This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 芉 is uncertain. It visually combines 艹 (grass) with 干, but the original meaning and component function are unclear.
No clear meaning is known, so a mnemonic is not possible. Focus on the visual shape: grass 艹 over 干.
For カイ, imagine a kite (カイト) tangled in the grass 艹 above a drying pole 干: kite -> カイ.