines 木 (tree) with 容, which likely contributes the sound. The character was created to name the evergreen mulberry tree.
The tree radical 木 beside 容 suggests a tree that is always full of leaves, like a container (容) that never empties. Picture an evergreen mulberry tree holding its green leaves all year.
For ヨウ, imagine a young evergreen mulberry tree: young -> ヨウ, and the tree stays green and youthful.