This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
蓈 combines the grass radical 艹 with 郎, but the exact historical development and original meaning are uncertain.
The top grass radical 艹 suggests a plant, and the bottom 郎 may hint at a sound or a place. Without a clear meaning, picture an unknown weed named after a person called 郎.
For コウ, imagine a plant so rare that only a 公 (こう, public official) knows its name: 公 -> コウ, and the official points to the mysterious grass.