This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 楻 is uncertain. It combines 木 (tree/wood) with 皇, but the original semantic and phonetic roles are unclear.
The left side 木 suggests wood, and the right side 皇 suggests something grand. Picture a grand wooden pole or beam, but remember its meaning is uncertain.
For コウ, imagine a grand wooden pole (楻) being carried by a strong kou (こう) sound: kou -> コウ.