This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 栙 is uncertain. It visually contains 木 (tree) and 夅, but the original meaning and component function are unclear.
The kanji has 木 (tree) on the left and 夅 on the right. Without a clear meaning, it is hard to build a reliable mnemonic.
For コウ, imagine a tree (木) with a coat (コート) hanging from its branches: coat -> コウ, though the meaning is uncertain.