This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
菨 combines 艹 (grass) with 妾, but the historical development and original meaning are uncertain.
The top 艹 suggests a plant, and the bottom 妾 is a shape that appears in other characters, but the overall meaning is not visually obvious.
For ショウ, imagine a rare plant with a showy flower: show -> ショウ, and the 艹 top reminds you it is a plant.