die (vegetation)
槁 centers on vegetation dying, withering, or drying up. It describes the state of plants that have lost life and moisture.
槁 combines 木 (tree/wood) with 高, which likely contributes the sound. The character is associated with dried or withered vegetation, but the exact historical development is uncertain.
A tall (高) tree (木) that has lost all its water and stands dried up: 槁 means vegetation dying or withering.
For コウ, imagine a withered tree in a cold (コウ) winter landscape: cold -> コウ, and the tree has died from the frost.