water chestnut
蔆 refers specifically to the water chestnut, an aquatic plant with edible corms. The meaning is narrow and concrete, centered on this particular plant.
蔆 combines the grass radical 艹 with 淩, which likely contributes the sound. The character is a phono-semantic compound for an aquatic plant.
The grass radical 艹 sits above water 氵 and a shape suggesting a plant with edible parts. Picture water chestnuts growing in a pond under grassy leaves.
For リョウ, imagine a water chestnut floating in a clear rill (small stream): rill -> リョウ, and the plant thrives in the water.