rose of Sharon, althea
葮 refers to a specific flowering plant: the rose of Sharon (althea). The meanings are botanical names for the same shrub, not separate conceptual ideas.
葮 combines the plant radical 艹 with 段, which likely provides the sound. The character was created to name the rose of Sharon plant.
The grass top 艹 marks this as a plant, and the bottom 段 looks like a tiered structure. Picture a rose of Sharon bush with layered blossoms like steps.
For ダン, imagine a rose of Sharon blooming in a dan (段, step) garden: dan -> ダン, and the plant grows on the terrace.