Ipomoea aquatica - used as a vegetable
蕹 refers specifically to Ipomoea aquatica, a leafy vegetable also known as water spinach. Its meaning is narrow and botanical, without a broader conceptual center.
蕹 combines the grass radical 艹 with 雍, which likely provides the sound. The character was created to name a specific plant, but the exact historical development is uncertain.
The grass radical 艹 sits above 雍, like a leafy vegetable growing in a cultivated field. Picture water spinach flourishing under the sun.
For ヨウ, imagine a gardener saying 'yo!' to the water spinach as it grows: yo -> ヨウ, and the leafy vegetable responds with fresh leaves.