pile up, store, hornwort
薀 centers on accumulation: piling things up, storing them, or gathering them together. The hornwort sense is a specific plant name that likely relates to its dense, piled growth habit.
薀 combines 艹 (grass/plant) with 溫, which likely contributes the sound and possibly the idea of warmth or accumulation. The exact historical development is uncertain.
The grass radical 艹 sits above 溫, which looks like a container with water and warmth. Picture a warm greenhouse where plants are piled up and stored in dense layers.
For ウン, imagine a huge pile of hornwort stored in a warm room, and the pile is so big it makes a deep 'oon' sound when you tap it: oon -> ウン.