scallions, onions, leeks
蔥 centers on allium vegetables with long, hollow, tube-like leaves: scallions, onions, and leeks. The character groups these together as a single conceptual category of pungent, layered greens.
蔥 combines the grass radical 艹 with 悤, which likely contributes the sound. The character is associated with scallions and similar plants, but the exact historical development of the lower component is uncertain.
The top 艹 shows it's a plant. The lower part 悤 looks like a bundle of hollow tubes with a heart-like base, just like scallions and leeks growing in a clump.
For ソウ, imagine a field of scallions swaying in the wind, making a soft 'sow' sound: sow -> ソウ, like the rustle of their leaves.