reed
葭 centers on the reed plant, a tall grass-like water plant.
葭 combines the grass radical 艹 with 叚, which likely contributes the sound. The character specifically refers to reeds.
The grass radical 艹 on top of 叚 suggests a plant. Picture a tall reed growing above a hidden base, like the grass covering the lower shape.
For カ, imagine a car driving through a field of reeds: car -> カ, and the reeds bend as the car passes.