This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
菪 combines the plant radical 艹 with 宕, which likely contributes sound. The exact historical development is uncertain.
The grass top 艹 over 宕 (which suggests a cave or dwelling) could be imagined as a plant growing near a rocky shelter, but the meaning is unclear.
For トウ, picture a toe (トウ) poking out from under a grassy plant: toe -> トウ, and the plant hides the rest.