coir-palm
桄 refers specifically to the coir-palm, a type of palm tree whose fibers are used for making ropes and mats.
桄 combines 木 (tree) with 光, which likely provides the sound. The character is used for a specific palm tree, but the exact historical development is uncertain.
The tree radical 木 beside 光 suggests a tree that provides light or useful material. Think of the coir-palm as a tree whose fibers shine with usefulness for making ropes.
For コウ, imagine a coir-palm tree glowing with a bright 光 (こう, light): the light from the palm cues コウ.