This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
莦 combines the grass radical 艹 with 肖, which likely provides a phonetic cue. The exact historical development and original meaning are uncertain.
The grass radical 艹 over 肖 suggests a plant, but the specific meaning is unknown. Picture a mysterious weed with no clear name.
For ソウ, imagine a tangled weed that makes you say 'sow!' in frustration: sow -> ソウ, as you try to untangle the messy plant.