hemp seeds, plant with abundant
蕡 centers on the fruit or seed of the hemp plant, and by extension a plant with abundant seeds or fruit. The given meanings are narrow and botanical; the kanji is rare and its standalone sense is limited to this specific plant context.
蕡 combines the plant radical 艹 with 賁, which likely contributes a sound element. The character is associated with hemp seeds and abundant plant growth, but the exact historical development is uncertain.
The top 艹 shows it is a plant, and the bottom 賁 looks like a container full of small things. Picture a hemp plant with a pod bursting with abundant seeds.
For フン, imagine the fun of watching hemp seeds pop open: fun -> フン, and the plant is full of abundant seeds.