grass
萹 centers on grass or grassy plants. The meaning is narrow and concrete, referring to a type of vegetation.
萹 combines the grass radical 艹 with 扁, which likely contributes the sound. The character is used for a type of grass or plant.
The grass top 艹 sits above 扁, like a flat grassy patch growing over a signboard. Picture a signboard in a field of grass: 萹 is grass.
For ヘン, imagine a hen pecking at the grass: hen -> ヘン, and the hen is hidden in the grass.