This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
葰 combines the grass radical 艹 with 俊, but the historical origin and meaning are unclear. It may have been a plant name or a variant form, but the exact development is uncertain.
The grass top 艹 over 俊 suggests a plant, but the specific meaning is unknown. Picture a rare, mysterious herb growing in a hidden garden.
For スイ, imagine a sweet (スイート) scent coming from the mysterious plant: sweet -> スイ, and the grass 艹 hides the source.