This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
葙 combines the grass radical 艹 with 相, but the exact historical development and original meaning are uncertain.
The grass radical 艹 over 相 suggests a plant, but the specific meaning is unclear. Picture an unknown herb with leaves that resemble the shape of 相.
For ショウ, imagine a showy, unknown plant: show -> ショウ, and the grass radical 艹 hints at a plant.